Jupiter at its closest opposition in nearly 50 years

Today at around 13:00 UTC, the Jupiter system will be in opposition, when the planet and its attendant moons are on the opposite side of the Earth's sky from the Sun.  This means that the planet will be visible as a brilliant star in the sky all night long, rise in the east at sunset and setting in the west at sunrise.  It also means that Earth is at its closest approach to Jupiter this year, with Jupiter only 591,560,000 kilometers (367,578,000 miles) away.  From the perspective of any amateur astronomers out there, this makes it a great time to take a look at Jupiter as it is nearly 50 seconds of arc in diameter in the night sky (though let's kill any "Jupiter hoax" in the bud, Jupiter's apparent diameter is still 1/36 of the apparent diameter of the Moon).  That is large enough to pick out Jupiter's cloud bands on even the most modest of telescopes.  Despite its great distant, its large size also makes it bright enough to easily pick out in the sky.

In addition to being the closest Earth will be to Jupiter all year, this is also the closest Earth has been to the giant planet since 1963.  That is because Jupiter is close to perihelion, its closest point in its orbit to the Sun.  As you can see in the graphic above, Jupiter's orbit is slightly eccentric, and right now it is closer to the Sun (and the Earth) than it would be on the opposite side of its orbit (near apohelion), which is slightly off the graphic that is centered on the Sun.  This makes this opposition a particularly good one to check out.  Don't forget though that even if you are unable to check out Jupiter tonight, it will still be an excellent target to view for the next couple of months, though it will become more and more a planet to view in the evening.

Many planetary astronomers have been taking advantage of the current opposition to take some great images of the giant planet and its moons, like the one at left.  I love the detail you can see in this image, taken on September 20 by astronomer Damian Peach.  The southern equatorial belt is still faded and there is no indication that it will reappear anytime soon.  A great place to check for more fresh images of Jupiter is the ALPO-Japan site, where astronomers from around the world post their latest and greatest shots. Another great site to check out is Cloudy Nights forum, which includes some great discussion of how these images are taken.

So please, definitely take this chance to look up at Jupiter!

The Errata of the Day

Happy Thursday everyone! Only one more day till Friday...  Just thought I would post a few quick notes:

• Jupiter and the Moon are particularly close tonight in the late evening and night sky.  The simulated view to the right shows the sky to the southeast at around 11:06pm local time (since the moon is moving slowly across the field of background stars, YMMV).  And nothing special happens right at 11:06pm, that's just when I pressed stop in Celestia ;-) The close proximity of the Moon to Jupiter (and Uranus) makes it a bit easier to locate those planets, so it is worth taking a look, even for those who don't have telescopes (like myself).  Jupiter is approaching opposition, which occurs on September 22.  On that date, Jupiter will be opposite the Sun from the Earth, meaning the planet (and its attendant moons) will rise over the horizon at sunset, reaches its highest point in the sky at local midnight, and set below the horizon at sunrise.  This is also the best opportunity to observe Jupiter because it will reaches its largest size in the night sky.  In fact, this will be Jupiter's closest approach to Earth than at any other time between 1963 and 2022.
• The Carnival of Space #168 is now up over at Weird Sciences.  Read up on Trojan kuiper belt objects, bistatic experiments on the Moon, and the Laser Interferometer Space Antenna (LISA).  I submitted my write up on a method for creating true color images of Io for this week's Carnival of Space.
• Emily Lakdawalla of the Planetary Society Blog is this week's featured Woman in Planetary Science.  I'll admit, I never read that blog much because, well, I'm not a woman in Planetary Science.  But after reading a few of the profiles, I see that many of the experiences are pretty universal, and can be useful for both men and women who are considering a career in this field.

Io@400 Part 5: The Starry Messenger

This week we are looking back at the discovery of Io and the other Galilean satellites by Galileo Galilei 400 years ago yesterday.  Yesterday, we took at Galileo's observations of the Jovian system with his refracting telescope in Padua, Venetian Republic in the first two months of 1610, including his discovery of the four moons that bear his name on January 7, 1610.  Today, in the fifth and final part of our series of posts in commemoration of the discovery of Io, we take a look at Galileo's book, Sidereus Nuncius, where he published his discoveries with the telescope.  We will also look at the contemporary reaction to the book, both from fellow scientists and from the dominant power in Italy at the time, the Catholic Church.

...for now we have not one planet only revolving about another, while both traverse a vast orbit about the Sun, but our sense of sight presents us four stars circling about Jupiter, like the Moon about the Earth...

- From Galileo's Sidereus Nuncius (trans. by Edward S. Carlos)

The Starry Messenger

As Galileo recorded his observations of the Jovian system in the first two months of 1610, Galileo set about writing a book reporting his discoveries to the world.  This book, Sidereus Nuncius or The Starry Messenger in English, was approved for publication by the Venetian church censors on March 1, completed on March 2 with one final Jovian observation, religious approval formalized on March 8 (probably after the proofs), and published on March 12 with a new forward dedicated to the Grand Duke of Tuscany.  Also during this time, he changed his suggested naming for the four new moons from the Cosmica Sidera (or the Cosmic Stars after Grand Duke Cosimo personally) to Medicea Sidera, or the Medicean Stars after Florence's ruling family as a whole.  All of these flatteries, the dedicated forward, the naming of the moons after the Medici, and the special bound version along with one of Galileo's telescopes sent to Cosimo through Tuscany's ambassador to Venice, Belisario Vinta, were intended to get in Cosimo's good graces, perhaps with a post at the University of Pisa and a better deal than the Venetians had given him in return.

Galileo's book became an instant best-seller, selling out the 550 copies printed its first week.  The imperial astronomer, Johannes Kepler, was given a copy of the book by a Galileo associate, Martin Hasdale, in mid-April 1610.  Kepler was encouraged to write a response to the book, which he did in Dissertatio cum Nuncio Sidereo (Conversation with the Starry Messenger).  In the book, Kepler praised Galileo's discovery, though he had not had a chance to confirm it, but regretted Galileo's lack of philosophical discourse in his treatise and that he didn't acknowledge the contributions of scientists like Copernicus and Giordano Bruno for expecting these kinds of observations.  In Italy, Galileo was facing a mixed reaction.  The Venetians, angered by Galileo turning his back on them and turning to the Medici, began a campaign to discredit Galileo.  They were his patrons after all, giving him a raise for his post at the University of Padua during the previous year after he showed his telescope to the Doge and the Segnoria.  The Venetian ambassador to Tuscany, Giovanni Bartoli, told Vinta that Galileo, "was being laughed at for having claimed new discoveries in the heavens and for having duped the government with a so-called invention that could have been bought anywhere for a few lire, said to be of the same quality as his" (from Stillman Drake's Galileo at Work, pgs. 158-9).  Note that Galileo gifted his telescope to the Venetian state, not claiming it to be his invention, but an innovation better than the version a merchant from the Netherlands was trying to sell.  Kepler requested that the imperial ambassador to Venice, Georg Fugger, send a copy of Sidereus Nuncius, to Emperor Rudolf II, to which Fugger responded that Galileo had written a pretty boring book without philosophy to back him up, and besides, he just stole the telescope design from the Dutch, so why bother.  Basically, Fugger was repeating the misinformation the Venetians were putting out there about Galileo.

Criticisms

In May 1610, Galileo was given an appointment as Chief Mathematician at the University of Pisa, where he would not be obligated to teach, giving him more time to write more books on his views on the universe and to act as the Philosopher and Mathematician to the Grand Duke of Tuscany.  These appointments came with a salary of 1000 florins.  So basically, Galileo received what he hoped to get out of the Tuscans, and he was able to return to his home country.  As Galileo traveled from Padua to Pisa in April and May, 1610, he began to pick up on the growing opposition to the Sidereus Nuncius and his discoveries.  At Bologna in late April, he visited with Giovanni Magini (who was passed over by Galileo for the appointment as chief mathematician at Padua in 1588) and his student Martin Horky.  Both Magini and Horky over the next few months would write letters and books describing Galileo's discoveries as ridiculous.  Horky would say that he failed to see the Jovian moons from Galileo's telescope when the latter visited Bologna (though in some sources I found, he does claim that he observed spots near Jupiter during Galileo's visit), and would write a book titled A Very Short Excursion against the Starry Messenger.  In Excursion, Horky argued that the spots were optical illusions produced by the imperfect glass used in Galileo's telescopes, akin to the colored halos that appeared around the stars and planets.  Francesco Sizzi would write similar objections, as well as noting a few theological ones, in his 1611 book, Dianoia Astronomica, Optica, Physica.  Further opposition would come from the chief theologians at Padua and Pisa, who refused to look through Galileo's telescope. Lodovico delle Colombe in 1611 wrote Against the Earth's Motion, refuting Galileo's interpretations. Colombe over the next few years would become one of Galileo's chief critics in Tuscany, forming the League of Pigeons (a play on Colombe's name, which means dove in Italian).

Much of the opposition to Galileo's discoveries came from the lack of verifiability.  With telescopes of high enough power to observe Jupiter's moons so rare, other astronomers were having difficulty replicating his discoveries, except for Simon Marius, but he didn't publish his findings for another four years.  Kepler finally had a high-enough powered telescope by October 1610, and was able to confirm the existence of Jupiter's moons.  The Jesuit astronomer of the Collegio Romano Christopher Clavius in Rome first disputed Galileo's findings for lack of proper instrumentation, but finally observed them as well by December 1610.  Other observers of the Galilean satellites during that second observing season included Thomas Harriot in England as well as Nicolas-Claude Fabri de Peiresc and Joseph Gaultier de la Vallette in France.  de Peiresc would later suggest individual names for the Galilean moons based on the names of Cosimo and his brothers, but those have not survived to the modern day.

Galileo's run-ins with critics of his discoveries, particularly his interpretation that they supported the Copernican model of the universe, would continue for the rest of his life.  This conclusion became particularly entrenched following his observation of the phases of Venus in late 1610.  At this point, the church predominantly supported Tycho Brahe's model which entailed the five known planets (Mercury, Venus, Mars, Jupiter, and Saturn) orbiting the Sun while the Sun and the Moon orbited an unmoving Earth.  This model retained the geocentrism and most importantly the geostatic positions of the Earth, which were discussed in the Bible, but was simpler and better supported by observations than the Aristotelian/Ptolemaic model where all celestial bodies orbited the Earth. Jesuit astronomers confirmed Galileo's observations using their own telescopes, but disagreed with him regarded his conclusions about what they said about the position of Earth and the Sun in the Universe.  The lack of observed stellar parallax at the time seemed to preclude a moving Earth, not to mention that Galileo's notion that tides were result of the Earth's motion were viewed as absurd.

Galileo would persist in his support for a heliocentric universe, as most dramatically presented in his 1632 book, Dialogue Concerning the Two Chief World Systems.  In that book, Galileo presented arguments for both the Copernican heliocentric model and the Tychonic hybrid model.  His attempt to be balanced in his coverage, as requested by Pope Urban VIII, a friend of Galileo's prior to his elevation to the papacy, backfired however, when it was perceived as having mostly pushed the heliocentric view, correcting the points of the geocentric Simplicio, and insulting his former friend, the Pope.  This apparent outright defense of the Copernican world view was seen as in violation of an order by Cardinal Roberto Bellarmino in 1616 (who was later canonized in 1930), which ordered him to only describe the heliocentric universe as a theory.  While it is now agreed that Galileo didn't intend to slight the pope (and Simplicio was meant not as an insult, but as a reference to famous Aristotelian philosopher Simplicius), Galileo's apparent offense to the Pope, who was already seen as not being tough enough against heretics, removed the huge support he had against his church critics.  For his support of the Copernican heliocentric world view and his support of atomism (the view that matter was composed of small, invisible particles, which contradicted the transubstantiation of the Eucharist), he was put on trial for heresy in Rome.  Galileo was found guilty, but managed to avoid life imprisonment or worse by confessing his actions and refuting his earlier heliocentrism.  As a result of this and his age, Galileo would receive house arrest, requiring him to remain at his Villa il Gioiello in Arcetri near Florence.

Thanks to the mathematical models of Johannes Kepler, which showed that the planets revolved around the Sun in elliptical orbits at speeds that were fastest at the perihelion point in their orbits, and Issac Newton, which provided a physical foundation for the Copernican/Kepler heliocentric model (gravity), the heliocentric model gained acceptance as the 17th Century progressed.

This is the last of my series on the discovery of the Galilean satellites by Galileo Galilei, the reaction to it among his contemporaries, and the scientific arguments regarding the place of the Earth in the Universe at the time.  I hope you all enjoyed it.  If you missed any of these articles, check out my post from Sunday which provides a nice index to this week's series.

Link: Galileo at Work: A Scientific Biography [books.google.com]
Link: The Galileo Affair [www.galilean-library.org]
Link: Sidereus Nuncius [www.liberliber.it]
Link: Sidereus Nuncius (translated to English) [hsci.cas.ou.edu]

Io@400 Part 4: 400 years ago today, Cosmica Sidera

This week we are looking back at the discovery of Io and the other Galilean satellites by Galileo Galilei 400 years ago today.  Yesterday, we took a look at Simon Marius's observations of the Jovian system and his claim that he observed the Galilean satellites as early as late November 1609.  Today, in part four of our series of posts in commemoration of the discovery of Io, we examine Galileo Galilei's discovery of the Galilean satellites on January 7, 1610, 400 years ago today, and his subsequent observations of the system during the first two months of 1610.

Behold therefore, four stars reserved for your famous name, and those not belong to the common and less conspicuous multitude of fixed stars which, with different motions among themselves, together hold their paths and orbs with marvelous speed around the planet Jupiter, the most glorious of all the planets, as if they were his own children, while all the while with one accord they complete all together mighty revolutions every twelve years around the center of the universe, that is, round the Sun.

- From Galileo's Sidereus Nuncius (trans. by Edward S. Carlos)

Three Little Stars

In early January 1610, Galileo had just finished up an incredible series of observations of the Moon, the Milky Way, and several stellar nebulae.  In the case of the Moon, his 20-power telescope revealed the Moon's cratered and mountainous surface, showing that it was not smooth and perfect as stated in the Aristotelian world view, but was in fact, quite rough.  Galileo discovered a multitude of stars, far more than could be seen with the naked eye, within the "clouds" of the Milky Way, on the belt of the constellation Orion, and within nebulae and star clusters such as the Orion Nebula, Beehive Cluster, and the Pleiades.  During the first week of January 1610, Galileo drafted a letter to Galileo's Florentine friend, Enea Piccolomini describing these observations along with details on how best to operate the telescope for astronomical viewing, for example, suggesting the use of a stand for the telescope to prevent inadvertent hand motions from ruining observations.  Galileo set aside the letter for a few days, picking it back up on the evening of January 7, 1610.

That evening, an hour after sunset, Galileo aimed his 20x telescope, with a newly masked objective lens, toward the planet Jupiter, at the time near opposition and in the midst of its retrograde loop in the night sky.  In this case he observed three stars forming a line with Jupiter, parallel to the ecliptic.  Two of these stars were to the lower left (east) of Jupiter and another star was to the upper right (west) of the planet.  He noted this in the conclusion of his letter to Piccolomini:

... and only this evening I have seen Jupiter accompanied by three fixed stars, totally invisible by their smallness, and the configuration was in this form [graphic shown matches the configuration displayed in the screenshot from Celestia at left] nor did they occypy more than one degree of longitude.  The planets are seen very rotund, like little full moons, and of a roundness bounded and without rays.  But the fixed stars do not appear so...

On January 7, 1610, Galileo discovered what we call today the Galilean moons, believing them at the time to be fixed stars behind Jupiter.  When I first read this passage, I assumed he was referring to the moons as "The planets" in that last section, hence my anniversary message earlier today.  However, re-reading that and understanding the timing of when that was written, the very night he made his first observation of these satellites, he was actually referring to planets like Venus, Jupiter, or Mars as "little full moons", and that these "fixed stars" do not appear like that.  This again drives home the point that on January 7, the day of discovery, Galileo hadn't figured out what he found, but it was enough for him to keep making observations over the following days.

A quick note: Galileo refers to having seen three little stars, not four.  In fact, he did see all four Galilean satellites.  Ganymede was the lone star to the west of Jupiter and Callisto the star furthest east.  The eastern star closest to Jupiter was actually two moons, Io and Europa, too close together for Galileo's low-powered telescope to separate.

Over the following days, Galileo continued to observe Jupiter.  On January 8, he found three moons again, this time all to Jupiter's west with about equal spacing (from Galileo's Sidereus Nuncius, with moon labels added by me):

This observation startled Galileo, "Yet my surprise began to be excited, how Jupiter could one day be found to the east of all the aforementioned fixed stars when the day before it had been west of two of them."  Jupiter was moving retrograde, as expected being at near opposition, and if anything, Jupiter should be to the west of all three stars.  Galileo even noted that he hadn't even considered the fact that all three were now closer together.  I should point out that a fourth Galilean satellite, Callisto, could have been seen to the far east of Jupiter, but was apparently outside Galileo's field of view and thus wasn't recorded.

After he was clouded out on January 9, he continued his observations on January 10, finding two stars (Callisto and a close conjunction of Ganymede and Europa) to the east of Jupiter.  Io was caught up in the glare of Jupiter in Galileo's telescope for him to observe it that night, as he suspected.  At this point, as he noted in his Sidereus Nuncius, Galileo realized that the orientations of the stars with respect to Jupiter changed, not due to the motion of Jupiter, which would have to be chaotic, first moving west, then east, then west again on successive nights, but due to the motions of these stars themselves.  On January 11, he noted two stars to the east of Jupiter (Ganymede and Callisto; Io and Europa were transiting Jupiter at the time and were thus lost in the planet's glare).  Galileo even noted the difference in brightness between Ganymede and Callisto.  With this observation, according to his Sidereus Nuncius, Galileo came to the conclusion that these were not stars at all, but little planets in orbit around Jupiter, just as the planets revolved around the Sun.  He also deduced that there were not three bodies moving about Jupiter, but four, even despite the fact that he hadn't solidly observed all four at once to this point.  Some historians, such as Stillman Drake, propose that these conclusions, that Jupiter was orbited by four little planets, were actually reach several days later, and that he simply put his those notes in the wrong place in his notebook, which were then further transcribed in his book two months later.

Cosmica Sidera

Galileo continued to jot down his observations on the draft of a letter he had written to Venetian Doge Leonardo Donato back in August 1609 (promising not to show his innovations to another foreign power...oh, like Tuscany...).  On January 12, Galileo observes Jupiter for a more extended, first observing the planet and two of its moons on each of the planet, Ganymede to the east and Europa to the west, then seeing Io emerge from Jupiter's glare to the planet's east.  Callisto remained hidden from view in the glare.  By this point, certainly Galileo could make the conclusion that the "stars" moved perceptively during a single observing run.  The following day, January 13, Galileo observed and recorded all four Galilean satellites for the first time, with Europa to the east of Jupiter and Ganymede, Io, and Callisto, respectively, to the west.  So it is on January 13 that Galileo could obviously conclude that there were four stars, instead of three.

I notice on Wikipedia that this is listed as the discovery date for Ganymede, while January 7 is listed for the other three. While January 13 was the date that Galileo first observed all four satellites at the same time, by that date, he had observed all four, though just not all at the same time.  You could reasonably argue that for any of the moons, that three were discovered on January 7 and the fourth on January 13, but the fact remains that on January 7, all four were visible to Galileo and noted by him (though two were too close to be separated).

Galileo's first detection of motion between the moons, evidence that the moons orbited Jupiter, came on January 15 when Galileo observed all four moons to the west of Jupiter.  Over the course of four hours that night, Galileo observed Io moving east and slowly become lost in the glare of Jupiter.  Ganymede and Europa, the middle of the two moons seen, were observed to come closer together as the night progressed.  From this and his earlier observations, Galileo was able to conclude that these four stars actually orbited the planet Jupiter, just as the planets orbit the Sun.

For Tomorrow

Galileo would continue to observe Jupiter and the moons he would christen the Cosmic Stars, after his former pupil and future employer, Cosimo II de' Medici, the Grand Duke of Tuscany, for the next month and a half, until March 2, 1610.  At that point, he published his findings, the rough terrain of the Moon, the countless stars of the Milky Way, and the new Jovian planets, in Venice as Sidereus Nuncius, or The Starry Messenger.  Tomorrow, January 8, we will discuss this ground-breaking book and its reception by the public, Church, and fellow scientists.

Link: Galileo's First Jupiter Observations (comparing Galileo's records with simulated views) [home.comcast.net]
Link: The Galileo Project | Satellites of Jupiter [galileo.rice.edu]
Link: JPL Galileo Discovery Page [www2.jpl.nasa.gov]

Io@400 Part 3: Simon Marius and the Mundi Iovialis

This week we are looking back at the discovery of Io and the other Galilean satellites by Galileo Galilei 400 years ago tomorrow.  Yesterday, we took a look at Galileo's first scientific observations with his new telescope in the fall of 1609 which included the viewing of mountains and craters on the Moon and stars fainter than what would normally be seen with the naked eye.  We also looked at Thomas Harriot's observations of the Moon in the summer of 1609, providing an example of the kind of tip of the iceberg the telescope's invention was in 1609.  Today, in part three of our series of posts in commemoration of the discovery of Io, we examine the research of another contemporary of Galileo's, Simon Marius, who claimed to have discovered the Galilean satellites before Galileo in December 1609.

Then for the first time I looked at Jupiter, who was in opposition to the Sun, and made out some tiny stars, sometimes following, sometimes preceding Jupiter in a straight line with him.  First, I thought that they were of the number of those fixed stars ... However, as Jupiter was then retrograding, and still I saw these stars accompanying him throughout December, I was at first much astonished; but by degrees arrived at the following view, namely, that these stars moved round Jupiter, just as the five solar planets revolve round the Sun.

- Simon Marius in the Mundus Jovialis (1614)

Meanwhile, in Germany...

In late 1608, as news of the Dutch discovery of the telescope spread across Europe, one of the inventors traveled to Frankfort in an attempt to market it.  The news of an instrument that made "the most distant objects be seen as though quite near" caught the attention of John Philip Fuchs, Lord Privy Councilor to the Margraves of Brandenburg-Ansbach.  He attempted to buy the telescope from the Dutch glass maker.  While he was allowed a demonstration of it, he found the price unacceptable.  Upon his return to Ansbach, Fuchs met with the court astronomer, Simon Marius (latinized from Simon Mayr), and they attempted to fashion their own telescope, but their were unable to get a working model since they didn't know how to polish glass and the lens makers in nearby Nuremberg didn't have the right tools.  By the summer of 1609, enough copies of the telescope were being made in the Netherlands that the prices had come down substantially since Fuchs first attempt to buy one the previous autumn.  With a newly purchased telescope, Simon Marius set to work making astronomical observations with it from both the home of the Lord Privy Councilor and at Marius's home at the Ansbach Observatory.

As described in his 1614 book, Mundus Iovialis anno M.DC.IX Detectus Ope Perspicilli Belgici (translated from the Latin title as "The World of Jupiter discovered in the year 1609 by means of a Dutch spy-glass"), Marius began to observe Jupiter in late November 1609 as the planet approach opposition, when Jupiter was opposite the Sun in the sky and thus allowing for more time for observations at night with Jupiter high in the sky.  It was during this observations that he noted at least three stars adjacent of Jupiter.  When the stars continued to move with Jupiter even when it was moving retrograde during opposition in December 1609, he realized that the "stars" were actually moving around Jupiter, just as the planets moved around the Sun.  When Marius came to this stunning realization, he began to record his observation on December 29, 1609, observing three of the moons to the west of Jupiter.  One thing to keep in mind about this observation was that Simon Marius, being a Lutheran, was using the Julian calender, just as the rest of the Protestant world used at the time.  December 29, 1609 in the Julian calendar is equivalent to January 8, 1610 in the Gregorian calendar, used by Galileo Galilei and those in other Catholic countries.  In fact, Simon Marius's description of the positions of the Galilean satellites in his first recorded observation match those of Galileo for January 8.

According to Marius in Mundus Iovialis, he continued to observe the Jupiter system until January 12 (January 22 for Galileo), at which point he determined that he was observing four bodies revolving around Jupiter. Following a trip to Swabia in Southwestern Germany, Marius returned in February 1610, and after discussing his observations with Fuchs, he was given permission to use the telescope on a more permanent at Marius's observatory.  This allowed Marius to make more continuous, detailed observations of Jupiter's satellites.  From the observations Marius acquired over the next few years from his observatory, eventually using an improved telescope of his own creation, he was able to determine that the four moons each orbited Jupiter in roughly circular orbits with different orbital velocities, with faster velocities for the innermost moons and slower velocities for the outermost.  Marius was also able to work out the orbital periods for each of the four moons, coming quite close to modern values for their periods. For example, his orbital period for Io was 1 day, 18 hours, 28 minutes, and 30 seconds, differing from the modern value by 56.5 seconds.  He noted that each of these moons orbit parallel to the ecliptic and that on occasion, the moons would appear either north or south of the equator of Jupiter, the result of Jupiter axial tilt (though not a conclusion Marius reached).  Marius would also note that Jupiter eclipses the inner moons during each orbit, causing Io and Europa to fade from view.

Galileo and the Mundus Iovialis

Simon Marius's treatise on his observations of the Jupiter system (as well as sunspots), Mundus Iovialis, was published in 1614, which included the first table of predicted positions for the satellites.  In his book, Marius acknowledged Galileo as the discoverer of the new worlds of Jupiter, "The credit, therefore, of the first discovery of these stars in Italy is deservedly assigned to Galileo and remains his."  Clearly, one could make the argument that he is quibbling when he says that Galileo was the discoverer IN ITALY, while he was the German discover of the moons.  Marius's supporters in Germany would later try to make the further claim that Marius was the sole discoverer, beating Galileo.

Galileo would respond to Marius's claims (or probably more correctly the claims of Marius's German (read: Lutheran) supporters) in his 1623 book, Il Saggiatore (or The Assayer).  First, Galileo and Marius had some history together.  Between 1602 and 1605, Simon Marius attended the University of Padua, becoming an important member in the German student society there, while Galileo was a professor there.  While a student, Marius also tutored a rich Milanese student named Baldessar Capra, where they would together observe a supernova in 1604.  After Marius returned to Ansbach in 1605, Capra remained at Padua, where he published a book in 1607 on the sector, which just so happened to be exactly the same as Galileo's mechanical compass.  After this blatant case of plagiarism, Capra was expelled from the University.  With Marius so connected to Capra as well as a result of his actions as the president of the German student's society, he was not well liked by Galileo.  So, in Il Saggiatore, published nine years after Marius's Mundus Iovialis, Galileo attacked Marius, claiming that Marius framed his student for a plagiarism he was responsible for, and saying:

Now four years after my Sidereus Nuncius appeared, this same fellow [Simon Marius] (in the habit of trying to ornament himself with other people's works) unblushingly made himself the author of the things I had discovered and printed in that book. Publishing under the title of Mundus Iovialis, he had the gall to claim that he had observed the Medicean planets which revolve about Jupiter before I had.  But note his sly way of attempting to establish his priority. I had written of making my first observation on the seventh of January, 1610. Along comes Marius, and, appropriating my very observations, he prints on the title page of his book (as well as in the opening pages) that he had made his observations in the year 1609. But he neglects to warn the reader that he is a Protestant, and hence had not accepted the Gregorian calendar. Now the seventh day of January, 1610, for us Catholics, is the same as the twenty-eighth day of December, 1609, for those heretics. And so much for his pretended priority of observation.

So Galileo clearly took offense, despite Marius's attempt in Part 1 of Mundus Iovialis to relieve that possible tension.

Now who should have credit?  Personally, I feel Galileo was right to point out the difference in calendar systems being employed between the two scientists.  Certainly the first observation Marius recorded on December 29, 1609 in the Julian Calendar matched the second of Galileo's observations from January 8, 1610 in the Gregorian Calendar.  Whether Galileo is right about Marius out right plagiarizing his work (again?), I don't think we can say.  Marius clearly was observing the Jupiter satellites starting at least in 1610, with enough observations for him to come up fairly accurate orbital periods for the satellites.  Still, based on the fact that Galileo has a  recorded observation, that has been verified using mathematical theories of the orbits of these moons, from the day before Marius's first recorded observation, gives Galileo the sole credit for the discovery.  Marius's claims that he was observing the moons as early as late November 1609 can't be verified because he didn't record what he saw.

Io, Europa, Ganymede, and Callisto

Regardless of who saw what, when, or who stole from whom, from the Mundus Iovialis, we do get the modern names for Jupiter's four largest moons.  Marius offers several naming schemes for the moons, calling them collectively the Brandenburg Stars (as a knock on Galileo calling them the Medicean Stars).  He also tried a naming scheme where he uses the names of the planets, for example "The Mercury of Jupiter" or "The Jupiter of Jupiter".  Finally, he offers a scheme suggested to Marius by Johannes Kepler in October 1613 while they were in Regensberg:

Jupiter is much blamed by the poets on account of his irregular loves.  Three maidens are specially mentioned as having been clandestinely courted by Jupiter with success.  Io, daughter of the River Inachus, Callisto of Lycaon, Europa of Agenor.  Then there was Ganymede, the handsome son of King Tros, whom Jupiter, having taken the form of an eagle, transported to heaven on his back, as poets fabulously tell, and notably Ovid.  I think therefore, that I shall not have done amiss if the First is called by me Io, the Second Europa, the Third, on account of its majesty of light, Ganymede, the Fourth Callisto.

For Tomorrow

Tomorrow, January 7, we will take a look at Galileo's observations of the Jupiter system, and how he came to view them as moons of Jupiter.

Link: Mundus Iovialis - Part 1 [adsabs.harvard.edu]
Link: Mundus Iovialis - Part 2 [adsabs.harvard.edu]
Link: Mundus Iovialis - Part 3 [adsabs.harvard.edu]
Link: Mundus Iovialis - Part 4 [adsabs.harvard.edu]
Link: Simon Marius [galileo.rice.edu]

Io@400 Part 2: Looking through the telescope in 1609

This week we are looking back at the discovery of Io and the other Galilean satellites by Galileo Galilei 400 years ago this Thursday.  Yesterday, we briefly examined the state of astronomy at the start of the 17th Century and the conflict between different world systems in play.  We also examined how Galileo first developed his telescopes based on a Dutch invention.  Today, in part two of our series of posts in commemoration of this significant event in the history of astronomy and science, we look back at Galileo's first scientific investigations with the telescope in the autumn of 1609.  We also look at Thomas Harriot's similar investigations that year.

These spots have never been observed by anyone before me; and from my observations of them, often repeated, I have been led to that opinion which I have expressed, namely, that I feel sure that the surface of the Moon is not perfectly smooth, free from inequalities and exactly spherical, as a large school of philosophers considers with the regard to the Moon and the other heavenly bodies, but that, on the contrary, it is full of inequalities, uneven, full of hollows and protuberances, just like the surface of the Earth itself, which is varied everywhere by lofty mountains and deep valleys.

- From Galileo's Sidereus Nuncius (trans. by Edward S. Carlos)

The Imperfect Moon

With a copy of his new telescope gifted to the Venetian Republic, Galileo then traveled to Florence in October 1609 to provide the Grand Duke of Tuscany, Cosimo II de' Medici, a demonstration of his 8x power telescope (which he created after receiving new lenses in mid-September 1609).  Cosimo II was a former pupil of Galileo and had become Grand Duke earlier in the year following the death of his father, Ferdinando I.  The visit was intended to smooth over relations with the ruler of his hometown after after he had given a copy of his telescope to a rival of Tuscany in north Italy in hopes that he might get employment at the University of Pisa.  During this visit, Galileo showed Cosimo the view of the Moon through his telescope.  This demonstration also provided support for Galileo's 1606 theory that the bright regions of the Moon contained tall mountains and the dark regions low plains.

Upon his return to Padua in late-October 1609, Galileo set to work creating a more-powerful telescope, this time with a 20x magnification, based on the lens blanks he had received a couple months earlier.  Galileo began his first detailed observations of the Moon, which he later published in Sidereus Nuncius, on December 1 (though other references suggest that he might have begun his first detailed observations in early October).  Again, he found shading on the Moon resulting from topography, hills and valleys that reminded Galileo of the hills and valleys of the Apennines of central Italy.  He correctly interpreted bright spots on the Moon's night-side as large mountains tall enough to reach into the sunlight, akin to sunlit mountains seen from the surrounding twilit plains.  He also correctly interpreted dark spots along the terminator, the dividing line between day and night, as valleys (in the case of the Moon, the floors of impact craters).  Over the next few weeks, Galileo continued to draw what he saw at the moon, noting sunlight slowly filling valley floors as well as the dark plains that seemed to be flat, devoid of topographic shading (though he noted that some were ringed by mountain chains).  These observations contradicted the current prevailing view of the Moon of perfect, smooth world.  The dark maria were attributed as simply being the result of differences in the density of the Moon.  Galileo's observations instead showed an imperfect world, more akin to the Earth with mountains he measured as four miles tall.  He even went so far as to suggest that the Lunar highlands were akin to terrestrial land areas and the maria were actually seas.

In his March 1610 treatise, Sidereus Nuncius, Galileo acknowledged a potential argument against the Moon having substantial terrain.  While he observed evidence for significant topography within the Moon's highlands, its limb appeared to be smooth even at 20x magnification.  With mountains as great as four miles in height, the limb should be lumpy.  Galileo proposed two explanations.  First, he suggested that limb appeared smooth because the mountains were more distant than the ones near the center of the disk and would appear smaller and foreshortened, akin to mountains in the far distance on Earth.  His other suggestion, which he would later accept as incorrect but one I am more familiar with on another moon, was that an atmosphere on the Moon would make the appearance of the limb indistinct, blurring features and topography as emission angle increases.  So in this case, he argued that the limb appeared smooth because we are seeing the edge of the Moon's atmosphere, not the physical horizon. Finally, in the Sidereus Nuncius, Galileo notes Earthshine on the nightside of the Moon, most visible when it is near new moon phase, as well as albedo markings on the dark maria (rays from craters such as Copernicus and Tycho).

During this period of late 1609, Galileo also observed background stars with his new "high" power telescope. He noted that his telescope did not magnify the appearance of stars, like the planets or the Moon.  Instead they seemed to brighten and become smaller.  His important discovery from these observations was the finding that telescopes seemed to bring out stars that were fainter than are visible with the naked eye.  For the Sidereus Nuncius, he highlighted three deep-sky features as demonstrations of this effect: the Orion constellation, the Pleiades star cluster, and the Milky Way.  In the first two cases, he noted that both areas of the sky had significantly more "invisible" stars than visible ones, charting 36 of these stars in addition to the six normally seen with the naked eye in the Pleiades cluster.  In the case of the Milky Way, he found that one what had previously been considered clouds were instead densely packed clusters of stars.  He found similar cases at other stellar "clouds", such as the Orion Nebula and the Beehive Cluster.

In early January 1610, Galileo wrote a letter to a friend in Florence detailing his discoveries to date with telescope, and prepared its conclusion on January 7, 1610 when he made an even more amazing discovery.

In England...

As I mentioned yesterday, news of the invention of the telescope in the Netherlands spread quickly across Europe, with 3x-powered scopes being demonstrated in the courts of Europe throughout 1609.  Word also spread to an English scientist named Thomas Harriot and in early 1609 he purchased a copy while he was working at the Syon House, the residence of the Earl of Northumberland (well, not at the time... the Earl was in prison at the time following the Gunpowder Plot of November 5, 1605).  Harriot recorded the first astronomical observations using a telescope in the summer of 1609, including several illustrations of the Moon.  However, these were not well known at the time as he failed to publish his observations, unlike other astronomers at the time like Galileo, Kepler, and Simon Marius.  These observations included detailed maps of the Moon that would not be matched for several more decades.  Later, he did publish a book on algebra and determined Snell's Law of Refraction independently.

For Tomorrow

One other astronomer who was using a telescope in 1609 and 1610 was Simon Marius.  Tomorrow, January 6, we will take a look at his claim that he was the discoverer of the four largest moons of Jupiter in December 1609.

Io@400 Part 1: Copernicus, Galileo, and the Telescope

This week we are looking back at the discovery of Io and the other Galilean satellites by Galileo Galilei 400 years ago this Thursday.  In part one of our series of posts in commemoration of this significant event in the history of astronomy and science, we look back at the invention of the telescope in the first decade of the 17th Century and how Galileo came to develop his own version and improve upon it.  We will also take a brief look at the state of astronomy prior to Galileo's discovery.

About ten months ago a report reached my ears that a Dutchman had constructed a telescope, by the aid of which visible objects, although at a great distance form the eye of the observer, where seen as if near; and some proofs of its most wonderful performances were reported, which some gave credence to, but other contradicted.

- From Galileo's Sidereus Nuncius (trans. by Edward S. Carlos)

The Copernican Revolution

Since the time of Aristotle and Claudius Ptolemaeus, the western view of the structure of the universe was geocentric, with the Moon, the Sun, and the other planets revolving around a stationary Earth with fixed stars in the background (or Firmamento).  Comets were viewed as atmospheric phenomenon, seen as portents of change. This system was the dominant theory in Europe with the backing of church theology that preached a Earth-centric universe with hell below and a perfect, unchanging heaven above.  This world view was revived in the west in large part to the works of Saint Thomas Aquinas, summarized in his book, Summa Theologica.  In the geocentric model, the planets were free of imperfections, all orbiting the Earth in circular orbits. However, these circular orbits did not explain the motion of the planets during opposition nor their changing brightness.  So an increasing complex system of epicycles and epicycles upon epicycles were developed in an attempt to better explain planetary motion while keeping the Earth at the center of the universe.

The late 16th and early 17th Centuries were important time periods in the history of astronomy and science. In 1543, Nicolaus Copernicus published his seminal work, De revolutionibus orbium coelestium, where he proposed a heliocentric universe, where the Earth moved around the Sun like the other planets in circular orbits.  However, to account for the apparent non-uniform motion of the planets (now known to be a result of the planets moving in elliptical orbits), Copernicus retained the epicycles of the Ptolemaic geocentric model. Additional models were developed during the 16th century, such as Thomas Digges's A Perfit Description of the Caelestiall Orbes (suggesting an "infinite" number of stars) and Tycho Brahe's modified geocentric model.  Brahe, while finding evidence such as supernovae and comets that revealed a changing sky, failed to detect stellar parallax, leading him to develop an alternative model, consisting of a geocentric universe with the Sun, stars, and Moon revolving around the Earth, but the other planets revolving around the Sun.

By the start of the 17th Century, there was significant controversy regarding the validity of the Copernican model, which contradicted both Catholic and Protestant theologies at the time.  For astronomers of the age, such as Brahe's former assistant Johannes Kepler and Florentine mathematician Galileo Galilei, it was important to find further evidence that would confirm or refute the various theories that were discussed: the Ptolemaic geocentric model, the Copernican heliocentric model, or the Tychonic hybrid model.  Kepler, then the imperial court mathematician under Emperor Rudolf II, developed a model to explain the non-uniform motion of Mars in his 1609 work, Astronomia Nova.  He proposed that the planets moved in elliptical orbits, rather the perfect circles of earlier models, and that planets moved faster when they were closer to the Sun (perihelion) than when they are farther away (apohelion).  This model provided a more accurate and elegant model to explain planetary motions than previous models that replied on complex systems of epicycles.

Enter the Spyglass

On October 2, 1608, Dutch authorities at The Hague received a patent from Middleburg lens-maker Hans Lippershey for an instrument, "for seeing things far away as if they were nearby."  This device used a pair of glass lenses, a convex object lens and a concave one for use as an eyepiece.  This rudimentary telescope allowed for 3x magnification of distant objects. The patent was ultimately rejected as a result of similar designs by other Dutch lens makers like Sacharias Jansen and Jacob Metius.  However, Lippershey was given a grant by his government to produce three more of the telescopes.  News of the invention would spread from visitors to the Dutch court, like a Siamese emissary who was shown the scope while they were at The Hague.

In 1609, word of the new invention reached astronomers like Thomas Harriot in England.  Galileo Galilei first heard about the telescope while in Venice in late July 1609.  There Venetian chief theologian Paolo Sarpi showed Galileo a letter from Jacques Badovere describing a copy of the instrument in Paris.  In early August 1609, Galileo received word of a a copy of the device with a foreign merchant in Padua, where Galileo worked as a mathematics professor, prompting him to rush home.  Unfortunately, by the time he arrived in Padua on August 3, the merchant had traveled on to Venice to sell his instrument.  Galileo worked out from Badovere and Sarpi's description of the telescope from the news he had received from the Netherlands the arrangement and shape of lens that would be required to magnify the appearance of distant objects.  His first design matched the magnification of Lippershey's instrument.

Galileo at the time was suffering from some financial hardships and had hoped to get a salary increase for his work at the University of Padua on behalf of the Venetian government.  He placed some hope if he managed to improve on the design of the telescope, he could receive a raise.  By mid-August, Galileo developed a telescope design with an 8x magnification.  With his friend Sarpi holding off on purchasing the foreign merchant's telescope after being put in charge by the Venetian government to purchase it, he set up a demonstration of his improved telescope for Doge Leonardo Donato and members of the Venetian Segnoria and Senate from atop the campanile in the Piazza San Marco on August 25, 1609.  He was able to show the telescopes ability to show ships 50 miles away as clearly as if they were 5 miles away, demonstrating the telescope's use as a naval instrument, very important for a naval power like the Venetian Republic.  Giving the telescope as a gift to the Venetian state, his appointment to the University of Padua was renewed and his salary would be permanently doubled from 520 to 1000 florins per year after the end of his current term.  While the pay raise initially sounded like a great solution to his financial woes, the fact that this salary would run for life with no prospect of future increases, Galileo would later look for employment elsewhere, such as the University of Pisa (which we will touch on more on Friday in Part 5).

For Tomorrow

Tomorrow, January 5, we will continue this series by looking at Galileo's first scientific observations with his telescope in late 1609.  We will also look at Thomas Harriot's work with the telescope that year.

July 22 Eclipse of Io by Ganymede

Later this morning, Io's trailing hemisphere will experience a total solar eclipse when Ganymede passes between Io and the Sun. The eclipse runs roughly from 13:34 to 13:39 UTC (14:08-14:14 UTC as seen from Earth). The video below was created in Celestia and shows the eclipse both from above Io, showing the shadow of Ganymede cross Io's surface, and from the surface of Io, showing Ganymede pass in front of the Sun.


Computer Animation of the total eclipse of the Sun by Jupiter's moon Ganymede over the trailing hemisphere of Io on July 22, 2009. First half shows a view from 4500 miles above Io's trailing hemisphere. Second half zooms in on the sun from an unnamed volcanic pit showing the total eclipse. The animation runs from 13:30 to 13:40 UTC on July 22, 2009.

EDIT 07/22/2009 10:06 AM: Fixed the title of the article, changing Jupiter to Ganymede. Obviously, a Jupiter eclipse is nothing special. A Ganymede one is.

Jupiter Impact Confirmed by IRTF

The impact of a small asteroid or comet into Jupiter's atmosphere, first observed by astronomer Anthony Wesley (let's be honest, anyone who takes the kinds of pictures he does of Jupiter is not an amateur), has been confirmed in observations taken by NASA's Infrared Telescope Facility (IRTF) atop Mauna Kea on the island of Hawaii. The image shown at left reveals the impact to be glowing quite brightly in the near-IR, in a methane absorption band at 1.65 microns

Based on additional images taken by ground-based telescopes, the impactor came in from below Jupiter, striking the South polar region sometime between 07:00 and 14:11 UTC on Jupiter's nightside. Several dark spots in addition to the main impact site are visible with a faint, fan-like plume deposit to the west and north of the impact site. Similar plume deposits were seen at Shoemaker-Levy 9 impacts 15 years ago this week in 1994.

EDIT 07/20/2009 06:23 PM: New Scientist has an article with an image taken by Keck II. The IR data from Keck seems to suggest the possibility of multiple impactors.

EDIT 07/20/2009 11:33 PM: Looks like the image from Keck II in the New Scientist article is a bit of a double exposure, making it look like multiple impact sites.

In case you missed it, an hour ago I posted some of my thoughts on this, the 40th Anniversary of the Apollo 11 landing. Don't forget to post a comment there about when you think the first humans will land on Io (never is a possible answer, but not one I necessarily agree with).

Link: New NASA Images Indicate Object Hits Jupiter [jpl.nasa.gov]

Impact observed on Jupiter

15 years ago, the world watched as broken-apart comet Shoemaker-Levy 9 impacted Jupiter's atmosphere in a series of event in July 1994. Well, a similar event seems to have occurred in the last day in Jupiter's south polar region. A new dark spot, similar in size and color to the Shoemaker-Levy 9 impacts was seen by ground-based observers on July 19.

Hopefully more observations will be acquired over the next few days to help confirm this discovery, but it looks quite plausible to me :)

The image at left was captured by Anthony Wesley on 19th July 2009 at 1554UTC from Murrumbateman Australia. The south pole is up, north pole down. The impact site is near the central meridian about an eighth of the way down.

EDIT 07/19/2009 9:23 PM: Wesley's website has been slashdotted so he has mirrored the page to another server. So if you are having trouble accessing the link above, check out http://jupiter.samba.org/

Ganymede Eclipse on Io Wednesday Morning

Tomorrow brings Io's most interesting eclipse this mutual event season as most of Io's trailing hemisphere (51° West-231° West) is plunged into darkness by Jupiter's largest moon, Ganymede. This is the culmination of a series of weekly eclipses by Ganymede on Io. With each weekly eclipse, the center of Ganymede's shadow appears further south on Io. Tomorrow, the center of Ganymede's shadow passes just north of Io's equator. The eclipse takes place tomorrow morning, July 15, between 10:45 and 10:50 UTC (3:45-4:50 MST) on Io. If you have a good telescope and want to try to observe this event, from Earth the penumbral shadow of Ganymede will reach Io at 11:19 UTC, totality will run from 11:21 to 11:25:28 UTC, and end of the eclipse comes at 11:27 UTC. The peak of the eclipse, as observed from Earth, comes at 11:23:14 UTC. The timing of this eclipse should make it a good observation target for observers in the western United States, western South America (like the European Southern Observatory), and Hawaii.

During the eclipse, Ganymede will appear 13' 17.2'' across in Io's sky (compared to our moon, which appears around 30' across in Earth's sky). The sun will appear 6' 19.9". Therefore, it is unlikely that the sun's corona would be seen during the eclipse except near the beginning and end. At its peak near Tol Ava Patera, the eclipse will last 1 minute and 55 seconds long.

For this eclipse, I've created a little fancier video using Celestia and Adobe Premiere. I think I am starting to get along with that latter software package...


Computer Animation of the total eclipse of the Sun by Jupiter's moon Ganymede over the trailing hemisphere of Io on July 15, 2009. First half shows a view from 4500 miles above Io's trailing hemisphere. Second half zooms in on the sun from east of Ra Patera showing the total eclipse. The animation runs from 10:40 to 10:55 UTC on July 15, 2009.

I have also created a nice map showing the area on Io that will experience this eclipse. You can download a full-res version here.

My post last year on this mutual event season should help provide some information on the science of these types of eclipse as well as occultations.

Notes from the Io Underground

Grrr... I hate when Mondays sneak up on you...

• The 111th Edition of the Carnival of Space is now online over at 21st Century Waves.
• Over the last few weeks, I have been presenting some of the mutual events in the Jupiter system, particularly solar eclipses on Io by either Ganymede or Callisto from the perspective of Io. However, a pair of amateur astronomers, John Sussenbach and Marc Delcroix, captured numerous observations of the solar eclipse on Io by Ganymede on June 24. Delcroix even plotted the brightness of Io versus time as the eclipse progressed. John Sussenbach also took a look at one of the the solar eclipses by Callisto on June 20.
• Plenty of important meetings related to the exploration of Io and the Jupiter system will be taking place this week. This includes a Europa/Jupiter System Mission Joint Science Definition Team (EJSM JSDT) meeting today, an Outer Planets Assessment Group (OPAG) meeting tomorrow, and the EJSM Instrument Workshop on Wednesday through Friday. I might be listening into the EJSM Instrument Workshop for at least some of the talks, but I haven't decided yet.
• Today's Astronomy Picture of the Day showing the volcano Arak Krakatoa erupting at night is pretty awesome. Definitely worth checking out.
• Van Kane has been keeping up with last week's Planetary Science Subcommittee and Decadal Survey Meetings. One of the key issues that seems to be coming to a head is the flat planetary science budget projected in the out years in the current budget proposal. This would cause increasing budget pressure on many projects, not only because of the lack of budget increases (resulting from the poor government revenues) and from cost overruns on some projects, including the albatross of planetary science, the Curiosity rover (née Mars Science Laboratory). According to Kane, Ed Weiler at the PSS meeting stated that there is not enough money in the Planetary Science Division budget projections to fund the Europa/Jupiter System Mission. As Kane stated in his blog, this is definitely bad news.
• Celestia version 1.6 was officially released late last week. Celestia is definitely one of the space simulator, particularly thanks to its support for NAIF Spice kernel files for spacecraft and planetary body trajectories and orientations. The software also has built-in video and screenshot support, which I often take full advantage of for this blog.
  

Today's Eclipses of Io by Callisto

Here are two videos showing today's total eclipses of the Sun by Callisto on Io's south polar region:


Computer Animation of the first of two total eclipses of the Sun by Jupiter's moon Callisto over the south polar region of Io on June 20, 2009. First half shows a view from 7,317 km above Io's southern trailing hemisphere. Second half zooms in on the sun from near Kurdalagon Patera showing the total eclipse. The animation runs from 04:10 to 04:56 UTC on June 20, 2009.


Computer Animation of the second of two total eclipses of the Sun by Jupiter's moon Callisto over the south polar region of Io on June 20, 2009. First half shows a view from 7,317 km above Io's southern trailing hemisphere. Second half zooms in on the sun from near Svarog Patera showing the total eclipse. The animation runs from 08:40 to 09:20 UTC on June 20, 2009.

An earlier partial eclipse occurred on June 19 over Io's southern leading hemisphere as well.

Eclipses of Io by Ganymede on June 16

Tomorrow (June 16), Io will experience three solar eclipses by its larger neighbor, Ganymede. Like those that took place last week, these eclipses are mostly visible over the north polar region of Io, though totality could be observed much further south than those last week (I say could because, obviously, no one is there to experience them). Luckily, thanks to software like Celestia, we don't have to let a little thing like a few hundred million kilometers stop us. Below is a video of an eclipse over the northern part of Io's anti-Jovian hemisphere that will take place in about three hours (1:08 am MST). Yes, like the nerd I am, I will watch it live on Celestia...

And completely off topic for a second, I know I have a few Iranian readers. All I want to say is good luck and stay safe! The world is pulling for ya!



In addition to this eclipse, there was an eclipse a few hours ago (5:52 pm MST) over the northern leading hemisphere and there will be one later on June 16 (3:39 pm MST) over the northern trailing hemisphere.

Another Total Eclipse by Ganymede Tomorrow

I completely forgot in my last post that there are TWO total solar eclipse by Ganymede tomorrow, in addition to the one around 08:50 UTC. The second, to occur around 18:50 UTC, will also cover the north polar region a little further to the west than the earlier eclipse, over the trailing hemisphere side of the north pole. Below is an animation of that eclipse:


Computer Animation of a second total eclipse of the Sun by Jupiter's moon Ganymede over the north polar region of Io on June 9, 2009. First half shows a view from 7,317 km above Io's northern trailing hemisphere. Second half zooms in on the sun from near the north pole showing the total eclipse. The animation runs from 18:30 to 19:15 UTC on June 9, 2009. This eclipse occurs nine hours after another total eclipse by Ganymede over a region to the east.

Today's Eclipses of Io by Ganymede

With the Jupiter system approaching vernal equinox on June 22, eclipses of the Sun on Io by the other satellites in the Jupiter system and other mutual events (such as occultations, when one satellite passes in front of another from Earth's perspective) have been occurring for the last few months. Tonight, the first total eclipse of the Sun by Ganymede this mutual event season will take place. A partial eclipse by Ganymede also occurred just 30 minutes ago (another partial eclipse occurred on June 1 as well). Below are two animations created in Celestia showing these two eclipses. For more information, check out my blog post from December about the mutual event season as well as my 365 Days of Astronomy podcast from last month.


Computer Animation of a partial eclipse of the Sun by Jupiter's moon Ganymede over the north polar region of Io on June 8, 2009. First half shows a view from 7,317 km above Io's northern leading hemisphere. Second half zooms in on the sun from near Io's north pole showing the partial eclipse. The animation runs from 20:38 to 21:08 UTC on June 8, 2009.



Computer Animation of a total eclipse of the Sun by Jupiter's moon Ganymede over the north polar region of Io on June 9, 2009. First half shows a view from 7,317 km above Io's northern anti-jovian hemisphere. Second half zooms in on the sun from near the eastern end of Lei-Kung Fluctus showing the total eclipse. The animation runs from 09:30 to 10:15 UTC on June 9, 2009.

These eclipses by Ganymede will occur every 8 days between now and mid-August with the most spectacular Ganymede eclipse occurring on July 15 when most of the trailing hemisphere of Io will be in the shadow of Ganymede.

My 365 Days of Astronomy Podcast

My podcast for the 365 Days of Astronomy should be online later today, if it isn't already. The topic of my podcast is "It's the Season for Eclipses in the Jupiter System." Here is the summary I submitted for this edition:

In 2009, as it does every six years, the Jupiter system experiences equinox. While Jupiter's low axial tilt makes the passing of season much less noticeable than on Earth, Mars, or Saturn, equinox does bring a season of eclipses for Jupiter's moons. From Earth, astronomers can observe these eclipses as well as occultations, when one moon passes in front of another. Today, we will discuss the useful science gained from eclipses and occultations as well as a few of the eclipses coming up in the Jupiter system, focusing primarily on Jupiter's innermost large moon, Io.

I cover both the equinoxes to be experienced by Jupiter and Saturn in the podcast, looking at the unique events that can be observed during the months surrounding equinox as well as look at the types of changes that occur in these two systems as the seasons change.

I hope you all have a chance to check it out! You can also read the blog post that was the genesis for this podcast.

Link: 365 Days of Astronomy - It's the Season for Eclipses in the Jupiter System [365daysofastronomy.org]

Astronomy on Io

With this being the International Year of Astronomy, I thought this would be a good time to do a post on Astronomy on Io. Obviously, there are no astronomers on Io (not yet anyway), but perhaps it would be insightful to take a look at a simulated view of Io's night sky and to explore the wonders one could see with the naked eye and through a telescope if one were on the satellite's surface (assuming one were protected from the elements).

Our make-believe observatory is located with in the Gish Bar Patera caldera, just south of the primary flow field, in an elevated area on the patera floor. Locally, the ground is flat and yellowish in color with 500-meter tall cliffs visible to the south curving around to the west and east. A 9-km-tall mountain would be visible to the north, reaching five degrees into the sky. Hopefully, though, for most of your observing, you wouldn't just be standing around outside, with all the radiation...

Jupiter

The largest object in our simulated sky is the planet Jupiter, a scant 350,000 km away. Because Io is tidally locked to Jupiter, much like our own Moon, Jupiter remains motionless in the sky, always resting on the eastern horizons from our observation point at Gish Bar Patera. Jupiter subtends almost 20° in Io's sky, 40 times the size of the Moon in Earth's sky. To give you an idea, an outstretched fist measures about 10° across, so Jupiter would appear 2 fists across. The great size of Jupiter in the sky would make Io an excellent platform to watch Jupiter's cloud formations as Jupiter rotates a little more than four times over the course of an Ionian day.

Like our own Moon, Jupiter goes through phases. At Gish Bar, "New Jupiter" occurs at daybreak. In fact, daybreak is delayed by more than an hour because Jupiter eclipses the Sun when the Sun would be rising above the horizons. Also, because of the presence of Jupiter on the eastern horizon, sunrise at Gish Bar would seem remarkably like sunrises on Earth, minus the beautiful colors in the sky. The sun's rays would be refracted through Jupiter's upper atmosphere as the day's eclipses draws to a close, bathing the landscape in reddish light. "First quarter Jupiter" would occur near noon at Gish Bar, "Full Jupiter" would occur just before sunset, and "Third quarter Jupiter" would occur near local midnight.

Io's orbit is not a perfect circle, so like our Moon in Earth's sky, Jupiter would very subtly grow and shrink in Io's sky over the course of a day. Currently, Io's closest point in its orbit around Jupiter (called perijove) occurs shortly after sunset at Gish Bar, when Jupiter is just past full phase. Apojove, when Io is furthest from Jupiter, occurs shortly after sunrise (again, presuming Jupiter weren't there to block the actual sunrise). The difference in distance between perijove and apojove amounts to a 1% difference in the size of Jupiter over the course of an Ionian day. In addition to these size differences, libration over the course of an Ionian day causes Jupiter to appear to rock slightly. This "rocking" is on the order of 4°.

Because of the size of Jupiter in the sky, and the brightness of its cloudtops, it would be easier to observe Io's night sky when Jupiter is at a high phase angle, when only a crescent is visible or less, such as during the middle of an eclipse, when the ground would also be dark from the lack of sunlight. While there would be no atmospheric scattering to prevent you from seeing the stars, moons, and planets in the sky, light from the Sun and light reflected from the ground and Jupiter would make it difficult to dark adapt your eyes, making all but the brightest of stars invisible to the naked eye.

Moons

While Jupiter is known to have at least 63 natural satellites, only eight of these would be visible to the naked eye from Io's surface: Metis, Amalthea, Adrastea, Thebe, Europa, Ganymede, Callisto, and Himalia.

Four satellites orbit Jupiter inside the orbit of Io. These moons - Metis, Amalthea, Adrastea, and Thebe - are all much smaller than Io and would appear as bright points of light in the sky. Amalthea, the largest of these inner moons, is big enough that the keenest eyes might be able to see its elongated shape while it transits across Jupiter. Each moon would rise in the east already transiting across the face of Jupiter and would set behind Jupiter several hours later. Because these moons orbit interior to Io, they would never stray very far from Jupiter, only 7° in the case of Metis and Adrastea, 15° for Amalthea, and 20° for Thebe.

Europa, the next moon out from Io, is the second largest object in Io's sky when it is closest to Io. From Gish Bar Patera, Europa would appear to rise above Jupiter shortly after perijove and would set one Ionian day later. At its furthest - just after conjunction and it rises above Jupiter - Europa is more than 1.075 million km away from Io and is only 10 arcminutes across in the sky, one-third the size of the Moon in Earth's sky. At its closest - at opposition before setting - Europa is only 256,000 km away and is 41.5 arcminutes across, one-third larger than the Moon in Earth's sky. Currently, Europa rises and sets shortly after sunset, though this shifts in time over the course of a Jovian year. Currently, Europa appears as a very thin crescent when it is at its largest but in three years, Europa will be nearly full at opposition. For the most part, from Gish Bar, the sub-Jovian hemisphere would be seen, though a bit of the anti-Jovian hemisphere (such as the ray crater Pwyll) would be visible when Europa was high in the sky.

Ganymede is the next largest object in Io's sky when it is closest to Io. From Gish Bar Patera, Ganymede would appear to rise above Jupiter and sets about 26 hours later. Because of the 4:1 resonance between their orbits, Ganymede rises and sets to a fairly predictable pattern. For example, on January 6 at 12:15 UTC, Ganymede will rise from behind Jupiter shortly after Io has reached the closest point in its orbit to Jupiter. At this point, Ganymede is 1.478 million km from Io and is 12 arcminutes across in Io's sky, about 2.5 times smaller than our Moon in Earth's sky. This occurs shortly after sunset on Io and Ganymede is just past full. Ganymede sets 26 hours later nearing half phase during the late morning hours at Gish Bar. The largest satellite in the solar system would be 644,000 km away and would appear 28 arcminutes across, slightly smaller than the Moon in Earth's sky. 28 hours later, a crescent Ganymede rises above Jupiter, and 26 hours later, at Io's perijove, a thin crescent Ganymede sets shortly after sunset. Another 30 hours later, on Jan. 11 at 05:09 UTC, a half-phase Ganymede rises above Jupiter shortly before noon and 26 hours later Ganymede sets a little past half phase. 30 hours later, on Jan. 13 at 13:30 UTC and a full Ganymede after it began, the cycle repeats as a nearly full Ganymede rises above Jupiter. Interestingly enough, 20 minutes later, an Ionian observer would be able to watch Ganymede's north polar region darken as it experiences a total solar eclipse by Europa.

Callisto, the furthest of the Galilean satellites from Jupiter, doesn't display such regular cycles as Ganymede and Europa as Callisto is not in an orbital resonance with Io. Callisto rises above Jupiter at a distance of 2.309 million km from Io and would appear 7.2 arcminutes across in Io's sky, about a quarter of the size of the Moon in Earth's sky. Callisto sets a little more than 22 hours later near opposition when Callisto is 1.472 million km from Io and is 11.2 arcseconds across, or about 3/8ths the size of the Moon in Earth's sky. Among the features that would be targets of interest for an Ionian observer would be the Valhalla impact basin on the western limb and the bright palimpsets Lofn and Heimdall on Callisto's south polar region.

Himalia, the largest of Jupiter's outer irregular satellites, would appear as a barely visible "star" near 5.5 mag. at its brightest. Jupiter's other irregular satellites, would be telescopic targets too faint to be visible to the naked eye.

Planets and the Sun

From Io, five additional planets would be visible to the naked eye most times of the year, with Uranus a naked eye target during opposition with that planet. Like Venus and Mercury from Earth, the terrestrial planets would stick fairly close to the Sun, with Mars appears as far as 17° from the Sun. The sun itself would appear much smaller than it does from Earth, only 6.5 arcminutes across, compared to 30 arcminutes from Earth.