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...
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.
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.