The Galilean Moons

Jupiter's four giant moons (Io, Europa, Ganymede, and Callisto) are useful targets for the webcam imager and for a variety of reasons. Jupiter is a low-contrast gaseous body with no high-contrast features like the Martian Syrtis Major, and no sharp-edged features like Saturn's rings. So what can you focus on? The planet's features and limb are just too fuzzy most of the time, unless you are using an infrared filter, but Io makes an excellent target. It is always relatively close to the planet (a quick search east or west will easily find it unless it is in front of or behind Jupiter) and is bright enough for a webcam on high gain to pick up. Yes, I know it is not a point source like a star, but, nevertheless, focusing back and forth on Io until you are happy it is as small a dot as you can get it is a good plan for the novice imager. The other Galilean moons can also be used, but they are usually much further away from the disc. In order of distance from Jupiter, the satellites Io, Europa, Ganymede, and Callisto orbit at a maximum angular distance of 2.3,

JETSTREAMS

JETSTREAMS

Figure 13.1. Jupiter's complex system of belts, zones, and currents, with the Great Red Spot to the right of the central meridian. The belts are dark features and the zones are bright features. The arrowheads indicate the direction of the jetstream currents on the planet which tend to lie along zone/belt boundaries. The Great Red Spot vortex rotates anti-clockwise. Jupiter Image: D. Peach. Legend from top, clockwise: SPR = South Polar Region; STZ/SSTB = South Temperate Zone/South South Temperate Belt; STB = South Temperate Belt; STrZ = South Tropical Zone; SEB = South Equatorial Belt; EZ + EB = Equatorial Zone + Belt; NEB = North Equatorial Belt; NTrZ = North Tropical Zone; NTB = North Temperate Belt; NTZ = North Temperate Zone; NNTB = North North Temperate Belt; NPR = North Polar Region. On the left hand side the terminology is the same, with s and n indicating North and South belt edge jetstream currents. NEC/SEC = North/South Equatorial Current.

Figure 13.2. A highly detailed webcam image of Jupiter taken by the veteran Japanese observer Isao Miyazaki on February 28, 2004, using a ToUcam Pro and his 40-cm f/6 Newtonian. The long-lived oval "BA" is seen in the top left. Image: I. Miyazaki.
Figure 13.3. Another webcam image of Jupiter by Isao Miyazaki, taken on March 11, 2004. Oval BA is in the top left quadrant and the Great Red Spot near the top left limb. The shadow of the Moon Callisto is about to leave the planet's disc. Image: I. Miyazaki.
Figure 13.4. A Jupiter image by the author taken on April 11, 2004, using a 30-m LX200 at f/22 and a ToUcam Pro webcam; 366 webcam frames were stacked. Image: M. Mobberley.
Figure 13.5. An excellent ToUcam Pro webcam image stack of Jupiter taken by Eric Ng from Hong Kong using a 250-mm f/6 Newtonian (at f/34.5) with William Royce Optics and a Vixen Atlux mounting. March 17, 2003. Image: Eric Ng.
Figure 13.6. One of Damian's finest images of Jupiter, taken on March 4, 2003 (23:29 UT), with a Celestron 11 at f/30 and a ToUcam Pro webcam. lo and its shadow can be seen on the planet. Image: Damian Peach.

3.7, 5.8, and 10.3 arc minutes respectively from the center of the planet's disc at a typical opposition (with Jupiter being roughly 45 arc-seconds in diameter). The time taken for each Moon to complete an orbit is 1.8, 3.5, 7.2, and 16.8 days, respectively. In the same order, the moons' diameters are 3,650, 3,130, 5,268 and, 4,806 kilometers, thus they have an angular diameter of 1.2, 1.0, 1.7, and 1.6 arc-seconds at an average opposition of Jupiter with the giant planet 4.2 A.U. from Earth. This tells us straight away that it should be possible, in good conditions, to resolve details on the Jovian Moons and, indeed, this is possible, as shown in Figure 13.7.

Observing and imaging the Galilean moons, as they transit the planet's disc, can be a challenging feat. Apart from when a Moon is passing over the darker limb regions, most of them will have a similar albedo to Jupiter's bright zones and, unless seeing is perfect, the Moon will become lost in the Jovian background. The one exception to this is Callisto, which is by far the darkest Moon with an albedo of 20%. (Io, Europa and Ganymede have albedos of 61, 64, and 42% compared to Jupiter's average albedo of 43%.) The first time you see Callisto transiting the Jovian disc you will be convinced it is a shadow of one of the moons! However, you will not see it cross the disc very often; with an orbital period of 16.8 days, it only spends a few hours per fortnight crossing the disk, if it crosses it at all (being 1.9 million kilometers from Jupiter, the orbital tilt often causes it to miss the planet entirely). Savour those moments when you see dark Callisto crossing the Jovian disc!

Another question which all this brings up is as follows: if we set a maximum limit of two minutes of time for collecting our Jupiter images (equivalent to almost

Figure 13.7. An extraordinary high-resolution image of Jupiter's largest Moon Ganymede, taken with a 280-mm Celestron 11 working at f/30, from Tenerife, with a ToUcam Pro webcam. The Moon has an angular diameter of only 1.7 arc-seconds! Image: Damian Peach.

0.5 arc-seconds of rotation drift on the middle of the disc) is this short enough to prevent the moons (or their shadows) elongating during the exposure run?

Anyone who enjoys observing Jovian satellite and shadow transits will have noticed that when Io, Europa, or Ganymede are transiting the central part of the giant planet's disc they appear almost to be part of the planet: atmosphere and satellite (on the face of it) appear to move at a similar rate! The most frequent transiting body is Io as it orbits in 1.77 days at 422,000 kilometers from the planet. The corresponding figures for Europa, Ganymede and Callisto are 3.55/671,000, 7.16/1,070,000, and 16.8/1,880,000. At mean opposition distances from Earth these four orbital speeds correspond to motions of 20, 16, 12.7, and 9.5 arc-seconds per hour, compared to the Jovian center's speed of 16 arc-seconds per hour. So, in a two minute imaging span at opposition, fast mover Io will drift two-thirds of an arc-second, with respect to the Jovian limb, but much more slowly with respect to the fast-moving central Jovian disc features. Essentially, in a two minute period the motion of Io (with respect to the Jovian limb) is roughly half Io's own diameter. This is irrelevant on all but the best resolution nights unless longer imaging spans are attempted to get more images to stack. Of course, if any of the four Galilean satellites are travelling over regions very near to the planet's limb, their relative motion (with respect to surface features) will appear faster due to foreshortening or the planet's smaller diameter at the poles. However, none of the big moons zip across the Jovian disc in the typical duration of an imaging window. Having said this, I have seen one very fine image by Damian Peach, using filters, where a double Io was visible because four minutes elapsed between the start of the first filter image (infrared) and the end of the second (blue). So Io can be seen to move in longer imaging runs, when seeing is good. Of course, to resolve detail on the satellites, Registax needs to be told to use the satellite itself as the reference stacking target.

Every six years or so the modest tilt of Jupiter's polar axis is aligned at right angles to the sun-Jupiter line (just as our Earth's polar axis is aligned at the spring and autumnal equinoxes). This results in all of the Jovian moons orbits appearing to be lined up, like looking at their orbital planes as if we were looking at the edge of a sheet of paper. This creates some interesting "photo opportunities" as the Jovian moons can mutually occult and eclipse one another. Under perfect seeing conditions it is possible to resolve details finely enough to show the shadow of one Moon passing over the other, or one Moon's limb cutting in front of another, as shown in Figure 13.8.

Is it possible to see two, three, or even four Jovian moons (and, more easily, their shadows) crossing the giant planet's disc simultaneously? Well, seeing two shadows crossing the disc is quite a common event if you have clear skies every night. However, in practice, how many people do? In reality, you might only observe such an event a few times a year if you are a keen observer, but with mainly cloudy skies. However, three shadows is practically a once-in-a-lifetime event. The student of Jupiter soon realizes that there are certain patterns in the revolution periods of the Jovian moons, just as there are in the Sun's own family of planets. Two revolutions of Io around Jupiter equal one revolution of Europa, and two revolutions of Europa equals one revolution of Ganymede. However, the pattern does not repeat for Ganymede and Callisto. The latter's revolution period is actually 2.3 times the former. These relationships mean that certain events involving the Jovian satellites repeat themselves every 3.6 or 7.2 days (the revolution periods of Europa and Ganymede). Even the novice observer notices the 7.2 day period pretty quickly. "Surely this same Europa-Ganymede event happened on this night last week?" you may find yourself asking. Yes, it did, but this week it is about three or four hours later. The 3.6 day Io-Europa repeat is harder to spot because the 0.6 day fraction tends to push a repeat performance into daylight

Figure 13.8. Another amazing image by Damian. In this webcam shot, taken on December 24th 2002, the Jovian Moon Callisto is being occulted by Io, i.e., Io is passing in front of Callisto. Io spans 1.2 arc-seconds and Callisto 1.6 arc-seconds. Left image at 03:06 UT; right image at 03:10 UT. Celestron 11 at f/30 and ToUcam Pro webcam. Image: Damian Peach.

Figure 13.8. Another amazing image by Damian. In this webcam shot, taken on December 24th 2002, the Jovian Moon Callisto is being occulted by Io, i.e., Io is passing in front of Callisto. Io spans 1.2 arc-seconds and Callisto 1.6 arc-seconds. Left image at 03:06 UT; right image at 03:10 UT. Celestron 11 at f/30 and ToUcam Pro webcam. Image: Damian Peach.

hours, plus, as many observers only find time to observe at weekends a repeat a week apart is much more likely to be noticed!

As I have already mentioned, seeing three satellite shadows cross the Jovian disc is very rare, and indeed, four simultaneous transits are impossible: Io, Europa, and Ganymede can never be simultaneously involved in these events and so triple events have to involve Callisto and two of the inner three moons. The Belgian mathematician and expert in spherical and mathematical astronomy, Jean Meeus, has painstakingly calculated all triple shadow events from 1900 to 2100 A.D. in his remarkable book Mathematical Astronomy Morsels (plus, there are sequels: More Mathematical Astronomy Morsels and Mathematical Astronomy Morsels 3). I would strongly recommend buying these fascinating books. Unfortunately, the next three events are some way off, specifically on October 12, 2013; June 3, 2014; and January 24, 2015. After that, you have to wait until March 20, 2032! As well as the situation of three satellite shadows crossing the Jovian disc, three satellites themselves can also appear on the disc at the same time. I have seen one such event: the triple transit of January 17/18, 2003. At its peak, five objects were on the Jovian disk simultaneously (see Figure 13.9 for an image by Damian Peach from that night). These five objects were the shadows of Io and Europa, plus Io and Europa themselves and dark Callisto, too. Three moons and two shadows! Actually, as I was observing in poor seeing, it looked like a triple shadow transit, as Callisto is so dark and bright Io and Europa were invisible to me. Capturing such rare events is a great challenge from a predominantly cloudy country and you will savour your memories and pictures of such occasions.

Figure 13.9. A very rare Jupiter image. Taken January 17, 2003 (23:39 UT), with a 280-mm aperture Celestron 11 SCT from Tenerife, it shows, from left to right on the planet, dark Callisto, lo's shadow, Io, Europa's shadow and, just off the limb, Europa itself. Note how the dark Moon Callisto looks like a shadow. Image: Damian Peach.

Figure 13.9. A very rare Jupiter image. Taken January 17, 2003 (23:39 UT), with a 280-mm aperture Celestron 11 SCT from Tenerife, it shows, from left to right on the planet, dark Callisto, lo's shadow, Io, Europa's shadow and, just off the limb, Europa itself. Note how the dark Moon Callisto looks like a shadow. Image: Damian Peach.

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