The comet has literally come and gone, yet Jupiter still remains. In 1996, I gave a talk at a meeting of the Astronomical Society of New South Wales on Comets and Solar System highlights for that year, and I was pleasantly surprised by the number of people who indicated that they had taken the time to observe Jupiter at some time during the last 6 months of 1995, a year after the comet crash. Clearly, the majority of amateurs don't discriminate when it comes to observing. Anything that's up in the sky is potential target material. Indeed, my best view of Jupiter came from Ilford, the Society's dark sky site in the early hours of January 20, 1991 during a period of what must have been exceptional seeing. Some members may recall the view. The following is from my article "Return To Ilford", which appeared in the February/March 1991 issue of Universe.
"I, let alone anyone else present at Ilford that night, had never seen so much detail on Jupiter. Looking closely at the two main belts, we could see that they were ragged.
"The belts, however, paled into insignificance when compared with the equatorial zone that lies between the two equatorial belts. David [Collis-Bird] described the equatorial zone as having been coloured by a child. Looking through the 20, 17 and 14 inch telescopes, there were streaks everywhere in this zone. Even through my 6 inch, the zone didn't look smooth.
"The appearance of Jupiter had an effect on everyone. It is rumoured that Steve [Mencinsky], who normally doesn't focus on anything within 10,000 light years, may indeed become a closet planetary observer [a prediction that in 1996 is becoming true]. Naturally, he denies this. The view also brought Joe Cauchi and David away from their photography to join the observers. Tony [Buckley] was still talking about Jupiter the following morning."
The memories of Jupiter in January 1991 are still strong. Looking at Jupiter through a telescope, three things will firstly come to mind :
Jupiter is a Gas Giant, and being gaseous and rotating quickly (in just under 10 hours), it bulges quite noticeably at the equator. Indeed, around the time of opposition, it will be 47 arcseconds across at the equator, but 44 arc seconds from pole to pole - a difference of 7%. Consequently, if you choose to do a sketching of the planet, your diagram will be more realistic if you draw the outline of the planet as a slight oval rather than a perfect circle.
So far, we've just discussed various areas that circle the planet. However, there are a number of individual features, some of which are transient in nature, that are well worth observing.
Remember of course that the Great Red Spot will not necessarily be visible when you first turn your telescope to Jupiter. As the planet rotates, there is a 50% chance that the Great Red Spot may be on the other side of the planet when you look. However, if you record the time that you observe the Great Red Spot, it is easy to calculate the approximate time of near-future observing sessions when you will sight the Great Red Spot as a multiple of Jupiter's 10hr rotation period (likewise, if you don't observe the Great Red Spot, you'll know when it won't be visible).
For the observer with a telescope 20cm and more in aperture (that's 8 inches for the old-timers), here's an interesting observational challenge: Interactions of the Great Red Spot with features on the southern edge of the South Equatorial Belt, in the South Tropical Zone, and on the northern edge of the South Temperate Belt, often result in noticeable accelerations and decelerations of the features when they encounter the Great Red Spot. Can you spot the changes?
1998 update : two of the white ovals have now merged. Jupiter is a changing environment!
Io, Europa, Ganymede, and Callisto are all visible with a small pair of binoculars, and a small telescope will provide a fine view of Jupiter and its four major moons, with their configuration changing noticeably over the space of an hour.
Careful study of the satellites at opposition will reveal that they are tiny discs rather than mere points of light. Owners of telescopes of aperture 25cm and upwards viewing under circumstances allowing for magnification of 400x plus, will reveal surface details under superb seeing. The best times to look are when the satellite is transiting Jupiter, since there is a reduced glare when seen against a light background. Indeed, Ganymede's polar frosts can be detected with a telescope of only 15cm aperture because they closely match the albedo of Jupiter's zones, giving Ganymede an elliptical rather than circular appearance.
If atmospheric transparency is near the best end of the scale, you can even try to detect the colours of Jupiter's satellites. Io is yellow, Europa is off-white (making it a difficult satellite to see during transits), Ganymede is tawny grey, and Callisto is a dusky blue-grey.
Those of you who own a yearbook or read such magazines as Astronomy and Sky & Telescope will see the monthly tables that show the movements of the Galilean satellites around Jupiter. Thus, you can easily determine which moon is which.
Looking at the diagrams, you'll notice that the orbital paths regularly cross Jupiter, and here is where some of the excitement following the satellites comes into play. Because the plane of the Galilean satellite orbits lies almost in the same plane as Earth's orbit, we see each satellite (except Callisto on occasions) pass directly in front of Jupiter and the behind it on each orbit.
The shadows cast by the satellites onto the face of Jupiter as they transit the disk can be seen as small black dots in any telescope. The satellites themselves appear as bright dots against Jupiter's dark belts. However, when they are in front of the brighter zonal areas, they become harder to distinguish unless you know exactly where to look. It is often best to follow a satellite as it starts to cross the planet's face. Prior to the date of opposition, the shadow precedes the transiting satellite ie. it falls on the disc before the transit begins. After opposition, the shadows fall on Jupiter after the transit, and remain on the planet even when the satellite itself has finished its transit.
On the other side of the orbit, the satellite is eclipsed by Jupiter. In the months leading up to opposition, a satellite approaching occultation can be seen to gradually fade away as it enters Jupiter's shadow. At opposition, the shadow is directly behind Jupiter as seen by us. After opposition, the shadow is now on the other side of Jupiter, and a satellite will reappear from eclipse at some distance from the limb of the planet.
All of these events (satellite transits, shadow transits, satellite eclipses, and satellite occultations) are listed in some yearbooks and magazines, so it's worth checking these tables to prepare for the night's viewing.
Once every six years, the plane of Jupiter's equator passes through the Sun, and at these times, we can see occultations and eclipses of Jupiter by each other. 1997 was the last time that this happened.
The study of Jupiter has once again returned to prominence, with the Galileo spacecraft currently in orbit around it, and returning images until the end of 1999. It's worthwhile taking a look for yourself. Indeed, keeping an eye on Jupiter, and following the movements of its satellites, should be incorporated into any night's observing plans. Best of all, observing Jupiter is so easy, and can be done at home with the simplest of telescopes. The satellite phenomena provide for exciting events as you see the Solar System "in action" - indeed, you're observing "a miniature Solar System". Set yourself a goal to undertake a simple exploration of the Jovian realm. You'll be surprised how rewarding and fascinating it is.
(Adapted from the July and August 1996 issues of Universe)