Lessons One through Four appeared in Starlite Issues # 138, June 1999, and Issues # 140 — 142, December 1999 — June 2000

On average, you can see a total eclipse of Earth’s Moon about once a year. But you can see an eclipse of one of Jupiter’s large moons about once a day. Brief mention of the dance of the Galilean moons was made in the Primer for the Beginner series. But there is much more that you can observe about them other than their continual switch of positions. And such observations take greater skill and patience than most people first imagine.

Basic Information

The reason you can see the four large moons in a nearly straight line across Jupiter is that each moon orbits in the plane of Jupiter’s equator, which is turned face-on to us. Actually, the tilt changes slightly over time. When it reaches the maximum of 3°, Callisto, the farthest moon from the planet, appears to pass slightly above or below Jupiter rather than in front of or behind it. Callisto revolves around Jupiter once every 16 days, 16 hours. The other three moons, Ganymede (7 days, 3 hours), Europa (3 days, 3 hours) and Io (1 day, 18 hours), always pass both in front of and behind Jupiter during each revolution. As a result of this phenomenon, we are able to observe occultations by Jupiter, transit shadows of its moons on the planet, and eclipses of the moons by the planet’s shadow. Critical to being able to make such observations is knowing the dates and times of such events. You can obtain such information from an astronomical almanac. A diagram showing this general information is available in both Sky & Telescope and Astronomy magazines. The specific times of such events are printed in Sky & Telescope, necessary for these more advanced observations. Abbreviations used there are: Oc for occultation

• Tr for transit
• Sh for shadow
• Ec for eclipse
• D for disappearance, when an occultation or eclipse begins
• R for reappearance, when an occultation or eclipse ends
• I for ingress, when a transit or shadow begins
• E for egress, when a transit or shadow ends

The times given are in Universal Time. For central Illinois, subtract six hours for Central Standard Time or five hours for Daylight Savings Time. Be aware that while these times are good approximations, they are not exact and may be off by several minutes.

Occultations

An occultation occurs when a moon passes behind Jupiter, while a transit occurs when it passes in front of the planet. Since you can not actually see such a transit from an amateur telescope, I will treat them the same for observing purposes. In viewing an occultation or transit, start several minutes ahead of schedule so you don’t miss the beginning. You can see the moon appear to come closer and closer to the planet. With good eyesight and conditions, you can see the moon appear as a dot attached to Jupiter. An interesting part of observing an occultation or transit is seeing the exact time the moon disappears or “blinks out” shortly thereafter. You should start at low power, then switch to high power to view all these events. It is much easier to observe if you have a motorized telescope, but it is not necessary to have one to be successful. It is far more challenging to see the exact second the moon reappears since you will not be able to follow the satellite to that point. It is still easier to see a reappearance of Jupiter’s moon during that planet’s occultation of it than during an occultation of a planet or star by our Moon. Even at high power, the disk of Jupiter appears small in comparison to the disk of our Moon. With our Moon, you need to know the approximate spot of reappearance of the occulted object, while you do not need that information for an occultation of a moon of Jupiter. You can also time the period it takes from disappearance to reappearance. If you are exact enough and have the appropriate equations and distances, you can calculate the approximate diameters of each of these moons.

Shadow Transits

While you cannot see any of the four moons transit Jupiter, you often can see the shadows of these moons transit across the planet. However, it is not as easy to see these transits as everyone assumes. I have been thwarted at seeing shadow transits more often than I have been successful. Like all the observations of these events in this article, seeing conditions play a critical role in being able to see these shadows. The shadow is very small in apparent size, more like a dark point than a dark spot. Very clear, dark, steady skies are necessary. Optic quality also plays a crucial role. If you cannot “see” a shadow on one try, keep trying when conditions are better. Practice and patience help. Shadow transits are not rare. On any clear night that Jupiter has risen, the possibility usually exists to see at least one shadow — especially if you are willing to stay up late enough. Less common is being able to see the shadows of two moons on the planet at the same time. It is much rarer to see a triple transit. On November 10, 1997, the shadows of three of Jupiter’s moons could be seen on Jupiter at the same time. Unfortunately, clouds prevented us from seeing that event here. Since these events are rare, you need to be “lucky” for all the favorable factors to occur simultaneously. Jupiter of course must be in our dark sky at the time. The area of the sky must be cloudless and the seeing conditions excellent. Finally, your schedule must be free to allow you the time to try. For all these factors to occur would indeed be rare. If they do, you will become one of the few people to observe this triple transit. And if you are an accomplished astrophotographer and capture the triple on film, I am confident you could get your photograph published in both major astronomy magazines. I am not sure that a quadruple shadow transit is even possible. If you are aware that it is, please let me know.

Eclipses of the Moons

We are also in a position to be able to see eclipses of the four moons. They occur in the same manner as our lunar eclipses, except these moons are eclipsed by Jupiter. We see these eclipses from the side. Because the apparent diameters of the four moons are so small and seem starlike to us, we cannot actually see the phases of these eclipses. However, you can still the effects of the phases. A particular moon gradually dims. Then at totality, you see nothing at all. More challenging is to see the split second when sunlight again strikes that moon. Gradually, the moon brightens until it reaches its normal magnitude, signaling the end of the eclipse. Because Io is so close to Jupiter, you can only see either the disappearance or reappearance of that moon during any particular eclipse. But also because it is so close, you see eclipses more often. The same is true of Ioan occultations, transits, and shadows. The greater the distance the moon is from Jupiter, the less frequently these events occur. Get a good calendar of these events and be patient. See how many of them you can observe and record over time.