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Seeing Jupiter's Moons

Week of September 8, 1997

image of Jupiter and moons [This image is a single frame of a GREAT animation made of several images of Jupiter and its moons by Antonio Cidadao. You can actually watch the moons orbit Jupiter! Click on the picture to be sent to his web site.]

I think it's safe to say that most people in modern times don't spend a lot of time looking up. Certainly the large number of UFO reports that turn out to be sightings of the planet Venus are an indicator of this; if more people were used to seeing it, then its uncanny brilliance looming low through the trees wouldn't be mistaken so often for a spacecraft following your car.

But ancient astronomers were used to looking up. Sometimes their lives depended on it; the season are tied in with the stars, and, for example, the annual flood of the Nile river can be predicted based on observations of the star Sirius. So it surprises me that no ancient texts describe the moons of Jupiter. Jupiter, right now as this snack is written, can be seen nearly all night long. It is very bright; after Venus and the Moon, it is the brightest object in the night sky. The view through a small telescope is stunning: Jupiter present itself as an easily discernible disk, and the four largest moons are also easy targets lined up in a row. Over the course of even one evening, you can watch the moons move a bit as they orbit the mighty planet.

What may surprise you is that the moons are so bright. If Jupiter were painted black, the four moons would be pretty easily seen with the naked eye from a dark site. Shining at fifth magnitude (about twice as bright as the unaided eye can detect), they would make an eerie sight, lined up in row. But Jupiter is there, and very bright. It is about two or three hundred times brighter than the moons, and they are swamped by its glare.

However, it is still possible to see at least one moon with the unaided eye. Callisto revolves around Jupiter with an orbital radius of about two million kilometers. If conditions are right, that translates into an apparent distance of 10 arc minutes or so, or about a third the diameter of the Moon as seen from the Earth. If you were to stand next to a building, or some other object with a long straight side, you could position yourself to just block the fierce glare from Jupiter. If you do this correctly, the moon Callisto might just be visible as a faint star just off to the side. This trick actually works; many amateur astronomers report seeing Callisto using just this method.

Certainly ancient astronomers spent quite a bit of time observing Jupiter; after all, many named it after their chief god (or was it vice-versa?). It amazes me then that no one looked at Jupiter just as it came around a building, or happened to be blocked by a branch, and notice that it was being followed by a faint star. Even a handful of follow-up observations would show the star to somehow be attached to the planet.

Of course, maybe some people did see it, but failed to report it, or did report it and weren't believed (how many hapless amateur astronomers had their heads cut off for treason against the state by nervous court astrologers back then, I wonder?). Certainly that still happens; a new type of lightning was discovered just a couple of years ago. Pilots had reported seeing it for years, but no one believed them until someone caught it on video. What other astronomical mysteries await someone just see them?

Want to know more about Jupiter? The Nine Planets is loaded with info. You can skip right to the Jupiter pages too.

©2008 Phil Plait. All Rights Reserved.

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