Moons and More Moons

At the SJAA astronomy class last month, I glibly commented that Saturn has the most moons of any planet in the solar system.

Turns out I was wrong: Jupiter has pulled way ahead in the moon race. According to a web site run by Scott Sheppard of the University of Hawaii, the latest satellites tally shows:

Jupiter 63
Saturn 46
Uranus 27
Neptune 13

Not all of these are named. The IAU, as of their conference in July of 2004, lists 101 known planetary satellites in the solar system, down to 1 km in diameter, and comments dryly, "At some time in the future it may be advisable to stop naming very small satellites." On the US Geological Survey's planetary nomenclature page (the USGS is apparently running a lot of the IAU's nomenclature programs these days) I count 48 named satellites of Jupiter, 34 for Saturn, 21 for Uranus and 8 for Neptune, a total of 111, so it may be that the IAU's web site is out of date.

And what about Earth? How many moons does it have?

Well, all of the web sites just mentioned list only one. But if you're a fan of the popular web site Astronomy Picture of the Day, you may have seen the April 30th entry, a beautiful astronaut photo from 1998 entitled "The Moons of Earth". It shows the Earth, our moon (looking quite small and distant) and, in the foreground, the Russian space station Mir.

What a tease! Mir and its successor the International Space Station don't count as moons, of course. They're fun to observe, and you can sometimes see a little detail in the ISS if you follow it smoothly with a good Dobsonian; but they're artificial satellites, not natural ones.

But every now and then, intriguing news items show up about "Earth's second moon", a little five kilometer rock called Cruithne (pronounced "croo-een-ya"). Asteroid 3753 Cruithne has a peculiar orbit which takes it both inside and outside the earth's orbit. Over a period of 770 years, if you take snapshots at just the right times, Cruithne traces a horseshoe shape around the earth as the earth orbits the sun.

This isn't, you have to admit, much like a normal moon, and calling Cruithne "Earth's second moon" seems like an unrealistic stretch. Still, Cruithne is an interesting object. It only makes a close approach with the Earth every 385 years: the next approach happens in 2285, at about magnitude 15.5, so it'll be a while before we get to observe Cruithne ourselves. Three other asteroids have been discovered with orbits resonant with the earth's; and Saturn has two moons, Janus and Epimetheus, which follow horseshoe orbits.

But moving along to objects with more normal orbits:

In June, Jupiter, with all 63 of its moons, moves into prime time. Two months past opposition, it's already high enough to observe at nightfall and remains well placed throughout the evening observing hours, though unfortunately it's so far south this summer that it never gets terribly high in the sky. You won't be able to see all those moons, but the brightest four: Io, Europa, Ganymede, and Callisto; make an ever-changing show that's always fun to watch.

This is a good month for double shadow transits, too, starting with a nice transit of Io, Europa, and their shadows, on June 2. Europa has already started its transit by dark, Io comes in at about 7:20, closely followed by Europa's shadow, and Io's shadow joins the party at about 8:30 pm.

An even better early-evening double transit happens on the 9th. Europa begins a transit at around 7:40, with Io joining in a little after 9:00. Then at about 10:10 the fun starts: Io's and Europa's shadows both appear nearly on top of each other. Can you see them as two distinct shadows? Can you see a size difference? Can you tell which shadow is which? As the shadows move across the disk of Jupiter, exiting a bit after 1 am, you'll be able to tell them apart by the different speeds at which they cross the disk, as Io's shadow overtakes Europa's and exits first. I haven't seen a transit like this myself; I'm looking forward to it!

Saturn can still be glimpsed, low in the evening twilight, but as June advances it gradually becomes lost in the sun's glare. Goodbye, Saturn - catch you next time around! Meanwhile, we'll have to content ourselves with more Cassini pictures.

Mercury emerges into the evening sky in the latter half of June, so this is a good time to catch it. It also has a conjunction with Saturn on June 26, on the following day, it makes an extremely close daytime approach to Venus, only .06 degrees of separation at 9am. Early risers that morning can still catch a lovely pair, faint Mercury and bright Venus; the more ambitious might want to keep tracking the inner planets as they rise, in order to see the closest pairing of these two planets since 1990. Daytime observation of planets works best if you set up in the shade. That way, you don't have to be so paranoid about accidentally pointing your telescope at the sun. Plan ahead and find a place that will still be in the shade when the close pairing happens, and don't forget to check that the telescope's objective will be shaded, not just the chair where you'll be sitting.

Pluto is at opposition on June 14, so this month begins a good time to look for it. Use a good chart - the one in the RASC Observer's Handbook is reliable, and some of the better star charting programs also give fairly good Pluto maps, though if you compare them you may be surprised at how much difference there is between any two Pluto charts. Pluto's orbit is surprisingly complicated (it's highly elliptical, and in addition, little Pluto is subject to perturbation by other objects) so it's much harder to calculate than the orbits of the inner planets, and since it's so faint, you need very accurate positions of faint stars in order to plot the correct star field to compare against Pluto. But that's part of the fun! To detect Pluto for sure, you have to observe it on several different nights, and notice whether the object you thought was Pluto has moved to a different place. If it moves, it's Pluto (or else you've discovered a new comet or asteroid and should contact the Minor Planet Center right away). So get yourself a chart, or several if you can, get yourself to a darksky with a sizeable scope (12" will do, 10" or less if you're really eagle eyed) and a little sketch pad, and take a look!

Mars is still in the morning sky, but it moves a bit farther from the sun this month, and early risers can probably catch it in the early dawn before the sunrise washes it out.