SKYWATCH PROJECT: JUPITER'S MOONS

EQUIPMENT REQUIRED: Good binoculars or a small telescope.

TIME REQUIRED: A short (10 minutes) observation on each of 5-10 nights, ideally as close in time as possible.

BACKGROUND: Jupiter has about 15 moons that we know of; four of them, the Galilean satellites, are visible in binoculars or a small telescope. In order of their distance from Jupiter, the moons are Io (I), Europa (II), Ganymede (III), and Callisto (IV). They are about as large as our moon, but move much more rapidly than Luna. Io makes one complete trip around Jupiter in less than 2 days while the others have periods of 3 to 16 days.

From the earth we see the orbits almost perfectly edge-on, so that the satellites seem to be spread out on a line. As the moons orbit, the configurations change; sometimes all of the moons will even be on one side. Note that because we see orbits projected edge-on, the moon that appears to be closest to Jupiter is not necessarily Io; it may be one of the others passing either in front of or behind the planet.

WHAT TO DO: Jupiter is now visible in the southeast after sunset and can be observed all night long as it moves westward through the southern sky. You should have no trouble finding Jupiter, as it is by far the brightest object in that part of the sky, and of course it does not twinkle.

You should observe the satellites sufficiently frequently that you can identify which moon is which. This will be easiest if you observe for 5-10 evenings in a row. If you use binoculars, be sure to steady your arms as well as possible. Each night, make a sketch of the moons' positions relative to Jupiter; if you have a small telescope, you can also use the diameter of Jupiter's disk as a handy "yardstick" for the distance of each moon from Jupiter. Try to measure the orbital period of at least one of the moons; then compare your measurement with the established value, available from your T.A.