Nearly 400 years ago Galileo pointed a crude telescope toward the heavens and documented three remarkable things in the sky, discoveries that dramatically affected the understanding of the universe at that stage in history. With his telescope Galileo was able to see that the Moon was not a perfect circle, that Venus had phases, and that Jupiter had satellites of its own. Each of these posed a threat to the current body of knowledge of that era - Jupiter's moons and Venus' phases challenged the geocentric view of the universe and the non-perfect Moon challenged the Aristotelian theory of perfectly circular shapes in the universe.
With a simple telescope or even binoculars you can recreate some of these discoveries in the coming weeks and months. I'll write more about Venus in a future post. In October and November the phase of Venus will gradually transition from gibbous to half and later in the year to a crescent. The Moon is easy enough to study and is a marvel to view in any telescope or binoculars with the spectacular rocky edge of the surface always intriguing.
However, for the coming months Jupiter is the dominant "star" in the evening sky. Jupiter calls out for investigation because of the interesting texture of its surface and because of the changing position of its four largest moons. Called the Galilean satellites, these moons are visible to us even in low magnification and are interesting to observe because they change location so quickly -- even within the course of a few hours. They regularly pass through the shadow of Jupiter, yielding eclipses on a frequent basis.
From the nearest to the furthest moon, the names of the four satellites are Io, Europa, Ganymede and Callisto. Io is so close to Jupiter that it orbits the giant planet in less than two Earth days. Europa takes exactly twice as long and Ganymede twice as long again. Every few hours the overall pattern of these four moons is changed sufficiently to see new moons emerge from behind Jupiter, others disappear, and the overall pattern spread apart and then contract into a group.
Sky and Telescope Magazine has an excellent article on the moons and a very helpful pop-up screen that you can view in your web browser. It shows you the relative position of the moons of Jupiter at any time and can make a star gazing evening into a treasure hunt that will give you the feeling of discovery that will rival that of Galileo. So take the time to look to the south for the brightest object you can see (about halfway up the horizon above due south after sunset) and try it for yourself.