Galilean Moons on Show

April 30, 2015

Jupiter is currently visible in the evening and will remain so until mid-August. Now is a good time to identify it and watch what happens over the coming weeks.

There are two important things to know about how planets appear to move in the night sky. First, the entire sky moves throughout the night (it's actually the Earth spinning but it looks like the sky is moving). Secondly, planets change their position relative to the stars a little bit each successive night.

At the moment Jupiter is the bright object almost due north around 7pm, moving towards the northwest horizon and setting before midnight. Over the next few weeks it appears slightly more to the west each night at the same time, and will set earlier.

You can see another bright object, Venus, between Jupiter and where the Sun sets in the west. Between now and the start of July these two planets will appear to move towards each other until they "meet" (we'll get back to that event closer to the time).

To the naked eye Jupiter looks like a single bright star but it's actually a planet with its own family of over 60 moons. In 1610, Galileo used the newly-invented telescope to see Jupiter's four biggest moons. This was a huge deal, proving for the first time that not everything in the Universe orbits the Earth. You can get an idea of what Galileo saw by using a small telescope or pair of binoculars.

The graphic shows the position of Jupiter at 7pm on May 1st, with a closeup view including the four "Galilean Moons". With a 4-inch telescope you might just be able to see the atmospheric bands on Jupiter, but even with cheap binoculars you have a good chance of seeing the moons.

Notice how the moons are in a line. This is because celestial objects tend to orbit their parent body in a disc and we're seeing Jupiter's "moon disc" side-on. You'll also notice that the moons' positions change quite quickly as they orbit. This is celestial mechanics in action, seen with your own eyes!

Your Night Sky, April/May 2015