91av

ON 14 April this year, the European Space Agency sent a new spacecraft off on its eight-year journey to Jupiter. The Jupiter Icy Moons Explorer (JUICE, illustrated above) will fly by three of Jupiter’s four biggest moons, Europa, Ganymede and Callisto, before going into orbit around Ganymede, the biggest moon in the solar system. This is an exciting mission because these icy moons, with their subsurface oceans, are some of the most promising places to look for life. And now that Jupiter is once again appearing in our night skies, we can see those icy moons ourselves.

The four biggest moons of Jupiter – Io, Europa, Ganymede and Callisto – are collectively known as the Galilean moons, because Galileo Galilei observed them in 1610, making them the first moons discovered beyond Earth. This was hundreds of years before Neptune, Uranus and Pluto were found. The fact they were discovered so long ago hopefully gives you some idea how easy it is to spot them, as long as you have access to a small telescope or large binoculars.

If you are using binoculars, they need to have at least seven times magnification to see these satellites. Your binoculars will be described by two numbers, usually written in the format “12×36”, for example. The first number is the magnification, so if this is a seven or above, you should be in luck. The second number is the diameter of the lenses, in millimetres, so a bigger number means a bigger field of view – but this isn’t so relevant here.

To look for Jupiter’s moons, we must first find Jupiter. It is usually easy to spot because, when visible, it becomes one of the brightest objects in the night sky. Throughout July, Jupiter will be rising in the early hours of the morning. But as the months go on, it starts to rise earlier and earlier until, in September, it will be rising about an hour after sunset. From October, and for the rest of the year, Jupiter will be visible as soon as the sun has set for a few hours. This is the same all over the world.

Once you have found Jupiter, look through your binoculars or telescope and you will see a few small spots of light very close to the planet. There will be up to four of these, and they will appear in a line. They might all be on one side of the planet or they might be on both sides. There might be fewer than four – in which case, some of the moons will either be in front of or behind Jupiter. The exact formation the moons appear in changes each day, depending on their paths of orbit around Jupiter. To work out what you have seen, you can use the Stellarium web software, plug in your location and time and zoom right into Jupiter.

I love looking at these moons because it reminds me the search for alien worlds doesn’t need to involve distant exoplanets: we have very exciting environments in our cosmic back garden.

Abigail Beall is a features editor at 91av and author of The Art of Urban Astronomy. @abbybeall

For other projects visit newscientist.com/maker.