Our Earth’s natural satellite, the Moon, is well placed in the sky during the BBC’s Stargazing Live event this week for observing with binoculars, a small telescope – or just your eyes alone.
But if you have that small telescope or some decent binoculars, you will be able to see more moons in the January night sky by having a look at Jupiter, the largest planet in our Solar System!
Jupiter’s four main moons, known as the Galilean Satellites, were first spotted by the famous astronomer Galileo way back in 1610. They are known as Io, Europa, Ganymede and Callisto they are easy to spot.
But first, let’s talk about our Moon. It is a blindingly obvious object in the sky, though it shines only by reflecting sunlight because it gives off no light of its own. As it orbits the Earth, once every 27 days or so, it goes through phases as the half of the Moon that is reflecting the sunlight changes its orientation towards us.
At New Moon, the Moon is roughly between us and the Sun so it is the far side that is reflecting the sunlight. At Full Moon, it lies on the other side of the Earth, opposite the Sun in the sky, and so the whole of the side of the Moon pointing at us is illuminated.
Over the three nights that Stargazing Live is showing on the BBC, the Moon’s phase will be gradually increasing, or waxing, because it will be midway between New and Full. At this time, the mountains and craters on the lunar surface are at their most spectacular because they are casting clearly visible shadows.
A pair of binoculars will show that the Moon is cratered, and make it easy to pick out the vast plains of lava known as maria, or lunar seas, though they are quite dry. You can see the Moon’s seas with just your eyes, and they make up the face features of the “Man in the Moon”.
The two images of the Moon above were taken six days and 11 days after New in the regular cycle. You can see that at 11 days old, as the Moon approaches Full phase, the region along the divide between the illuminated and the dark side still shows the craters and mountains clearly, but the lack of shadows is hiding the relief of the part of the Moon that is more in sunlight. At Full Moon, the shadows are completely lost and it becomes harder to see most craters and mountains.
For more about observing the Moon, see our own Moonwatch pages. Go here is you want to see the Moon as it appears on the first night of Stargazing Live.
If your skies are clear, now is a great time to see something else in the sky – mighty Jupiter. You really can’t miss it as it is the brightest thing in the night sky at the moment after the Moon, rising in the East at around sunset! And if you have those binoculars or a small telescope handy, you can use them to see more moons – those Galilean satellites.
Point your telescope at Jupiter with a low-power eyepiece, and you will see them strung out like beads in a line on either side of Jupiter. Their positions change rapidly so they can be on either side of the planet. And you may not see all four if one or more are on the far side of or in front of Jupiter.
On Tuesday evening as Stargazing Live is being broadcast, only two of the moons will be visible for a time with the others hidden. But they soon become visible again as the pattern swiftly changes. Sometimes with higher-powered telescopes, you can see the shadows of the moons cast onto Jupiter’s cloudtops.
If you have binoculars and can hold them very steady, either by leaning against a wall or by mounting them on a tripod, you can spot the Galilean moons with those. In fact they would be just bright enough to see with the unaided eye from a dark site if Jupiter was not to bright as to drown them out with its glare.
Note: I wrote a version of this article for the Society for Popular Astronomy’s website.