Earthshine
If you have a clear view of the western horizon tomorrow (26th January) just after sunset then you should be able to see a thin crescent Moon very close to the planet Venus.
As you look at the Moon you will be able to see the crescent shape that is being lit by the Sun but you should also be able to make out the rest of the Moon’s face, much fainter but still visible bathed in a ghostly light. This is Earthshine.
If you have ever been in a very dark area at the time of the full Moon you will have noticed just how brightly it can shine and even light your way. The Moon’s surface is made from really quite dark material but can still shine brightly in the night sky. The Earth on the other hand is covered in white clouds, it has white polar ice caps and reflective oceans and it is also much bigger than the Moon. So for anyone standing on the Moon the Earth appears as a very large, bright beacon in the sky, much brighter than a full Moon can ever appear from Earth. The bright Earth lights up the dark areas of the Moon so much that we can even see it from Earth.
All of the Apollo astronauts who went to the Moon have mentioned how overwhelmed they were by just how big and bright the Earth appeared, and, when they went to the Moon, it was obviously ‘daytime’ with the Sun in the sky too. Had they been there when the Sun was below the horizon then the Earth could have probably lit their way.
As the Moon keeps one face permanently turned toward the Earth then, as seen from the Moon, the Earth is constantly in the same place in the sky – it is only the phase of the Earth that changes as the Moon orbits us. When we see the Moon in its young phase – as it will be tomorrow night– then the Earth appears full from the Moon and shines at its brightest.
Mercury and Venus are the only two planets that don’t have moons. Mars has two tiny ‘potato shaped’ moons called Phobos and Deimos which are probably asteroids that were captured by Mars’s gravity. Jupiter has 63 moons, the largest of which, Ganymede is the biggest moon in the Solar System and is actually bigger than the planet Mercury. If you look at Jupiter with binoculars you should be able to make out its four largest moons Ganymede, Europa, Io and Callisto as they circle the planet. Jupiter is currently very prominent in the night sky shining brighter than any star. On 30th January the Moon will be just a few degrees above Jupiter.
Saturn has 62 moons (and counting). Saturn’s largest moon is called Titan and even has its own atmosphere of mainly nitrogen gas. The atmosphere on Titan is so dense and the gravity so low that any future astronauts who land on Titan would be able to fly through the air by simply flapping their arms!
Uranus has a family of 27 moons and Neptune has 13. Even tiny Pluto–which is no longer classed as a planet – has three Moons called Charon, Nix and Hydra. That makes a total of at least 171 moons in our solar system and almost every one different from the next.
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By Ken Campbell
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