The Moon – Our Nearest Neighbour In Space
by Kevin Brown : last updated: September 24, 2008If you’re new here, you may want to subscribe to the site RSS feed or Podcast channel. Thanks for visiting!
The Moon is a familiar sight in the sky and it is our nearest neighbour in Space, being only (in astronomical terms) 250,000 miles away.
It is thought that the Moon was probably once part of the Earth, until some cataclysmic event broke it off.
Now the Moon and the Earth rotate about about their common centre of gravity. This point of rotation is actually within the Earth, because the Earth is so much more massive, but it is nearer the surface than the centre.
Crescent Moon
The Moon is about one third the diameter of the Earth and it orbits around it every 27.3 Earth days. This is called its “sidereal period”.
However, this is not the same as the time between each New Moon, which is over 29 days, because of the Earth’s rotation about the Sun.
In the time it has taken the Moon to go once around the Earth, the Earth has covered approximately one twelveth of its orbit around the Sun. So the Moon has to “catch-up”, so to speak, one twelveth of its orbit around the Earth (just over 2 days), in order to reach the same Sun-Moon-Earth relative alignment for the New Moon to appear.
Most people will be aware of the “far side” of the Moon which we never see from Earth. This is because the Moon rotates on its axis with the same period that it orbits around Earth. Consequently, it always presents the same side towards the Earth.
The far side is not actually the “dark side” of the Moon – it is actually illuminated by the Sun, but we cannot see it.
The Moon has no atmosphere and its surface has been hit by many objects over time, resulting in its heavily cratered condition.
Seen from Earth, there are obvious light and dark areas. In the past, before telescopes were developed, these were thought to be seas (or Mare from Latin). We now know there is no water on the Moon and these “seas” are just relatively smooth areas of the surface.
Apollo 11 Tranquility Base
This was of course, confirmed when the Apollo 11 space mission landed in one of them (Mare Tranquilitatis or Sea of Tranquility).
Further Apollo landing missions followed, but the Moon remains the only space body that humans have set foot upon.
The Moon is an excellent object to observe with binoculars or a small telescope. It is often bright and easy to see.
Looking through a telescope, you can see the craters and ridges so clearly, that it is possible to imagine you are actually there, flying over the surface and looking down!
Some people think full-Moon must be best for astronomy, but this is just not so. Near to full-Moon, the Sun does not cast shadows on the lunar surface, meaning that you cannot see much detail.
The best time to observe is around half-Moon. With this degree of illumination there is plenty to see, the relief features are defined by shadow and it is not too bright.
