Full Moon facts

Full Moon facts
  • Tonight's full moon will give you a perfect chance to learn some of its features

Tonight's full moon will give you a perfect chance to learn some of its features. The full moon is directly opposite the Sun in the sky and so rises tonight at 18.15, exactly the same time that the Sun sets. If you can catch it just as it is rising it will appear much larger than when it is overhead. This is just an optical illusion, nobody is really sure why this happens but it is thought that it is because you can see objects such as trees and buildings in the foreground to give a sense of perspective.

The moon shines purely by reflecting sunlight off its surface and while the full moon may look very bright, rock from the surface brought back to Earth by the Apollo astronauts is really quite dark in colour, almost black.

Contrary to popular belief there is no “dark side of the moon”. The full moon occurs because the Sun is shining directly onto the side that is facing us; as the moon orbits around the Earth, the Sun will light different areas of it. When it has travelled halfway round, and is between the Sun and the Earth, then the side that faces us is in darkness and the far side that we can never see from Earth, is in full light.

Although we think of the moon as keeping one side permanently facing the Earth we are able to see about 59% of the moon during its orbit around us. This is because the moon wobbles slightly as it travels and so we can sometimes glimpse a little more around the edges. We call this effect libration of longitude.

It takes 27 days, 13 hours, 18 minutes and 37.4 seconds for the moon to complete one orbit around the Earth. But because the Earth itself is moving around the Sun, the moon has to travel a little farther before it returns directly opposite the Sun and so the time between one full moon and the next is 29 days, 12 hours, 44 minutes and 2.7 seconds. Think of it this way, the minute and hour hands of a clock begin together at 12. It takes the minute hand one hour to return to 12 but it takes one hour and about 5 minutes for it to catch up with the hour hand that has now moved on to the number 1.

There are roughly 300,000 craters over half a mile wide on the side facing us and many of them are named after famous people. It is the job of the International Astronomical Union (IAU) to officially accept proposals for the naming of craters and other astronomical discoveries; so romantic as it may seem, you cannot name a star or crater after your loved one.