Earth and Moon

The Moon is such a familiar sight in our skies that we rarely stop to think about it.

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People talk about 'the dark side of the Moon', and there are plenty of pubs called the Half Moon, but most of us probably don't even think about why we get the phases of the Moon.Yet at one time, before the days of electric or even gas light, knowing the phase of the Moon was important to everyone.

It is never truly dark at night in this area now. There's always plenty of city light reflected down from the sky. But before streetlights and house lights, a dark and cloudy night really was dark. You wouldn't see anything at all without a lantern. So the Moon provided welcome light at night, and you might have to get home before the Moon set. One famous group of scientists and engineers in Birmingham called themselves the Lunar Society, not because they studied the Moon but because they met at full Moon, when they could see their way home after the meeting.

The Moon's phases are caused simply by the changing angle at which sunlight strikes it as the Moon orbits the Earth every 29½ days. When it's an evening crescent Moon, it's backlit by the Sun, and you only see it over in the western sky after sunset. At half Moon, a few days later, the Sun is sideways on to it. Then a week after that, the Moon is full, and it is opposite the Sun in the sky, rising at sunset. Following that we don't see it in the evening sky at all, and it soon becomes a half Moon in the early morning sky. Try watching the Moon night by night and you'll see that it rises about ¾ hour later each night, with a slightly different phase.

Incidentally, there's no truth in the belief that the Moon is bigger when it's low in the sky. It just looks that way, probably because we are comparing it with nearby objects. And the belief that there are more accidents or emergencies at full Moon is also a myth. This is because the Moon looks nearly full for several days on either side of the actual full Moon, so it's more obvious and there's a greater chance that people will make a link.

In winter, the full Moon is high in the sky, the opposite of the Sun, and in summer it's always low in the sky at night when the Sun is high up during the day. The Harvest Moon is the nearest full Moon to the September equinox, and is called this because its rising just after sunset helped the farmers to gather in the harvest. Normally, the Moon rises about 50 minutes later each night, but at that time of year it rises about only 30 minutes later night after night, giving added illumination. The Hunter's Moon is the October full Moon for a similar reason.

Credit: NASA - The Earth photographed from lunar orbit in 1968 from the Apollo 8 spacecraft

Page last updated: 30 Mar 2022