Everybody knows that the Moon changes its face. Sometimes we have Full Moons, other times it’s a Half-moon, or a crescent, or there’s no Moon at all. And everybody knows that these are called the phases of the Moon. But not everybody knows that these faces follow a regular cycle, or that the common descriptions are misleading at best. A proper understanding of what’s actually happening to make the moon show different faces, and the nature of the “Dark Side of the Moon”, is quite rare.
The motion of the Moon
So here’s the first thing you need to understand. The moon is an unusually large satellite of the Earth (most planet’s satellites are much smaller than their parent), and it orbits very far from the earth (again, compared to most satellites in the Solar System). Depending on your point of view, it takes roughly either 27 days or 29 days to orbit the earth. Against a fixed reference point (say, a distant star), the moon takes 27.3 days to revolve around the earth and this is called the Sidereal Period. However, if we count from Full Moon to Full Moon, we find it takes 29.5 days: the Synodic Period. The reason for this discrepancy is the fact that, in those 27.3 days the earth has shuffled along around the sun — slightly less than one twelfth of the way, in fact, since 27.3 days is a little less than one month. In that time, the Sun appears to move in the sky (relative to the stars – this movement, incidentally, is the basis of astrology’s horoscope birthsigns), so that the phases are a little behind what we expect. What have the phases got to do with the Sun?
The Sun’s influence on the Phases
First, we need to know about the phases. The moon has four basic phases: New, 1st Quarter, Full (or 2nd Quarter), 3rd Quarter, and back to New. A New moon is when the moon is completely blank. People commonly refer to the crescent moon a few days before and after this as a New Moon, but this is only because you can’t really a see the true new moon as it is completely dark and so close to the sun that the glare hides it. 1st Quarter is named because the moon is now a quarter of the way through it’s cycle, and looks like a “Half-moon”. Full moon is when the whole face of the moon is illuminated and Third Quarter is “The other Half-moon”. If the phase is busy changing towards Full, so that the face of the moon is growing bigger, then we say that the moon is “Waxing”, and if it is in the opposite situation then we say that it is “Waning”.
The Phases of the Moon
Now that these definitions are out of the way, we can think about the Sun. Everything in the solar system is (usually) lit up by the sun. The side facing the Sun is bright, and the side facing away from the Sun is dark. We describe these two sides of the Earth as the day side, and the night side. On the moon, I call them the Light Side, and the Dark Side. Most astronomers will explain that there is no dark side, but this is because people confuse the near/far sides with the light/dark sides. Obviously different parts of the moon will be dark at different points in the synodic cycle, since as it moves around the earth, it’s presenting different sides to the sun. It is the movement of the light/dark side that causes the phases. At full moon, we are directly facing the light side. At New moon, we are facing the dark side. At 1st and 3rd Quarter, we are facing exactly between the two sides. It’s all quite simple. But what was that about Near and Far sides?
The Dark side of the Moon versus the Far Side
Interesting fact about the moon: The time it takes to orbit the earth is exactly the same as the time it takes to rotate on its own axis. This means that we only ever see one side of the moon! If you’re having trouble visualising how this works, try “orbiting” a tennis ball around your head and turning it in such a way that you only ever see the one side. It’s a simple concept to demonstrate, but not so easy to explain!. The side of the Moon that always faces us is called the Near Side, and the side that faces away is the Far Side. Light side and Dark side, are always changing, but Near side and Far side are fixed.
One last thing…
At any particular phase, the moon is always at the same place in the sky relative to the sun. This is why you only ever see crescent moons near sunset or sunrise, close to the sun, and why the full moon always rises just as the sun sets, and sets just as the sun rises. Here’s an intellectual exercise for you: Try to visualise the moon, earth and sun in space and work out where all three are relative to each other at each phase. You’ll find the answer on most astronomy textbooks, or this wikipedia page