The Earth and Moon
Eclipses, both solar and lunar, are a result of the movements of the Sun, Moon, and Earth. This page takes a very brief look at our local bit of the solar system, and explains the basics of how the Earth and Moon move relative to the Sun.
The Sun and its Companions
The Sun is the centre of our solar system, and by far the dominant object in it. As well as providing the light and heat which power all life on Earth, it is by far the most massive object in the solar system: more than 300,000 times heavier than the Earth, and over 700 times heavier than all the planets put together. This huge mass acts as the anchor for the whole system: all of the other objects in the solar system -- planets, asteroids, moons, etc. -- are in orbit around the Sun, either directly, or indirectly as moons of other objects.
The main other objects in the solar system are the nine planets, each of which circles the Sun in its own orbit. Our own Earth is the third planet out from the Sun. Most of the planets have moons in orbit about them, and Earth is no exception; our Moon orbits around us once per month, and so gets carried along with us in our journey around the Sun.
Our Moon is fairly unremarkable, except for its size; it is one of the larger moons in the solar system. The main reason it looks so big to us, though, is simply that it is very close; the Moon is just 384 thousand km away. By comparison, the Sun is 150 million km away.
The Earth and Moon
Although the whole solar system is fascinating, the other planets don't play a role in eclipses. For our purposes, we're interested in the Sun, the Earth, and the Moon, and how they relate to each other.
This diagram shows the Sun, Earth, and Moon, and how their orbits work. The scale is hugely exaggerated, as a scale diagram would be hideously impractical (think about the sizes and distances above, and you'll see what I mean):
In this diagram, the Sun is shown as the orange ball. The Sun is the centre of the solar system, so when discussing the structure of the solar system, it's appropriate to think of the Sun as sitting still (though it does actually move within our galaxy).
Orbiting the Sun, I have shown the Earth, with its orbit shown as a blue circle -- the other planets are not shown. The view is from above the Earth's north pole, and from this point of view, the Earth goes round the Sun anti-clockwise, as shown by the arrow on the orbit. The direction of the Earth's rotation is shown by the curved blue arrow beside the Earth; again, as seen from above the north pole, this is anti-clockwise.
The Moon is the only natural object orbiting the Earth (there are thousands of man-made satellites). I have shown it here as the white ball, and its orbit as the white line; its direction of rotation is shown as the small white arrow. Again, you can see that the Moon's orbit, and its direction of rotation, are anti-clockwise as seen from above the north pole.
So -- bearing in mind the terribly distorted scale -- this is how our Earth and Moon relate to each other, and to the Sun; and it is the motions shown in this diagram that cause eclipses. But before we get into that, let's look at how this works in practice.
Day and Night
Given the motion of the Earth as described above, we can see how night and day work. Here's a picture that shows how the Earth is lit up by the Sun:
As you can see, at any given time, half the Earth is illuminated by the Sun -- that is the half of the Earth on which people can see the Sun, and this is the day side. The other half of the Earth receives no direct light from the Sun, and hence is in darkness; looking at it from our Earthly point of view, the Sun isn't visible because it's round behind the other side of the Earth. This is the night side.
Day and night aren't permanent, of course; we're used to the Sun rising, moving across the sky, then setting, then rising again. This happens because the Earth is rotating, as shown by the arrow in the diagram above, so that over the course of 24 hours -- which is how long it takes the Earth to do one complete rotation -- every part of the Earth sees a day and a night. It is the rotation of the Earth that makes the Sun appear to move across the sky, when it is actually sitting still.
Somebody standing on the Earth would be on the side facing the Sun at noon. The rotation of the Earth would carry them anti-clockwise towards the point marked sunset; from their point of view, the Sun would appear to be going the other way, and to be getting lower in the sky. When our observer reached the sunset point, the Sun would be exactly on the western horizon from their point of view; then they would move around to the night side.
At midnight they would be on the side of the Earth opposite the Sun; then they would move on towards the point marked sunrise, where they would see the Sun appear on the opposite (eastern) horizon. As the Earth continues rotating, they would see the Sun climb in the sky until they were once again directly in line with it at noon.
Phases of the Moon
The Moon is -- by far -- the brightest object in the night sky, far outshining all the stars and other planets. So what makes it glow? Well, in fact, it doesn't "glow" at all -- like all the other planets, the Moon is only visible because it reflects light from our Sun, the only large-scale source of light in our solar system. (The stars, of course, are Suns in their own right, and in their own solar systems). The Moon is very bright simply because it is pretty big, and very close to us compared to the other planets, and so it reflects a lot of sunlight our way.
Every month, though, something happens -- the Moon shrinks, first to a half moon, then a crescent, then vanishes entirely at the New Moon. Then, it starts growing again -- from crescent, to half, and then to Full Moon. These are called the phases of the Moon -- but what makes them happen?
It's a not uncommon misconception that the Moon goes through these phases because of the shadow of the Earth. This isn't true, though -- if you look at the diagram above, you will see that the Moon couldn't be in the Earth's shadow for more than a small portion of its orbit at most. In reality, if we were to look at a properly scaled diagram, we would see that the Earth is even smaller in proportion to the Moon's orbit, and so the Moon wouldn't be in the Earth's shadow for more than a few hours a month. Since the phases of the Moon last for a whole month, something else must be causing them.
The answer is that the Moon is only half-lit; and we're seeing different angles of the half-lit Moon as it orbits around the Earth. Just like the Earth, only the side of the Moon facing the Sun is lit by it; the rest is in darkness. When we look at the Moon, depending on the angle it is at, we may see the lit side, or the unlit side, or part of the lit side and part of the unlit side; and this is what causes phases.
The Moon goes through a complete cycle of phases -- from New to Full and back to New -- in one orbit around the Earth; this is a lunar month, which is 29.5 days on average. (Getting a lot more technical, this is actually a synodic month). This picture shows how the Moon is illuminated by the Sun at different times during a lunar month as the Moon orbits the Earth:
Starting at the New Moon, we can see that the Moon is on the same side of the Earth as the Sun. This means two things:
Waxing Crescent Moon
As the Moon moves anti-clockwise in its orbit, it moves away from the line between the Earth and Sun. As it does so, it is no longer lined up perfectly to turn its dark side towards us, and a bit of the light side becomes visible; this appears to us as a crescent Moon. Look at the diagram above, and you should be able to see how Earth's view of the crescent Moon is mostly dark, but with a sliver of light. Because the Moon is growing from the New Moon, this is called the waxing crescent ("waxing" is an old word for "growing").
You'll also notice that while the crescent Moon can be seen mostly by people on the daylight side of the Earth, it's also visible from the night side just after the sunset point. This means that the crescent Moon can be seen from mid-morning, when it rises, through to just after sunset, when it sets, if the sky is clear enough. With the Moon being close to the Sun, it might be drowned out by the Sun's light in daytime, particularly if it's a little hazy.
At night, though, the crescent Moon can be a beautiful sight; and if you take the time to have a look at it, you may see something even more beautiful. Because the dark side of the Moon is mostly facing the lit side of the Earth, it actually gets illuminated by reflected Earthlight. Although this is much dimmer than the Sunlight which is lighting up the crescent, you can quite often see the whole Moon cradled in the arms of the crescent Moon, lit up by Earthlight. When you see this, you are looking at light that has travelled from the Sun, then bounced of the Earth, then bounced again off the Moon, and travelled back to the Earth, to be seen by you. This is something to look out for around dusk in the days after the New Moon.
As the Moon continues anti-clockwise in its orbit, it moves around to the point marked First Quarter. Now, the term "quarter" here refers to the lunar month -- we are a quarter of the way in to the lunar month at this stage. However, looking at the diagram, you can see that people on the Earth looking "down" at the First Quarter Moon see part of the lit side, and part of the unlit side; in fact, what they see is a half Moon.
As you can see, the First Quarter Moon can be seen by some people on the daylight side of the Earth, as well as by some on the night side -- in fact, the First Quarter Moon can be seen from noon, when it rises, through to midnight, when it sets, if the sky is clear enough.
Waxing Gibbous Moon
At the next stage, we can see that the Moon is on the opposite side of the Earth from the Sun. This means two things: the dark side of the Earth -- which is to say, the part which is currently in night-time -- is the part which can see the Moon; and it sees the whole lit side of the Moon. This is a Full Moon, which rises at Sunset, and sets at Sunrise. We are now half way through a lunar month.
Waning Gibbous Moon
The second half of the lunar Month is the reverse of the first half. As the Moon moves past Full, it starts to shrink, becoming a gibbous Moon; this time, the waning gibbous moon, as it's shrinking ("waning"). This Moon is visible from early night-time to mid-morning, if the sky is clear enough to see it in daytime.
The Moon again reaches the half-Moon stage at Last Quarter (ie. the last quarter of the lunar month). At this stage, we again see half the Moon lit, and again the Moon is visible from both the day and night sides of the Earth; looking at the diagram above, you'll see that the Moon is visible from the midnight point, to Sunrise, through to noon.
Waning Crescent Moon
Finally, the crescent Moon shrinks to nothing at the next New Moon, and the cycle starts all over again.
So what about eclipses? And what about the shadows of the Earth and Moon that we mentioned earlier?
Well, as we'll see, these subjects are closely related. Now that you know about the basic mechanics of the Earth and Moon, we can go on to look at how eclipses work.
Copyright (C) 1995-2006 Ian Cameron Smith.
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