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Earth, Moon, and Sun? Which orbits around which?


It's yet another of those things that is assumed obvious and everyone knows, but really, a great many people don't know whether the moon goes around the earth or the other way around, and they don't know whether the earth is in orbit around the sun. The situation has been far from clear historically, but now it's a well known set of facts which scientists understand. Other folk, well some of them just guess. So anyway, here are the facts:

The moon goes around the earth.

The earth (with the moon going around it) goes around the sun.

In cosmological terms, the earth and moon are close neighbours, and that's easy to spot by the fact that there were some magnificent moon landings by Nasa astronauts in the late 1960s and early 1970s. It is much easier to land on the moon than on various other celestial objects, and one of the reasons for this is the relative closeness of the moon, at 239,000 miles. There are cars on the roads that have done more miles than that!

The earth rotates on its own axis once per day, or thereabouts. With the sun in the distance, if you're at a particular place on the earth, half of the time the sun isn't in the sky, and that's when it's night!

Let's draw a map of the moon-earth-sun system to scale. This is an imaginary map, drawn to scale, not an OS Map! Imagine you're on a golf course which is the right sort of scale for drawing maps on. In this scale map, the earth is drawn the size of a golf ball, like those little globes you can get in bargain stores. Put your globe on the teeing-off point. Now the moon, at this scale the size of a marble, goes on the fairway about 4 feet away from the globe of the earth.

The moon goes around the earth once a moonth, and it is in orbit, which is just as well as it stays up there. How high the moon? A quarter of a million miles. Or, 4ft on this scale map.

So where's the sun on this map, and why is this crazy thought-experiment on a golf course? At this scale, with the earth the size of a golfball, the sun is on the green, about five hundred yards down-range of the earth-moon system.

On this scale the sun is about 13ft across, like a zorb-ball. Notice how much bigger than the golf ball earth this is.

This may all seem odd, because from an earthly perspective when you look at the sky, the sun and moon look about "the same size". In truth the sun is 400 times bigger, but is 400 times further away.

This similarity in apparent size of the moon and sun is what makes total solar eclipses so good, and so worthy of the tourist business. However, catch it while you can! The moon is receding further away from the earth, and some time in the future it will not be possible to see total eclipses without going on a flight into space!

The earth goes around the sun once per year. It keeps on going, round and round, as it is in orbit. You can see how the size relationship works. The earth spins round once per day, but that's just on its own axis, which at this scale is quite small. The moon, in the map only the size of a marble and 4ft from the golf-ball sized earth, orbits the earth once a month, and the earth goes around the sun (over there at the 18th hole) once per year.


Useful? Helpful? [response]. There are loads of other interesting pages like this. Well worth having a browse through the index to see what you can find!


More mysteries resolved at pages connected from... The Earth (such as why is the Earth round)