Our notions of the motions in the universe have been built up over several thousands years of observations; in other words, it has taken humans thousands of years to unravel the motions. This is made difficult because we are part of the system itself. However, we do now know with certainty a number of things:

1.The universe is filled will about 100 billion galaxies. Each galaxy is a collection of about 100 billion individual stars of which SUN is one> This means that there are about 1,000,000,000,000,000,000,000,000 stars in the universe.

2. All galaxies are rushing AWAY FROM EACH OTHER. The analogy here is as follows: take a ballon, blow it up part way and use a magic marker to put some dots(galaxies) on the surface. Now continue to blow up the balloon. You will note that NO MATTER WHAT DOT YOU ARE ON, ALL THE DOTS SEEM TO MOVE AWAY FROM EACH OTHER. This is what is meant by the BIG BANG...since that time 15 billion years ago, all matter in the universe is moving apart. This is the style of motion at the largest scale. However, at smaller scales there are other motions.

One of these is the SUN's. The sun is moving around the center of our galaxy, the MILKY WAY, once every 250 million years. The planets tag along with the sun.

Because the gravity of the sun is so large, the planets revolve about it much as the moon revolves around the earth. But all these motions are occuring at the same time...so really the moon like the earth is going around the sun at the same time the earth moves about the sun, at the same time the sun moves around the center of the MILKY WAY, at the same time the milky way galaxy moves apart from other galaxies, as part of the expansion due to the big bang.

(To find out more about all this, check out the popular magazine called ASTRONOMY; it is for high school students but smart 6th graders can benefit)

One can always describe a motion in a large number of ways...however, when considering the sun earth system, one logical way is to define the center of gravity of the system and to desribe the motion of both the sun and earth as moving around the common center of gravity. Now since the Sun is so much more massive than the earth, the common center of gravity is PRACTICALLY coincident with the sun itself. So, the best way to describe the motion is to imagine the earth moving about the center of the sun.

How do we know that the Earth revolves around the Sun? Before Copernicus, Westerners believed that the Earth was at the center of the universe and that the Sun revolved around the Earth, as did the stars and the planets. The Sun, however, does not merely rise in the east and set in the west. You can see for yourself that the Sun only rises directly in the east on the equinoxes: at all other times of the year, it rises in the northeast (summer) or southeast (winter). Also, the Sun moves with respect to the stars: the Sun wanders through the 12 constellations of the zodiac, coming back to its starting point after 1 year.

The complicated motion of the Sun forced the ancients to develop a very complicated model of the Sun orbiting the Earth. Eventually people realized that all the motions of the Sun could be explained very simply with

A) the Earth rotating on a tilted axis once every 24 hours and,

B) the Earth orbiting the Sun once per year.

You are right that you are free to choose the "center" of your universe wherever you like. However, the explanation you will have to come with for the motions of the stars and planets may become so horribly complicated that you may wish you had chosen a more natural place to be your center. So basically, we say the Earth goes around the Sun because it's much, much easier than the other way around.

Questions: If you wanted to say that the Sun orbited the Earth once every 24 hours, what other motions would you have to add to make it set in the northeast in the summer and the southeast in the winter? At the North Pole, the sun NEVER sets during the summer -- it just circles around the sky, fairly close to the horizon. How would this be explained by the Sun circling the Earth?

You are right that the "center" of anything can be relative. We commonly think of the sun as the center of our solar system, but our solar system moves relative to the center of our galaxy, and our galaxy moves relative to other galaxies, etc.

The important underlying point has to do with gravity. The force of gravity causes any two objects with mass to attract EACH OTHER. This means that the sun is also moved a little bit by the earth, but the sun has much greater mass, so it seems that the sun is fixed while the earth is revolving around it. If the earth and sun had equal masses then they would revolve around each other with the center of revolution half way in between.

Think about swinging a ball on a long string around your head. If the ball is very light (compared to you), then the ball will not tug you out of place at all. But if you swing a bowling ball, you will have to lean back away from the ball to keep your balance, because the ball is pulling very hard on you.

The best answer I can think of about the sun revolving about the Earth dilema is that

1) If you think of the center of mass of the solar system, the Sun is very close to the center of mass, while the Earth orbits at a much greater distance. So if you were an alien viewing our solar system from some other star, it would look to you like the Earth is rotating about the sun. The Sun orbits about the center of mass of the solar system too, but if I remember right, the center of mass of the solar system lies within the sun, so the sun doesn't have a very big orbit: it just wobbles a bit with the motion of the other planets. One of the reasons it took me so long to reply is I was waiting for a response from a physics and astronomy instructor about that last point to confirm it, but I haven't heard back.

2) It is absolutely true that in an Earth-centered reference frame the sun rotates about the Earth, but, the other planets do not have orbits around the Earth. From a sun-centered reference frame all planets revolve around the sun, so that's another reason the sun-centered reference frame is more convenient. If you track a planet's motion across the night sky over the course of months you will see it retrogress, or go backwards from its normal direction for a few days or so at a time. When astronomers (like Galileo? I wish I remembered) first noticed this they knew it was evidence that the planets do not revolve around the Earth. You could look for planet retrogression as an assignment for your class by asking your students to track some planets across the night sky with the rest of the stars as a reference frame. I did that once when I was a student in Minnesota. It's amazing what you can see with just the naked eye and a pencil and paper!

I bet COSMOS by Carl Sagan would be a reference that would have information about retrogression.

Specifically, the earth and sun revolve around a mutual center of mass (which is very close to the center of the sun). No, the sun is not fixed, but is in its own orbit around the center of the galaxy (the milky way) -- which is also in motion. How we know that the earth orbits the sun and not the reverse is that relativity assures us that physical laws are similar in any moving frame -- however, objects in a circular orbit are actually in an accelerating frame. It is thus possible to make a local measurement of that acceleration. For example, if you took a pail of water and spun it, the surface would assume a parabolic shape. Similarly, if you were to stand at the north pole with a large pail, the surface would deform a small but measurable amount due to the earth's rotation. It is in fact possible to measure this rotation and indeed the rotation of the earth about the sun directly, with careful measurements.

Recently, the measurements of the cosmological black body radiation have been shown to indicate a slight red shift indicating a net motion relative to the observable universe as we understand it. So, with a universe so full of stuff, even constant velocity is not completely relative!