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The Question

(Submitted February 25, 1998)

If the moon goes around the earth from left to right, and the earth rotates from left to right around the Sun, does the the Sun go around in the galaxy from left to right, too? Is this a pattern? Is our galaxy going around something?

Oh and by the way I'm in 6th grade and my whole science class (we rotate for science) has been debating about for a week.

The Answer

What an excellent question! The direction of rotation is very important in astronomy because it gives us some clue about the how things were formed in the first place. One thing to remember is that things can rotate in any direction. Think about throwing a ball with topspin, backspin, and sidespin. A good tennis player can hit a ball that is spinning in any direction (up, down, left, right, kind-of-up-and-right, etc ...). (Show this to your class with a real ball.)

We often think of rotation using the "right hand rule". Take your right hand so your thumb on the axis of rotation and your figures point in the direction of the rotation.

Example: A counter-clockwise spinning top will rotate "up" according to the "right hand rule". A clockwise spinning top rotates down.

Exercise: Find a Globe and have everybody in your class put their hands in the right way for each of the different rotation compared to the globe.

So lets go over it:

1. The Earth rotates on its axis from West to East with its axis in the North/South direction (by definition of North and South). So it is rotating "due North" because of the right hand rule.

2. The Earth revolves around the Sun about 23 degrees from "due north". (This is why we have seasons!)

3. The Moon revolves around the Earth about 5 degrees from the direction the Earth revolves around the Sun. The Moon's rotation is about 1.5 degrees from the direction the Earth revolves around the Sun.

4. All the planets revolve in the same general direction, with Pluto's orbit being the most inclined (17 degrees). Their axes of rotation are more diverse, Uranus and Pluto rotate 'on their sides' and Venus's axis points towards the South.

5. Our Galaxy, on the other hand, is completely different. The Sun revolves around the galaxy in a totally different direction. Using that "right hand rule" you need to point your thumb toward the "South Galactic Pole." This is located above the southern hemisphere, at 27 degrees south latitude. So the rotation axis of the Galaxy is tilted by 117 degrees from the rotation axis of the Earth.

You can see this at night, by noting that the Milky Way (the disk of the galaxy) is always across the sky in some funny direction. Never due East-West.

6. Our Galaxy, the Milky Way, is also moving in some funny direction (completely unrelated to any of the other directions) as it orbits the other galaxies nearby.

I hope this helps. And please, take out the globe and have each person in your class point his/her thumb in the right direction for each example (1-5) above.

Also, you can take out a protractor and make a drawing of each of the 3 main axes. Remember, from the Earth's axis: 23 degrees and 117 degrees.

Good luck!

Jonathan Keohane
for Ask an Astrophysicist

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