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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 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 a High-Energy Astronomer
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