Ask A Scientist Astronomy Archive

name         Sean M.
status       other
age          15

Question -   My question is involving the Earth's orbit of the Sun, but it is 
difficult to explain without using a drawing of some sort to describe it. 
Generally, when people picture the Earth orbiting the Sun, they picture the 
Sun as a stationary body in the center and the Earth rotating around at a 
constant speed. But as we know, the Sun is not stationary, as it is also 
moving throughout the Milky Way. Picture the Sun drawn towards the top of 
a piece of paper, and the Earth drawn just above it. Now, draw a dotted line 
in a circular path to the left side of the paper, and draw the sun again. Have 
this distance represent six months of motion for the Sun. Since it takes the 
Earth twelve months to make one full orbit around the sun, then six months 
would be one-half of an orbit. Since the Earth started as being drawn on top 
of the Sun at the beginning of this picture, it should end up being drawn below 
the Sun after the six months of motion. However, this would force the Earth to 
travel a much greater distance. And, for the Earth to make it back above the 
Sun in our drawing after another six months, the Sun would have the greater 
distance to travel. Can the Earth accelerate and decelerate throughout space? 
If so, it would have to give off some kind of energy, according to the Law of 
Conservation of Energy. It can not do work without expelling energy.
-------------------------------------------------------------------------
Imagine that you are in an airplane in constant level flight.  There is no 
experiment that you can do inside the airplane that would tell you if you were 
in motion or at rest.  You behave as if all is at rest.  The flight attendant does 
not call up to the pilot to ask her how fast she is flying the plane in order to 
figure out how to pour the coffee.

DO NOT DO THIS:
Take a yo-yo out and twirl it about your head at a constant rate.  No problem.  
But the plane is moving!  In your frame of reference, it is uniform circular 
motion.  Someone on the ground would not see uniform circular motion, but rather 
some sort of cycloid or epicyclical motion.

So is the case with us orbiting the sun as the sun orbits the galactic center.

---Nathan A. Unterman
===============================================================