Expert: Steven LJ Russo - 12/27/2005

Question
Let me begin by saying how grateful I am for this service.

I recently watched the series "Origins" hosted by Neil De Grasse Tyson.  While watching models of solar systems and planetesimals hurling themselves through space, I began to wonder what keeps the ones that are fixed, fixed.  Why can I look up into the sky night after night and see the same stars in the same positions (seasonal changes not withstanding)?  I look at the same constellations that the ancient greeks gazed at?  If there is no gravity in space, what keeps any one object in a fixed position?  Does our solar system travel through space en masse, and if not, why not?  The sun keeps us in place, but what keeps the sun in place?

Thank you so much for your time.

Sincerely,

A.J. Bodnar


Answer
AJ:

Good program, but I may be "jaded", as Neil is a friend of mine!

First of all to clear up a very common misconception.  There IS Gravity in space.  Actually, there is NO SUCH THING as "No Gravity".  It is like the "glue" that holds the whole universe together.  We often hear the astronauts talk about "Zero G", but that is just an expression that really means "very little gravity".

Nothing is "fixed" in space.  The planets move around the Sun, the whole Solar System Moves around the Galaxy, and the whole galaxy moves through the Universe.  Actually,a all of this "movement" amounts to you moving about a million miles per hour through space!

The stars "appear" fixed because they are too far away to see their movement.  Yes, the Big Dipper looks the same now as it did to the "ancients", because a few thousand years is a drop in the bucket in astronomical time.  However, if you could live for about 60,000 years, none of the constellations would look the same, as the stars are all moving in different directions at different speeds in relation to each other.

Steve