Ask A Scientist General Science Archive

Author:     bragee
Text:       Why is the earth round?

Response #:  1 of 1
Author:      Robert Topper
Text:        An interesting question.  Now, the first question that comes to 
my suspicious mind is "IS the earth round?"  And the answer is no, at least 
not perfectly round.  But it is round compared to your average cube, that is 
for sure!  Now, my understanding of how solar systems form is pretty 
inadequate; all I can tell you is what I see on "Nova."  But basically, I 
guess it has to do with gravity, which is a CENTRAL FORCE.  What that means is 
that gravity pulls things together along a straight line, and the more massive 
one of the things is, the stronger it pulls.  When the earth was forming, it 
was basically a big ball of dust left over from an exploded sun (a supernova).  
Gravity caused a lot of the matter to condense into a big fiery ball (our 
sun), but some of the dust was far enough away from the Sun so that the 
stronger forces were between planetary dust particles and not the sun.  SO, 
they swirled together into a ball.  You can see something similar when you 
make a painting with one of those "spin-art" machines; you put a card on the 
spinner, which whirls around real fast, and then you squirt paint on the card.  
The whirling causes the paint to go out away from the center (this is like the 
explosion of the supernova) and droplets go out along an interesting path 
until they stop, which happens when the spinning force is equal and opposite 
to the force which attaches paint to the card (this is like gravity).  Now, if 
the force attaching paint to the card were not so strong,eventually the drops 
would get bigger - "planets."