Re: How can I prove that the earth is round?

People long ago thought the Earth was flat and didn't move.  This idea held 
on for centuries and a few people today still think it is true.  But, the 
idea does make some intuitive sense.  When I step outside, I don't feel the 
Earth moving under me.  When I'm in a car, I do feel it moving.  Also, the 
Earth looks flat to me.  I don't see a big curve or drop off at the edge of 
the horizon.  But, these mistakes were created because people weren't paying 
close attention.

In an old book that I have called Lessons in Astronomy by Charles A. Young 
from 1896, he listed some ways we can show that the Earth is round.  First, 
It can be sailed around.  It can also be flown around, but this book was 
written before we had airplanes.  Second, the appearance of vessels coming 
in from the sea indicates that the surface is everywhere convex.  What this 
means is that if you stood on a shoreline and watched ove the sea, you would 
see a ship rising over the horizon.  First you would see the mast, then the 
bow, then the whole ship.  It would appear that it was coming over a hill.  
But, if you asked the crew of the ship, they would have thought they were 
always traveling over a flat surface.

Also, not everyone long ago thought the Earth was flat.  An Arab man (whose 
name I can't remember) learned that at noon on a particular day in a village 
just south of him one could see the sun shining at the bottom of a well.  
But, in his village you couldn't.  He thought that the only way this could 
be true is if the Earth was round and was shining directly over the village 
just to the south of his.  To prove his case, he hired a man to walk from 
his village to the one in the south and measured his steps.  With that 
information, he not only showed that the Earth is round, but also calculated 
the circumference of the Earth and got it right!  All of this long before we 
had computers, satellites, and lasers.