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Q.What is the minimum distance form earth from where one can see that earh is round in shape and moving with a high velocity.
- Vijay (age 48)
People noticed ships gradually vanishing beneath the horizon a long time ago. They interpreted that to mean that the Earth is round. So you could say that the minimum height to observe the roundness is about zero. Of course, the higher you go the more you get a complete picture. At a height comparable to the Earth's radius (about 6000 kilometers) you can see a pretty obvious ball-like picture.
As for the 'velocity' of the Earth, there is no particular correct value. That's the key idea of relativity, as discovered by Galileo and refined by Einstein. Each observer will assign a value dependent on his own state of motion.
(published on 07/27/09)
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