The earth is round because it's obvious -
1) No other kind of
stable bodies appear in the space but spheres. Flat earth would be contradicting all the evidence we have been observing.
2) There's absolutely not a single reason why earth should be flat instead of it being a sphere.
3) There's not a single evidence about earth being flat. You can't observe it from the ground, because earth is too big.
4) People have been noticing that earth is round long before governments, Nasa or similar associations existed. Thus the motive of the conspiracy is totally unknown.
+ The people, who noticed it first were scholars, philosophers, astronomers, travelers, sailors and other scientists. The round earth was discovered in different parts of the world by different cultures.
The motive of bringing up such 'lie' in the first place is unlikely to exist.
5) All the known science expects the world to be round and succeeds in its experiments, bringing us great technology and scientific achievements.
6) There are excursions to the south pole very often. Why don't you visit it yourself to make sure?
http://wikitravel.org/en/South_Pole7) Gravitational anomalies on the earth correlate with the round earth model. (Higher gravity at the poles and lower gravity at the equator and high mountains)
8 ) Earth's magnetic field expects rotating iron nucleus.
9) The lack of understanding gravity does not disprove the round earth, like lack of understanding how life began doesn't disprove life. It's just one more thing we haven't figured out yet.
10) Lots of people have been to international space station and other stations. It's unlikely that the conspiracy could hold together. They have families and relatives.
11) The sun and moon are not discs 3000miles away. We have technology to measure this. The perspective and viewing angle would distort the size and shape too much if this was the case anyway.
People from Brazil and people from US would see the approaching sun from different angle and with different size. However the sun appears approximately the same size from every part of the world.
The setting sun would appear to minimize instead of going behind the horizon. Moon couldn't be a disc because it's shadow is always on the opposite side to the sun. A simple telescope can show you the moon's
actual shape.