I like to back up dog's statements. Nothing is 100% certain, even my belief that the earth is round (although it is veeeeeery close to 100%).
I also would like to point out the simplicity of the 'theory of gravity' Just take the following equation:
It states the attractive, gravitational force between two masses 'M' and 'm' given a distance 'd' between them. With this simple equation, you can calculate:
- the motion of the planets in our solar system around the sun
- the motion of the planet's moos around the planets
- the motion of asteroids
- the motion of comets
- the motion of satellites around the earth
- the motion of probes travelling through our solar system
- solar eclipses
- lunar eclipses
I even did these calculations myself on a computer. They worked! So try it yourself!
However, this equation does not hold when including other forces (as this equation only describes the
gravitational force). Low-earth satellites are decelerated by the earth's atmosphere and smaller objects in the solar system experience a small force from the particle wind coming from the sun. And for much greater distances, one has to consider cosmological effects.
But for all the 'basic' things we observe, like the moon, the sun, sunset, sunrise, eclipses, position of planets etc., the
only equation you need is the one posted on top of this thread. No need for strange, complicated and unexplainable movements of the stars and planets over the 'flat' earth. Just this simple and elegant equation to explain almost everything you see on the sky!