I have been interested in astronomy since I was two years old (I knew the names of all the moons of Uranus, the fact that there were only 15 back then was irrelevant :P) but even now at nearly 16 something confuses me: Planets orbit the Sun (or stars orbiting a galaxy, or moons orbiting a planet, whatever) because the force of gravity pulls on them - the closer they are to the Sun, the faster they have to move because they feel a stronger pull of gravity. However gravity depends on both distance and mass, so if Jupiter was as close to the Sun as Mercury is (assuming it could hold on to its mass being so close to the Sun, which it obviously couldn't as it's liquid and gas) wouldn't it travel faster? However doesn't Kepler's third law state that the velocity of a planet in orbit is proportional to its distance from the sun? (One is squared, one is cubed, I forget which.) This makes no sense to me; so Mercury and Jupiter at the same distance would travel at the same speed and have the same period? What about mass? Also, I always thought Dark Matter was needed to explain the rotation curves of galaxies because the extra mass around the galaxy created a stronger gravitational pull and caused the matter round the edges to move faster than it should if all there was was visible matter? And if that's not true, then can someone explain how Dark Matter creates galaxy rotation curves? Thanks.