Astronomy or Planetary Science
Venus is the closest planet to Earth, but it does not have oceans or human life like Earth. Depending on their relative positions in their orbits, Mercury, Earth, and Mars each swap that title between them. For example, when on opposite sides of their orbits, Venus and Earth can be more than 150 million miles apart, while Mars and Mercury can be as close as 74 and 31 million miles from Venus, respectively. It's a result of the circular nature of their orbits. The closest planet to Venus either earth or mercury depending where the planets are in their orbits.
No, it has changed over time. The original atmosphere of the Earth was mostly carbon dioxide with some water vapour, ammonia and methane, and was probably formed by volcanic outgassing. Living things introduced oxygen, and were one of the causes of reduced carbon dioxide. The ammonia then reacted with the oxygen.
A lot of this is just due to rounding. Most sunrise/sunset tables are in whole minutes, so the changes are pretty choppy. But there is an astronomical reason. If the earth's orbit were a perfect circle, we would see the same change in sunrise and sunset every day. For example, on a given day in the fall, sunrise would be 90 seconds later than the day before, and sunset would be 90 seconds earlier. But it's not a circle, it's slightly elliptical, so the speed we move around the sun changes throughout the year. This causes solar time to be a little off from clock time, and by a different amount each day. This is reflected in the analemma, the figure-8 thingy often found on globes. Also try searching on that word, and you will find a bunch of interesting stuff.
I assume you mean Barringer Crater in Arizona. 50.000 years.
well I'm assuming they meant the crater that killed the dinosaurs. about 65 millions years ago. :P