What is a day like on a planet?

Answer 1

It depends on the planet and the observer's position on the planet. Most observers on Earth would note the Sun rise in the east and set in the west; the reverse would be true on Venus, which orbits 'upside-down' (retrograde motion) with the sun rising in the west and setting in the east (and would seem to happen only about twice in the year). On Neptune, which orbits on its side, a polar observer would not the Sun seem to occupy a spiral path in the sky, spinning in ever-widening circles from a fixed point in the sky until it was so wide it made a 360-degree trip above the horizon, then descended below the horizon for half the year, reappearing later this time in decreasing circles until it was back at the same point all day. For Mercury, which has spin-orbit resonance (and an eccentric orbit) the sun would rise, get larger as it rose, and moved more and more slowly until at the zenith it seemed to begin going backwards the way it came (this being the point of Mercury's closest approach to the Sun where its spin is slightly faster than its orbit) after which the Sun would again double back and proceed the same direction until it set.