Observing and understanding motions in the sky is what led people to understand the layout of the Solar System, and to understand how the Earth moves in space. Objects can appear to move in the sky either because they are actually moving, or because they reflect our motion through space For stars, almost all apparent motion comes from reflex motion, because they are so far away that their intrinsic motion appears very small. For planets, apparent motion in the sky comes from a combination of their intrinsic motion (around the Sun) and their reflex motion. The rotation of the Earth on its axis causes all objects to appear to move around the sky once each day. The apparent motion of a star to an observer which arises from the Earth's rotation depends on the location of the observer on Earth, and the location of the star relative to Earth's rotation axis. A diagram shows the idea Stars appear to travel in circles around the celestial sphere. Objects near the poles travel in small circles; far from the poles, they travel in large circles. The orientation of the circles as seen by observers depends on their latitude on Earth. Depending on your location, some stars are visible all of the time, others rise and set, and some are never visible. (Note that when I say visible all of the time, I mean they could be seen in the sky all the time if sunlight didn't overwhelm them during daylight!). Example: view from the North Pole Example: view from the equator Example: view from northern hemisphere, e.g. Las Cruces Example: a time lapse movie Day and night occur because of the same rotation of the Earth, which makes the Sun appear to move around the sky once each day. The motion of the stars and the Sun can be used to tell time. We define a day to be the length of time it takes for the Sun to come back to the same position in the sky. It takes about 24 hours for the stars to return back to the same position in the sky. Consequently, one can measure time by the fractional distance a star has gone in its full circle around the sky. This is best done for stars which never set, i.e., stars near the north celestial pole. It is made easier because there happens to be a star located almost at the location of the pole, called the North Star. This movie shows the idea.
