A traditional unit of time marked by one rotation of the Earth on its axis. Different cultures have had different ideas about when to mark the start of the day, whether it should be from one sunset to the next, one sunrise to the next, etc. (Astronomers sometimes consider the day to run from one noon to the next, and today we usually mark the day from one midnight to the next.) This unit is called the solar day, since it is defined by observations of the sun. However, the solar day varies in length, because the speed of the Earth in its orbit varies, with the Earth moving faster when it is closer to the Sun. The result is that solar days are longer during the Northern Hemisphere winter and shorter during the Northern Hemisphere summer, because, despite what Northern Hemisphere residents might think, the Earth is actually closer to the Sun during the Northern Hemisphere winter. Because of this irregularity in the length of actual days, what our clocks attempt to measure is the average period of time between two successive noons, that is, between two successive appearances of the Sun on the meridian (the imaginary line joining due north to due south across the sky). Astronomers call this period of time the mean solar day. By long tradition, solar and mean solar days were divided into 24 hours, or 1440 minutes, or 86 400 seconds