1. 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.
   2. a unit of time generally equal to exactly 24 hours, or 1440 minutes, or 86 400 seconds. This unit is meant to be equivalent (for all practical purposes) with the mean solar day. But there is a small problem with making this equivalence. The second has its own scientific definition based on the frequency of a certain radiation from cesium atoms and having nothing to do with the Earth's rotation. The reason for making this scientific definition of the second is that the length of the mean solar day is not constant from one year to the next. The gravitational attraction of the Moon is very gradually slowing the Earth's rotation, so that each day is a tiny bit longer, about 40 nanoseconds on the average, than the previous day. This lengthening adds one second to the length of the day about every 60 000 years. During the first decades of the 21st century the actual length of the mean solar day will be about 86 400.002 seconds. For this reason, time as kept by our best clocks runs a bit faster than time as kept by the Earth. To keep clock time and sun time in step (within 0.9 seconds), every so often a leap second must be added at the end of a day. This is done at midnight Universal Time either on June 30 or on December 31. Because the rate of slowing is erratic, the need for these leap seconds cannot be predicted more than a few months in advance. (Link: explanation of leap seconds from the U.S. Naval Observatory.)
   3. a civil unit of time, the period during which all times have officially the same date (see above). Like day2, this unit is generally equal to exactly 24 hours, or 1440 minutes, or 86 400 seconds, with a rare extension by addition of a leap second. However, changes in time reckoning also affect the length of the civil day. A day on which "summer" or "daylight" time is adopted has only 23 hours, and a day on which the reckoning reverts to "winter" or "standard" time has 25 hours. Similar adjustments may occur if a locality switches from one time zone to a different zone.

Dictionary of units of measurement. 2015.

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