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.)