The Leap Second
The little known leap second is not directly connected with the leap year. Leap years arise from the rotation of the Earth around the sun. Leap seconds involve the rotation of the Earth around its own axis.Our traditional, common-sense view of time has always been based around the solar day, that is the time between one noon and the next. The solar day varies in length during the year but averages 24 hours. For practical purposes we use this "mean solar day" (UT1) of a constant 24 hours. The second is then defined as the length of a day divided by 86400 (that number being 24 x 60 x 60).
Modern atomic clocks can measure time extremely accurately. This atomic time measurement (TA1) is the basis of the international time standard known as Coordinated Universal Time (UTC).
Unfortunately mother nature isn't as rigid as an atomic clock! The rotation of the Earth - and hence the length of the mean solar day - changes. Over the last few centuries the day has been slowly getting longer. The changes are tiny, too small for mere humans to notice, however they result in a divergence between UTC and Solar Time than can be awkward for astronomers and others who rely on accurate timing.
For this reason the leap second was introduced. Whenever UTC and the solar day are diverging by more than 0.9 seconds, a leap second is either added to or subtracted from UTC in order to bring them in line. When leap seconds are necessary, they are applied at the end of June or December.
In 2005 the US proposed scrapping the leap second. Such a move would make life easier for manufacturers of GPS systems and similar equipment but would cause chaos for astronomers.