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Many believe there are 86,400 seconds in a day. While that is mostly true, it is not totally true. Instead, there are approximately 86,400.002 seconds in each solar day. While that may not seem like a significant difference, over a year's time, that almost equals one full second. To come up with a solution to this discrepancy in time, the leap second was invented.
A leap second, much like a leap year, makes a correction in time. Due to the fact there is no calendar or clock that synchronizes exactly to the Earth's activities, these corrections are needed from time to time. A leap second is inserted into the clock nearly once a year.
The leap second is, in reality, much more common than a leap year, but not as many people take notice of it. The reason for this is because it is a minuscule length of time in most people's minds. However, international standards say it is important to keep coordinated universal time, which is the time we all live by, in sync with solar time.
Every time a leap second has been implemented, it has either taken place on the last day of June or the last day of December. So for some, they may be starting their New Year's Eve celebrations a second or so early. Since its implementation, the longest the world has gone without the insertion of a leap second is 7 years, from 1998 to 2005.
There was one period of time where two leap seconds took place six months apart. That was between the first leap second on 31 December 1971, and the second one of 30 June 1972. Since that time, every leap second has been at least one year apart. One year between leap seconds is the most common interval, followed by 1.5 years, and then two years. There was also one time when the leap seconds were three years apart.
While some believe that a leap second is necessary because the Earth is slowing down in its revolutions, that is only partially true. While tides do have a slowing effect on the Earth, even without tides there would still be a small discrepancy. Still, some claim that the solar day has only been slower than the clock day since 1820. Conversely, the solar day was shorter than the clock day before that year. Even so, it has only been since 1971 that a concerted effort has been made to clear up the discrepancy.