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Length of the tropical year, defined as the average interval between vernal equinoxes. This calendar year was the objective of the Gregorian calendar reform, which finalized the calendar as we use it today.
365 days, 5 hours, 49 minutes (365.2424 Universal days)Lengthening of the vernal equinox year over the last two millennia
About 10 seconds (0.0001 universal days per year)Variation of this length in the next few millennia
less than 5 secondsThe earliest known date
4236 B.C.E., the founding of the Egyptian calendarAncient Egyptian calendar year
365Date Emperor Huangdi invented the Chinese calendar
2637 B.C.E.Early Chinese year
354 days (lunar year) with days added at intervals to keep the Chinese lunar calendar aligned with the seasons--------
Slowing of the vernal equinox yearThe length of the year has increased slightly over the millennia for a variety of reasons. These include: the gradual slowing of the Earth's rotation, slow changes in the Earth's orbit due to other planets and the moon, as well as regular effects due to precession of the Earth's axis of rotation every 26,000 years.
Measures of the yearThere is a subtle but important difference in two primary measures of the year, used by our calendar and by astronomers. The year mentioned above is the length of the tropical year defined as the mean interval between vernal equinoxes (1582-2000 C.E.) : 365 days, 5 hours, 49 minutes (365.2424 Universal days). Another measure of the year often used is the astronomer's mean tropical year, defined as 365 days, 5 hours, 48 minutes, 45 seconds.
Atomic timeThe measurement of time is currently determined by an international consortium based in France which averages the time from approximately 220 atomic clocks in over two dozen countries. The atomic clock is the only object that both tells time and generates a precise time scale.
Historically, the calculation of time has been based on the position of the earth relative to the sun using noon, when the sun is highest in the sky, as a marker. The length of the second, which corresponds to the length of time required for 9,192,631,770 cycles of the Cesium atom at zero magnetic field, was determined near the end of the 19th century; this second is thus equivalent to the second defined by the fraction 1/31 556 925.97 47 of the year 1900. In 1967, the official second was set as equal to an average second of Earth's rotation time; the calculation of the average is necessary due to the fact that the earth rotates at a slightly irregular rate.
Today, time is determined by counting official seconds. This is subject to slight measurement inaccuracies; thus, the international community calculates a stable time by averaging accumulated seconds from several clocks worldwide. Next, this figure is compared to a few highly accurate laboratory measurements of the second. Every month, the official world time is adjusted by a few nanoseconds. Politically, time is a cooperative venture; and, by making time an international endeavor, the international community benefits from the combined resources of many laboratories.
Leap seconds in universal time coordinated (UTC)World time is typically adjusted every year by adding what is called a "leap second." Because the time calculated by the position of the sun differs from the time calculated by the atomic standard, it is occasionally necessary to adjust international time standards to match the position of the Earth.
The rotational speed of the Earth changes slightly for several reasons, some of which are not fully understood. Large scale movements of water and changes in the atmosphere affect the Earth's angular momentum. Tidal friction from the moon, which results in the rise of tides in the ocean, diminishes the speed of rotation. Physical processes occurring on or within the Earth also affect the earth's rotation.
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