|
|
|
|
|
|
|
Since the Earth moves about one degree in its orbit around the Sun in the course of one day, the Sun appears to be progressively displaced against the "fixed" stars. Hence, the sidereal (star) day of about 23 hours and 56 minutes is shorter than the solar day, which includes this extra relative motion. |
|
|
|
|
|
|
|
|
Since the Earth's orbit around the Sun is elliptical, not circular, the Earth moves at a different relative speed at different parts of its orbit. Hence, the solar day does not always have the same length. For this reason, the mean solar day of 24 hours is used. |
|
|
|
|
|
|
|
|
Universal time (UTC) and therefore Local Civil Time (LCT) are related to solar motion. For the purposes of civil time keeping, a constant day of exactly 24 hours is used. |
|
|
|
|
|
|
|
|
The invention of highly accurate atomic clocks had, as one consequence, the discovery that the rotation of the Earth itself is not strictly uniform. Rather, it shows small erratic fluctuations. The mechanism behind this variation is not well understood. |
|
|
|
|
|
|
|
|
Astronomers need a uniform measure of time. Before 1984, this was Ephemeris Time (ET). After 1984, atomic clocks were used to provide Terrestrial Dynamic Time (TDT). |
|
|
|
|
|
|
|
|
A measure of the variation in Earth's rotation is provided by the concept of "delta T," which is the difference between TDT and Universal Time (UT). UT is tied directly to the Earth's rotational period, whereas TDT is related to international atomic time (TAI). |
|
|
|
|
|
|
|
|
One practical effect of this variability is that accurate prediction of the timing of astronomical events in terms of UT or Greenwich Sidereal Time (GST) is subject to unpredictable error for future events. The value of delta T is about one minute for the year A.D. 2000, but its value for the year A.D. 2500, for example, is not known. |
|
|
|
|
|
|
|
|
Hence the accurate prediction of events such as the new moon in terms of a civil calendar is not really possible. In turn, calendars that depend upon actual astronomical events for their inception dates (like the Chinese) are impossible to construct accurately for years in the distant future. |
|
|
|
|
|
|
|
|
There are, likewise, several kinds of year: |
|
|
|
|
|
|
|
|
Anomalistic The interval of time between two successive passes through perigee. The value is 365.259 Mean Solar Days (MSDs). |
|
|
|
|
|
|
|
|
Besselian The period of one complete revolution in right ascension of the mean Sun, as defined by Newcomb. This definition has not been used since 1984. |
|
|
|
|
|
|
|
|
Civil The length of the year in whole days as defined and used by a calendar. |
|
|
|
|
|