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Julian within its name indicates that other Julian epochs can be a number of Julian years of 365.25 days each before or after J2000.0. For example, the future epoch J2100.0 will be exactly 36,525 days (one Julian century) from J2000.0 at 12:00 TT on January 1, 2100 (the dates will still agree because the Gregorian century 2000–2100 will have ...
TT differs from Geocentric Coordinate Time (TCG) by a constant rate. Formally it is defined by the equation = +, where TT and TCG are linear counts of SI seconds in Terrestrial Time and Geocentric Coordinate Time respectively, is the constant difference in the rates of the two time scales, and is a constant to resolve the epochs (see below).
The unit was redefined in terms of the tropical year at 1900.0 instead of the sidereal year; [12] and the standard second was defined first as 1/31556925.975 of the tropical year at 1900.0, [12] [14] and then as the slightly modified fraction 1/31556925.9747 instead, [15] finally being redefined in 1967/8 in terms of the cesium atomic clock ...
The window of totality is just 3 minutes and 38 seconds, with variance for location, and begins at 3:20 p.m. The moon will pass completely away from the view of the sun by 4:33 p.m.
Clock time and calendar time have duodecimal or sexagesimal orders of magnitude rather than decimal, e.g., a year is 12 months, and a minute is 60 seconds. The smallest meaningful increment of time is the Planck time ―the time light takes to traverse the Planck distance , many decimal orders of magnitude smaller than a second.
The March equinox itself precesses slowly westward relative to the fixed stars, completing one revolution in about 25,800 years, so the misnamed "sidereal" day ("sidereal" is derived from the Latin sidus meaning "star") is 0.0084 seconds shorter than the stellar day, Earth's actual period of rotation relative to the fixed stars. [3]
Barycentric Dynamical Time (TDB, from the French Temps Dynamique Barycentrique) is a relativistic coordinate time scale, intended for astronomical use as a time standard to take account of time dilation [1] when calculating orbits and astronomical ephemerides of planets, asteroids, comets and interplanetary spacecraft in the Solar System.
Years affected are those which divide by 100 without remainder but do not divide by 400 without remainder (e.g., 1900 and 2100 but not 2000). No guidance is provided about conversion of dates before March 5, -500, or after February 29, 2100 (both being Julian dates).