Search results
Results From The WOW.Com Content Network
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 ...
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 unit of TT is the SI second, the definition of which is based currently on the caesium atomic clock, [3] but TT is not itself defined by atomic clocks. It is a theoretical ideal, and real clocks can only approximate it. TT is distinct from the time scale often used as a basis for civil purposes, Coordinated Universal Time (UTC).
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 ERA may be converted to other units; for example, the Astronomical Almanac for the Year 2017 tabulated it in degrees, minutes, and seconds. [12] As an example, the Astronomical Almanac for the Year 2017 gave the ERA at 0 h 1 January 2017 UT1 as 100° 37′ 12.4365″. [13]
A Gregorian year, which takes into account the 100 vs. 400 leap year exception rule of the Gregorian calendar, is 365.2425 days (the average length of a year over a 400–year cycle), resulting in 0.1 years being a period of 36.52425 days (3 155 695.2 seconds; 36 days, 12 hours, 34 minutes, 55.2 seconds).
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).
As of the beginning of 2011, the difference between TDB and TCB is about 16.6 seconds. TDB = TCB − L B ×(JD TCB − T 0 )×86400 + TDB 0 where L B = 1.550519768 × 10 −8 , TDB 0 = −6.55 × 10 −5 s, T 0 = 2443144.5003725, and JD TCB is the TCB Julian date (that is, a quantity which was equal to T 0 on 1977 January 1 00:00:00 TAI at the ...