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In most cases, the base unit is seconds or years. Prefixes are not usually used with a base unit of years. Therefore, it is said "a million years" instead of "a megayear". 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.
10 times the length of the previous cosmological decade, with CD 1 beginning either 10 seconds or 10 years after the Big Bang, depending on the definition. eon: 10 9 yr: Also refers to an indefinite period of time, otherwise is 1 000 000 000 years. kalpa: 4.32 × 10 9 yr: Used in Hindu mythology. About 4 320 000 000 years. exasecond: 10 18 s ...
1.67 minutes (or 1 minute 40 seconds) 10 3: kilosecond: 1 000: 16.7 minutes (or 16 minutes and 40 seconds) 10 6: megasecond: 1 000 000: 11.6 days (or 11 days, 13 hours, 46 minutes and 40 seconds) 10 9: gigasecond: 1 000 000 000: 31.7 years (or 31 years, 252 days, 1 hour, 46 minutes, 40 seconds, assuming that there are 7 leap years in the interval)
In the year −2000 (2001 BCE) the May maximum was +12 minutes and a couple seconds while the November maximum was just less than 10 minutes. The secular change is evident when one compares a current graph of the equation of time (see below) with one from 2000 years ago, e.g., one constructed from the data of Ptolemy. [25]
Some common units of time in seconds are: a minute is 60 seconds; an hour is 3,600 seconds; a day is 86,400 seconds; a week is 604,800 seconds; a year (other than leap years) is 31,536,000 seconds; and a century averages 3,155,695,200 seconds; with all of the above excluding any possible leap seconds.
In contrast, the Julian year is defined in terms of the SI unit one second, so is as accurate as that unit and is constant. It approximates both the sidereal year and the tropical year to about ±0.008 days. The Julian year is the basis of the definition of the light-year as a unit of measurement of distance. [2]
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).
The sidereal year is 20 min 24.5 s longer than the mean tropical year at J2000.0 (365.242 190 402 ephemeris days). [1] At present, the rate of axial precession corresponds to a period of 25,772 years, [3] so sidereal year is longer than tropical year by 1,224.5 seconds (20 min 24.5 s, ~365.24219*86400/25772).