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A leap year occurs every fourth year during which a leap day is intercalated into the month of February. The name "Leap Day" is applied to the added day. In astronomy, the Julian year is a unit of time defined as 365.25 days, each of exactly 86 400 seconds (SI base unit), totaling exactly 31,557,600 seconds in the Julian astronomical year. [4] [5]
The year, defined in the Gregorian calendar as 365.2425 days has to be adjusted with leap days and leap seconds. Consequently, these units are now all defined for scientific purposes as multiples of seconds. Units of time based on orders of magnitude of the second include the nanosecond and the millisecond.
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.
They can also be called the early, middle, or late parts of the year. In the Gregorian calendar: First quadrimester, early year: January 1 – April 30 (120 days or 121 days in leap years) Second quadrimester, mid-year: May 1 – August 31 (122 days) Third quadrimester, late year: September 1 – December 31 (121 days)
Between 1972 and 2020, a leap second has been inserted about every 21 months, on average. However, the spacing is quite irregular and apparently increasing: there were no leap seconds in the six-year interval between 1 January 1999, and 31 December 2004, but there were nine leap seconds in the eight years 1972–1979.
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. [1]
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).