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Time and Date of Vernal Equinox

The information described on this web page is implemented in the 4x3 Atmosphere-Ocean Model and the simulations (C480, C483, C484, C490, C493, C494) used by the 4x3 Model sections of this web site.

The date of vernal equinox and the number of days in February are political decisions when choosing to use the Gregorian calendar. In this calendar, February has 28 days most years; every fourth year (1988 A.D., 1992, 1996) February has 29 days; every century (1700, 1800, 1900) February has 28 days; and every fourth century (1600, 2000, 2400) February has 29 days. The Model assumes that this 400 year cycle is repeated indefinitely, that the tropical year is (365*400+97)/400 = 365.2425 days, and that vernal equinox occurs exactly on March 20, 7:30 GMT every four hundred years (including year 2000 A.D.).

This last decision is based on comparing the Model's time of vernal equinox with observations. The Table shows Model times and actual times of vernal equinox published in NASA Reference Publication 1349 [1994 October] for years 1995 to 2010 and in Explanatory Statement to the Ephemeris [19??] for years 1903, 2000 and 2096.


Produce a table of Model orbital events (equinoxes, solstices, perihelion, aphelion) for selected years between -999 and 9999 A.D. There are two large sources of error to this table. If the Model's tropical year (365.2425 days) is incorrect by .0003 days, then the error in orbital events would be 2.88 hours every 400 years. The existence of the Moon can cause a few minutes inaccuracy in the Model's equinoxes and solstices and as much as a couple of days inaccuracy in the Model's perihelion and aphelion.

Enter minimum year, maximum year, and yearly increment:


The following programs were used to calculate the time of vernal equinox and other orbital events.
VERNAL.SUB For a given Year, calculate an approximate time of vernal equinox in Days measured from 2000 January 1, hour 0.
DtoYMD.SUB For a given Day measured from 2000 January 1, hour 0, determine the Year (A.D.), Month, and Date.
DtoYMDHM.SUB For a given Day measured from 2000 January 1, hour 0, determine the Year (A.D.), Month, Date, Hour, and Minute.
ORBPAR.SUB For a given Year, calculate the three orbital parameters: Eccentricity, Obliquity, and Longitude of Perihelion.
ORBIT.SUB For given orbital parameters and a given Day measured from 2000 January 1, hour 0, determine the distance to the Sun, the declination angle of the Sun, and an unnormalized Equation of Time.
SREVENTS.FOR For a range of Years (A.D.), display the times of the equinoxes, the solstices, perihelion, and aphelion.

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Curator: Gary L. Russell . . . . . 2005/02/09/11:55:54