ModelE AR5 Simulations: Past Climate Change and Future Climate Predictions

Time and Date of Vernal Equinox

For the Model E AR5 simulations, all years have exactly 365 days and vernal equinox always occurs on March 21, hour 0. Future versions of Model E will use the more precise time of vernal equinox as described on this web page.

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. This web page 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 web page's time of vernal equinox with observations. The Table shows web page 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 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 web page'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. Ignoring the existence of the Moon can cause a few minutes inaccuracy in the web page's equinoxes and solstices and as much as a couple of days inaccuracy in the web page'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.

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.


Please address questions about these AR5 webpages to Dr. Gary Russell.

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