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Why is the equinox date not the same each year?

Question submitted by Mike
(Salmo, BC)

While the September equinox occurs on September 22 in 2008 and 2009, it occurs on September 23 in 2010 and 2011 (UTC). The September equinox has also occurred on September 24(UTC), with the last occurrence on that date being 1931. The next time a September 24 equinox occurs will be in the year 2303. Moreover, a September 21 equinox will occur in 2092.

There are a few explanations on why the equinox dates differ in the Gregorian calendar. The varying dates of the equinox are mainly due to the calendar system – most western countries use the Gregorian calendar, which has 365 days in a year, or 366 days in a leap year. According to the National Maritime Museum, the equinoxes generally occur about six hours later each year, with a jump of a day (backwards) on leap years. An extra day is added in a leap year to minimize a gradual drift of the equinox date through the seasons.

As for the tropical year, it is approximately 365.242199 days, but varies from year to year because of the influence of other planets. A tropical year is the length of time that the sun takes to return to the same position in the cycle of seasons, as seen from earth. The exact orbital and daily rotational motion of the Earth, such as the “wobble” in the earth's axis (precession), also contributes to the changing solstice dates.

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