Equinox variation

01 August, 2010

The Sun - in the prime of its life.
The Sun - in the prime of its life.
Image: SOHO (ESA & NASA)
Source: SOHO (ESA & NASA)

Question: Why have the times of solstices and equinoxes this year changed from last year?

Answer: The date and time of the equinoxes and solstices change slightly because the Earth's orbit around the Sun is not precisely one year. The equinoxes and solstices happen when the Earth reaches a precise point in its orbit around the Sun. However the beginning of the year does not happen at the exact same point every year. We count years as being either 365 or 366 days long, but the orbit actually takes about 365.242 days to go from one March equinox to the next. Thus, in a non-leap year the equinox will be about 0.242 days (approximately 5 hours 50 minutes) later than the previous equinox, while in a leap-year it will be 0.758 days (18 hours 10 minutes) earlier.

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