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The NPR News item and podcast Spring Starts Today All Over America, Which Is Weird includes the following:

But why isn't the time of the equinox the same each year?

The short answer is that the time and the date are imperfect human constructs that we use to keep track of our planet's movements.

The longer answer involves leap years.

All of this is caused simply by the fact that the spin of the Earth doesn't divide evenly into one year," says Michelle Thaller, an astrophysicist turned space communications expert at NASA.

One spin of the Earth around its axis is one day. "The problem is we're happily spinning on our axis, and the Earth is going around the Sun, but one year — one complete path around the Sun — isn't an even, exact number of days. In fact, it's 365.24 [days]."

Wikipedia gives Earth's orbital period as 365.256363004 days and I have always remembered it to be 365.2564 which is the same value as Wikipedia, just to fewer digits.

So is a year really about 365.24 days or closer to 365.2564 days?

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  • $\begingroup$ One way to look at it: If a "year" was 365.2564 days long, our calendar would typically need a leap year every four years that contain one extra day, but would occasionally need double leap years, years in which two extra days are added. Instead we typically have single day leap years every four years, but occasionally skip leap years (e.g., 1700, 1800, and 1900 were not leap years), making the mean calendar year a bit less than 365.25 days long. $\endgroup$ – David Hammen Mar 21 at 15:24
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Both are correct.

The Sidereal Year is the length of time it takes Earth to complete an orbit around the sun, relative to the fixed stars. It is 365.2564 days.

The Tropical Year is the length of time it takes for the Sun to complete a cycle around the Ecliptic and return to the position in the cycle of seasons; e.g. from Vernal Equinox to Vernal Equinox. It's about 365.24217 days, about 20 minutes shorter than the Sidereal year because of the precession of the equinoxes. This is the year that the Gregorian Calendar is attempting to emulate.

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A distiction has to be made between the length of a year, and the period of the orbit.

For human purposes, a year is the time it takes for one full cycle of the seasons. It is the time between one vernal Equinox and the next. Seasons are caused by the tilt of the Earth, and if the tilt was fixed, then this time period would be the the time it took for the Earth to orbit the sun: 365.2564 days (in which each day is 86400 seconds)

However the direction of the Earth's axis (relative to the stars) isn't fixed. It precesses in a circle. This means that the moment of the vernal equinox isn't at the same point in the orbit each year. In fact we reach the vernal equinox in slightly less time than it takes to orbit the sun. The time between successive equinoxes is 365.24219 days.

One mean tropical year is 365.24219 days. One sidereal year is 365.256363 days.

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