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Why Does the March Equinox Fall on Different Dates?

Many cultures claim March 21 as the date of the March equinox. In reality, the equinox can happen on March 19, 20, or 21.

Flowers growing through the frozen ground.

Spring or March equinox?

©bigstockphoto.com/sauletas

Note: All dates and times in this article refer to the equinox in UTC time, unless otherwise stated. Due to time zone differences, the equinox may occur a day later in locations ahead of UTC and a day earlier in locations behind UTC.

Sun Directly Above the Equator

An equinox is the exact instant when the Sun is directly overhead the Equator and the Earth's rotational axis is tilted neither towards nor away from the Sun. In technical terms, this means that at the instant of the equinox, the Earth's celestial equator, which is the equator's imaginary projection into space, intersects with the center of the Sun.

In any given calendar year, this happens twice, first around March 20 (March equinox) and then again around September 22 (September equinox).

Many people around the world celebrate the whole day, usually March 21, as the March equinox. In reality, however, it occurs at a specific moment in time and its date at any given location is determined by the exact instant when the Sun is overhead the Equator.

Varying Equinox Dates

The March equinox can happen on March 19, 20, or 21. The last time the March equinox was on March 21 (in UTC) was in 2007. It will happen again in 2101.

Due to time zone differences, the equinox may occur a day earlier at locations that are behind UTC. Take the example of mainland United States. While locations following UTC have seen a March 21 Equinox in 2003 and 2007, there is no March 21 equinox in mainland US in the 21st century!

Between 2020 and 2048, March 19 equinoxes will happen every leap year in Central, Pacific, and Mountain time zones in the United States. In the same period, the years between the leap years will see a March 20 equinox.

In 2024, the March equinox will fall on March 19 in all time zones in the United States. This will be the earliest spring equinox in the country since 1896.

In locations that are ahead of UTC, the March equinoxes this century will fall on March 19, 20, or 21.

Why Do the Dates Change?

YearMarch Equinox in
UTC
March Equinox in Sydney
(UTC+11)
March Equinox in New York
(UTC-4)
2020March 20, 03:49March 20, 14:49 March 19, 23:49
2021March 20, 09:37March 20, 20:37March 20, 05:37
2022March 20, 15:33March 21, 02:33March 20, 11:33
2023March 20, 21:24March 21, 08:24March 20, 17:24
2024March 20, 03:06March 20, 14:06March 19, 23:06

The March equinox would occur on the same day every year if the Earth took exactly 365 days to make a complete revolution around the Sun. But this is not the case. It takes the Earth about 365.25 days on average to go around the Sun once. The Gregorian Calendar accounts for this by adding an extra day—the leap day—almost every 4 years. This means that each March equinox occurs about 6 hours later than the previous year's March equinox. This is why the date of the equinox can change from year to year.

Here's an example that will make this clearer: the 2022 March equinox occurred on March 20 at 15:33 UTC. The 2023 equinox will take place at 21:24 UTC, about 6 hours later than in the previous year. 2024 is a leap year. The March equinox will happen at 03:06 UTC, about 18 hours before the time of the equinox in 2023. For 2025 and 2026, both common years, the gap between each equinox is about 6 hours again.

The equinox dates for these years do not change for locations observing a local time equivalent to UTC, but for locations that are ahead and behind UTC, this 6-hour difference (18 hours in a leap year) means a potential change in its date.

Take the case of Sydney, Australia, which is 11 hours ahead of UTC in March. Here, the 2022 March equinox occurred on March 21 at 02:33 (2:33 am) AEDT. In 2023, it takes place on March 21 at 08:24 (8:24 am) AEDT, 5 hours and 51 minutes later than in 2022. In 2024, which is a leap year, it is 14:06 (2:49 pm) AEDT on March 20 in Sydney when the equinox occurs. This is almost 18 hours earlier than the time of the 2023 March equinox.

New York City, which is 4 hours behind UTC in March, celebrated its 2022 spring equinox at 11:33 (11:33 am) on March 20. In 2023, the city celebrates equinox at 17:24 (5:24 pm) EDT on March 20, about 6 hours later than the time of the equinox in 2022. In 2024, a leap year, it is almost 18 hours earlier, on March 19 at 23:06 (11:06 pm).