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.
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.
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 2020, the March equinox fell on March 19 in all time zones in the United States, making it 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?
|Year||March Equinox in|
|March Equinox in Sydney|
|March Equinox in New York|
|2018||March 20, 16:15||March 21, 03:15||March 20, 12:15|
|2019||March 20, 21:58||March 21, 08:58||March 20, 17:58|
|2020||March 20, 03:49||March 20, 14:49||March 19, 23:49|
|2021||March 20, 09:37||March 20, 20:37||March 20, 05:37|
|2022||March 20, 15:33||March 21, 02:33||March 20, 11:33|
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 2018 March equinox occurred on March 20 at 16:15 UTC. The 2019 equinox took place at 21:58 UTC, about 6 hours later than in the previous year. 2020 was a leap year. The March equinox took place at 03:49 UTC, about 18 hours before the time of the equinox in 2019. For 2021 and 2022, 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 2018 March equinox occurred on March 21 at 03:15 (3:15 am) AEDT. In 2019, it took place on March 21 at 08:58 (8:58 am) AEDT, 5 hours and 43 minutes later than in 2018. In 2020, which was a leap year, it was 14:49 (2:49 pm) AEDT on March 20 in Sydney when the equinox occurred. This was almost 18 hours earlier than the time of the 2019 March equinox.
New York City, which is 4 hours behind UTC, celebrated its 2018 spring equinox at 12:15 (12:15 pm) on March 2. In 2019, the city celebrated equinox at 17:58 (5:58 pm) EDT on March 20, about 6 hours later than the time of the equinox in 2018. In 2020, a leap year, it was almost 18 hours earlier, on March 19 at 23:49 (11:49 pm).