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. 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!
Fun Fact: Between 2016 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 locations that are ahead of UTC, the March equinoxes this century will fall on March 19, 20, or 21.
Why Do the Dates Change?
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 2017 March equinox occurred on March 20 at 10:28 UTC. The 2018 equinox took place at 16:15 UTC and the 2019 equinox will occur at 21:58 UTC, each about 6 hours later than in the previous year. 2020 is a leap year. Then, the March equinox will take place at 03:49 UTC, about 18 hours before the time of the equinox in 2019.
The equinox dates for 2017, 2018, 2019, and 2020 will 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 the date of the March equinox.
Take the case of Sydney, Australia, which is 11 hours ahead of UTC in March. Here, the 2017 March equinox occurred on March 21 at 21:28 (9:28 pm) AEDT. In 2018, it took place on March 21 at 03:15 (3:15 am) AEDT, about 5 hours and 30 minutes later than in 2017. In 2019, it will be 08:58 (8:58 am) AEDT on March 21 in Sydney when the equinox occurs. In 2020, which is a leap year, the equinox in Sydney's local time will occur at 14:49 (2:49 pm) on March 20. This is almost 18 hours earlier than the time of the 2019 March equinox.
New York City, which is 4 hours behind UTC, celebrated its 2017 spring equinox at 06:28 (6:28 am) on March 20. In 2018, the city celebrated equinox at 12:15 (12:15 pm) on March 20, 6 hours later than the time of the equinox in 2017. In 2019, the city will have its vernal equinox on March 20 at 17:58 (5:58 pm) EDT and in 2020, a leap year, it will be almost 18 hours earlier, on March 19 at 23:49 (11:49 pm).
|Year||March Equinox in UTC||March Equinox in Sydney (UTC+11)||March Equinox in New York (UTC-4)|
|2017||March 20, 10:28||March 20, 21:28||March 20, 06:28|
|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|