September equinox 2021

The September equinox 2021 is on Wednesday, September 22, 2021 (in 163 days).  Calendar for 2021

Please note: The dates given on this page are based on Coordinated Universal Time (UTC), which for practical purposes is equivalent to Greenwich Mean Time (GMT). While the September equinox occurs at the same moment in time all over the world, the date and local time differ from location to location depending on a location's time zone. For more details see the explanations below.

The September equinox, also called southward equinox, is the moment in time (not a day-long event) when the Sun stands directly above the equator while crossing from the north to the south. For the Northern Hemisphere (where nearly 90% of the world's population live) it is the autumn equinox (fall equinox) as it is the moment when summer ends and autumn (fall) begins, while for the Southern Hemisphere it is the vernal equinox (spring equinox), the moment when winter ends and spring begins. Up until the September equinox the Sun rises and sets more to the north of the equator, and afterwards it rises and sets more to the south.

The September equinox usually occurs every year between September 21 and 24. The dates given on this page are based on Coordinated Universal Time (UTC), which for practical purposes is equivalent to Greenwich Mean Time (GMT - the time zone of the United Kingdom). While the September equinox occurs at the same moment in time all over the world, the date and local time differ from place to place depending on the year and a location's time zone. For locations that are ahead of UTC (further east) it may fall on the day after, and for locations that are behind UTC (further west) it may fall on the day before. To find out the exact date and time of the September equinox 2021 in your area use this seasons calculator.

The September equinox is one of four days (two equinoxes and two solstices) throughout the year that mark the beginning of a new season. The other days are the March equinox, the June solstice and the December solstice.

The word "equinox" is derived from Latin and means "equal night". On the day of an equinox, day and night are of approximately equal length all over the world, as the Earth's rotational axis is neither tilted away from nor towards the Sun. At all other times the length of day and night will be different.


The exact moment of the September equinox through the centuries

Graph demonstrating the movement (caused by leap shifting) of the exact moment of the September equinox between 1900 and 2520. X axis (bottom, horizontal): Calendar years, Y axis (left, vertical): day in September (21 to 23).

In a non-leap year, the moment of the September equinox is about 5 hours 49 minutes after that moment in the previous year (in UTC), and in a leap year it is about 18 hours 11 minutes before that moment in the previous year, as can be seen in this graph. In this way the time of the September equinox constantly shifts back and forth so it always stays between September 21 and 24. This four year cycle is clearly visible in the graph.

To adjust for the gradual movement backwards on the calendar (as visible for the period 1900 to 2099), in years divisible by 100 (1900, 2100, 2200, 2300 and 2500 on the graph), which should be leap years as they are divisible by 4, the leap day is omitted (except when they are divisible by 400, eg. 2000 and 2400) and the moment of the September equinox moves forward in time compared to the same moment in the year before.

Further reading:

• September equinox on Wikipedia
• Equinoxes in general on Wikipedia

Overview: seasons, equinoxes and solstices

Dates shown apply to the Northern Hemisphere and are based on UTC (GMT)

Season	Start	End
Spring	Spring equinox (vernal equinox, March equinox) March 19 to 21	Summer solstice (June solstice) June 20 to 22
Summer	Summer solstice (June solstice) June 20 to 22	Autumn equinox (Fall equinox, September equinox) September 21 to 24
Autumn (Fall)	Autumn equinox (Fall equinox, September equinox) September 21 to 24	Winter solstice (December solstice) December 20 to 23
Winter	Winter solstice (December solstice) December 20 to 23	Spring equinox (vernal equinox, March equinox) March 19 to 21

The September equinox 2022 is on Friday, September 23, 2022 (in 529 days).  Calendar for 2022

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