Temperatures may still be warm outside but today is the first day of fall.
Sept. 22 is the autumnal equinox, the time when the sun crosses the celestial equator, the imaginary line in the sky that corresponds to the earth’s equator. The autumnal equinox – also known as the September or southward equinox – occurs between the 21st and 24th of September each year.
On the equinox, the sun rises directly in the east and sets directly in the west with a location above the equator illuminating the Earth from pole to pole. Before the autumnal equinox, the sun rises and sets more to the north; afterwards it shifts more southward.
The autumnal equinox is the first day of fall north of the equator; it’s the first day of spring in the southern half of the world.
When is the start of fall?
The autumnal equinox, the start of fall, will officially arrive at 10:21 a.m. EDT on Thursday.
Here’s a breakdown of fall’s official arrival for other times zones:
Central Daylight Time: 9:21 a.m.
Mountain Daylight Time: 8:21 a.m.
Pacific Daylight Time: 7:21 a.m.
After the equinox – Latin for “equal night” – the days in the Northern Hemisphere will get shorter until the winter solstice in December. On Thursday, the sun will rise at 6:44 a.m, EDT and set at 6:52 p.m. EDT. The exact equal day and night won’t actually occur until a few days later on Sept. 25 with sunrise at 6:47 a.m. EDT and sunset at 6:47 p.m.
The autumnal equinox is the second that occurs each year. The first is the vernal equinox which occurs around March 20 or 21 and marks the beginning of spring.
While the autumnal equinox marks the beginning of fall for astronomical watchers, it’s not often used by meteorologists and climatologists. As the National Oceanic the Atmospheric Administration explains, the difference is because the astronomical seasons are based on the position of the Earth in relation to the sun, while the meteorological seasons are based on the annual temperature cycle.
