The autumnal (or fall) equinox, which usually falls on Sept. 22 or 23, is technically not a day-long astronomical event. It’s a brief moment in time when the sun appears directly over the Earth’s equator before crossing into the Southern Hemisphere. Like the spring equinox in March, the fall equinox is one of only two days each year when most of the Earth experiences about 12 hours of daylight and 12 hours of darkness. Day and night are nearly equal because we are at a point in our orbit when neither hemisphere is tilted away from or toward the sun.
In the Northern Hemisphere, the autumnal equinox means we are entering the dark season and inching closer toward winter. On Sept. 22, Washington sees 12 hours and 8 minutes of daylight, with the sun rising at 6:56 a.m. and setting at 7:04 p.m.
Friday will be our last sunset in the 7 o’clock hour this year. Beginning Sunday (Sept. 26), the length of day dips to 11 hours and 59 minutes. Not until March 17, 2022, will the sun again spend more than 12 hours above the horizon.
Dwindling daylight, cooler temperatures
For the next three months, until the winter solstice, the days will continue to get shorter as the sun traces a shorter and lower path across the sky. The location of sunrise and sunset will also edge closer to the southern horizon until December. During the equinox, the sun rises perfectly due east and sets due west everywhere on Earth except near the North and South Pole.
Though our days have been getting shorter since the summer solstice in June, our earlier sunsets become quite noticeable near the fall equinox, when daylight vanishes at its fastest pace of the year.
Washington, D.C., loses one hour and 15 minutes of daylight in September, or about 2 minutes and 30 seconds per day. We started the month with 13 hours and one minute of daylight, but by Oct. 1, the length of day shrinks to 11 hours and 46 minutes. Daylight reaches its annual shortest length of 9 hours and 26 minutes on Dec. 21, the winter solstice.
As the days get shorter and the sun sinks lower in the sky, temperatures also start to drop. D.C.'s average high temperature on Sept. 22 is still a toasty 79 degrees, and our average overnight low is 62 (these are both two degrees warmer than our old 1981-2020 averages).
By Halloween, however, D.C.'s normal high and low dip to 64 degrees and 46 degrees, respectively. While October and November are now both warmer than in the past, these two months have not warmed quite as much as September. In the new climate normals, the average November temperature in Washington has increased only 0.3 degrees, while September is now 1.4 degrees warmer. This suggests that the fall transition season is getting slightly truncated as summer warmth lingers deeper into September.
While cooler weather is inevitable in the months ahead — and we’ll get a taste of autumn this weekend — on balance, the season may end up on the warm side. In its latest seasonal outlook, the National Oceanic and Atmospheric Administration favors above-normal temperatures across the Mid-Atlantic and a sizable portion of the Lower 48 through December.
Even if fall ends up being mild, the fall equinox is a reminder that cooler days and longer nights are upon us. As temperatures tumble and the leaves begin to turn vibrant shades of red, orange and yellow, it can only mean winter is not far behind.