This week we officially say goodbye to summer and hello to fall. (Erinn Shirley via Flickr)

Summer warmth has visited many parts of the country this September, but fall is finally in the air as we near the autumnal equinox. This year’s equinox occurs Sept. 23 at 4:21 a.m. Eastern time. At that time, the sun can be seen directly overhead along Earth’s equator, marking the beginning of astronomical fall in the Northern Hemisphere. 

The fall equinox is one of only two days each year when all points on Earth outside the polar regions see the sun rise and set due east and due west along the horizon. And since neither hemisphere is tilted toward the sun, all latitudes see almost exactly 12 hours of daylight and darkness (though not quite).

During the equinoxes, all latitudes receive equal periods of daylight and darkness. The sun appears directly overhead at exactly the equator -- zero degrees latitude -- over the western Indian Ocean at 8:22 a.m. GMT. (timeanddate.com)
During the equinoxes, all latitudes receive equal periods of daylight and darkness. The sun appears directly overhead at exactly the equator — zero degrees latitude — over the western Indian Ocean at 8:22 a.m. GMT. (timeanddate.com)

Dusk is arriving noticeably earlier than it did a few weeks ago. In the Northern Hemisphere, daylight dwindles at its fastest clip in the weeks just before and after the September equinox. Washington, D.C., loses 2 minutes and 30 seconds of daylight each day, while cities across the northern tier lose close to 3 minutes or more.

But if you are an astute observer of the sun’s daily and monthly movements, you might notice that sunrise and sunset times do not change equally. Take a look at the chart below and it’s clear we’re losing much more daylight in the evening than in the morning. For example, Washington, D.C., loses a good 46 minutes of evening daylight in September, but only 26 minutes in the morning. What’s going on here?

(Justin Grieser, data from timeanddate.com)
(Justin Grieser; data from timeanddate.com)

This discrepancy happens because the Earth is tilted 23.5 degrees on its axis and follows an elliptical (non-circular) orbit around the sun. In early autumn, these factors combine to make our days a few seconds shorter than 24 hours, in terms of the time it takes the sun to cross directly over our location — or pass over our longitude — from one day to the next. The sun therefore appears to move faster than our human clocks. We see this by the fact that solar noon, the time when the sun reaches its maximum height each day, drifts 10 minutes earlier during the month of September.

How does this affect the time of sunrise and sunset? In the mornings, it means sunrise doesn’t move later as quickly as we would otherwise expect, since the sun takes a bit less than 24 hours to reach the same point in the sky each day. However, in the evenings, the time of sunset will shift dramatically earlier due to our shorter days, and the sun’s apparent movement ahead of our normal clocks.

Dwindling daylight, falling temperatures

September often still feels like a summer month in warmer climates, but those eager to dig out jackets and fleece won’t have much longer to wait. As the days continue to shorten and the sun stays lower in the sky, temperatures begin to drop significantly.

Most major cities see average high temperatures drop at least 10 degrees between September and October. In the nation’s capital, even our 80-degree days are finally getting numbered, making room for more comfortable 60s and low 70s.

(Justin Grieser, data from NOAA)
(Justin Grieser/NOAA)

That said, fall may still take its time arriving this year. NOAA favors above-normal temperatures for much of the country into the first half of October. But whether autumnal or not, the September equinox means we’ll be seeing more night than day these next six months.