Chilly mornings, shorter days and longer nights. It’s certainly felt like fall lately, as crisp autumnal air more typical of October settled into our area over the weekend. Tuesday’s autumnal equinox is another reminder that summer is now officially behind us.

Astronomers use the equinox to mark the transition from summer to fall in the Northern Hemisphere, which happens around Sept. 22 or 23 every year. While many of us think of the first day of fall as a full calendar day, the equinox itself is a rather fleeting astronomical event. It happens at a precise moment when the sun’s direct rays are straight over Earth’s equator. This year’s equinox is at 9:30 a.m. Eastern time on Sept. 22.

The autumnal equinox is roughly the halfway point between our longest and shortest days of the year. Like the spring equinox in March, it’s one of only two days of the year when day and night are about 12 hours long everywhere on Earth. The equinoxes are also the only times when every place on Earth — unless you’re near the North or South Pole — will see sunrise at due east and sunset due west along the horizon.

Over the next three months, you’ll notice the sun rising and setting closer to the southern horizon as the sun traces a shorter and lower path across the sky.

Rapidly losing daylight

Though the days have steadily been getting shorter since the summer solstice, the earlier arrival of dusk becomes especially noticeable by September. That’s because the fall equinox marks the point when we lose daylight the fastest here in Earth’s mid-latitudes.

In the nation’s capital, we lose 2 minutes and 30 seconds of daylight each day in mid-September. More northern cities like Minneapolis and Seattle lose over three minutes of sunlight per day, while places closer to the equator see less day-to-day variation. Miami, for example, loses only 90 seconds of daylight per day.

The loss of sunlight is the main reason trees burst into brilliant shades of red, orange and yellow before dropping their leaves for the winter.

Equal day and night — not quite

Even though “equinox” originates from the Latin words aequus (equal) and nox (night), day and night are only approximately equal in length.

In the District, sunrise Tuesday is at 6:56 a.m. and sunset is at 7:04 p.m., bringing us 12 hours and 8 minutes of daylight. Not until Sept. 25 do sunrise and sunset fall exactly 12 hours apart, at 6:59 a.m. and 6:59 p.m., respectively.

The not-so-equal daylight happens for two reasons: One is that Earth’s atmosphere can refract, or bend, the sun’s light, which can cause the sun to appear higher in the sky than it is. The other is how we define the length of the day. As CWG’s Matt Cappucci explained earlier this year:

“Sunrise is defined as the moment the upper limb of the sun pokes over the horizon — the first hints of sunlight. Sunset occurs when the last glimmers of sunlight disappear below the horizon. Because we’re taking the first-up, last-down approach to defining day length, rather than tracking when a single point on the sun is above the horizon, our day is a couple of minutes longer than 12 hours.”

The exact amount of daylight depends on your distance from the equator. At higher latitudes, such as northern Alaska, the sun is up for more than 12 hours and 20 minutes on the equinoxes.

Washington’s coolest fall equinox in years

In Washington, Tuesday morning’s low of 48 degrees marked the coolest fall equinox since 1999, when the low was also 48. Our coldest start to astronomical fall occurred back in 1904, when it was 36 degrees on Sept. 23 — that reading remains D.C.'s coldest temperature ever recorded in the month of September.

In recent years, the first day of fall has often felt more summerlike. In 2019, D.C. was a toasty 94 degrees on the fall equinox (not far behind the record of 98 set on Sept. 23, 1895). This year will be the first equinox below 80 degrees since 2014, when the high was 76 degrees.

D.C.'s normal high temperature around the fall equinox is 77 degrees, and our average low is 60. In our warming climate, however, the first day of fall has been trending warmer. Since 2000, D.C.'s average high and low temperatures on the fall equinox have been about 82 and 64 degrees, respectively.

The beauty of autumn is its transitional nature. As the leaves begin to turn, we can expect a mix of warmer and cooler days as the last vestiges of summer compete with cold Arctic air spilling out of Canada. Eventually, cold air will gain the upper hand as the shorter days and longer nights remind us that winter lies ahead.