Fall equinox: explaining the first day of autumn

WEATHER NEWS: Fall equinox: explaining the first day of autumn

There are only two days each year when daylight and darkness are in near-perfect harmony everywhere on Earth.

One of them happens Thursday: The autumnal equinox arrives at 9:04 p.m. Eastern time, which marks the astronomical transition from summer to fall in the Northern Hemisphere (and winter to spring south of the equator).

What happens on the equinox?

The autumnal (fall) equinox is the halfway point between our longest and shortest days of the year, and usually falls on Sept. 22 or 23. Technically, an equinox is not a day-long astronomical event. It’s a brief moment in time when the sun appears directly over the Earth’s equator.

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. In the Northern Hemisphere, daylight will continue to dwindle until the winter solstice, as the sun traces a shorter and lower path across the sky. The diminishing sunlight is the main reason trees burst into brilliant shades of red, orange and yellow before dropping their leaves for the winter.

The location of sunrise and sunset will also edge closer to the southern horizon until December. During the equinox, the sun rises due east and sets due west everywhere on Earth except near the North and South poles.

Not quite equal day and night

Though “equinox” comes from the Latin words “aequus” (equal) and “nox” (night), all places on Earth actually see more than 12 hours of daylight on the equinox.

Washington sees about 12 hours, 8 minutes of daylight on the equinox (sunrise on Friday, the first full day of fall, is at 6:56 a.m. and sunset at 7:03 p.m.). However, the “equilux” — the day when sunrise and sunset are closest to 12 hours apart — happens a few days later.

In most of the United States, the equilux is Sept. 25 or 26. Not until March 17 will the sun again grace our skies for at least 12 hours.

Why the equinox has more than 12 hours of daylight

There are two reasons we see more than 12 hours of daylight during the equinox.

One is how we measure the length of a day. The sun appears as a lumbering disk, not a discrete point in the sky. Sunrise occurs as soon as the sun’s upper edge appears on the horizon, while sunset doesn’t happen until the sun’s upper edge completely dips below it. “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,” Capital Weather Gang’s Matthew Cappucci explained in 2020.

The second reason we see more than 12 hours of daylight is because the Earth’s atmosphere can refract, or bend, the sun’s light. This allows us to see the sun even when it’s technically below the horizon. The amount of refraction depends on atmospheric pressure and temperature. “[W]hen we see the sun as a reddish-orange ball just sitting on the horizon, we’re looking at an optical illusion. It is actually completely below the horizon,” Space.com explained in a recent article.

These two factors — how we measure the length of day and atmospheric refraction — add several minutes of daylight to the equinox — from 12 hours, 6 minutes near the equator to about 12 hours, 20 minutes in Earth’s polar regions.

The autumnal equinox is when we experience the fastest loss of daylight, although the rate of change depends on how far you live from the equator. Near the fall equinox, Washington loses 2 minutes, 30 seconds of daylight per day, while Miami loses only 90 seconds. At higher latitudes the loss of light is more dramatic: Seattle sees daylight vanish by nearly 3½ minutes each day, and in Anchorage, the difference is more than 5½ minutes.

As if on cue, our first full day of fall will certainly feel the part. A strong cold front arriving Thursday will bring crisp, autumnal weather to D.C. and the Mid-Atlantic. The National Oceanic and Atmospheric Administration expects below-normal temperatures for most areas east of the Mississippi River during the last week of September.

What’s in store for the rest of the season? While colder weather is inevitable as we head toward winter, fall on balance should still lean warmer than normal, as we can expect in our warming climate. Much like last year, NOAA is again forecasting a warmer-than-average fall across most of the Lower 48.

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