There are only two times a year when daylight and darkness almost perfectly harmonize anywhere on earth.
One of them occurs on Thursday: the Autumnal Equinox comes at 21:04 ET and marks the astronomical transition from summer to autumn in the northern hemisphere (and from winter to spring south of the equator).
What happens at the equinox?
The autumnal (autumn) equinox is halfway between our longest and shortest days of the year and usually falls on September 22nd or 23rd. Technically, an equinox is not an all-day astronomical event. It is the brief moment when the sun appears directly over the earth’s equator.
Like the March vernal equinox, this is one of only two times a year when day and night are roughly 12 hours long anywhere on Earth. In the northern hemisphere, daylight will continue to decrease until the winter solstice as the sun follows a shorter and lower path across the sky. Dwindling sunlight is the number one reason trees explode in brilliant reds, oranges, and yellows before shedding their leaves for the winter.
The point of sunrise and sunset is closer to the southern horizon until December. During the equinox, the sun rises in the east and sets in the west everywhere on earth except at the north and south poles.
Day and night are not exactly the same
Although “equinox” comes from the Latin words “aequus” (equal) and “nox” (night), all places on Earth actually see more than 12 hours of daylight at the equinox.
Washington sees about 12 hours and 8 minutes of daylight at the equinox (sunrise on Friday, the first full day of fall, is at 6:56 am and sunset is at 7:03 pm). “Equilux”—the day when sunrise and sunset are closest at 12 hours—occurs a few days later.
In most of the United States, the equinox is on September 25th or 26th. Only on March 17th will the sun decorate our sky again for at least 12 hours.
Map showing the date of the autumn equinox in 2022. This is the date closest to exactly 12 hours of daylight. Note: It changes from year to year. pic.twitter.com/cKdgmUHDql
— Brian Brettschneider (@Climatologist49) September 21, 2022
Why does the equinox have more than 12 hours of daylight?
There are two reasons why we see more than 12 hours of daylight during the equinox.
One of them is measuring the length of the day. The sun appears as a stationary disk, not a single point in the sky. Sunrise occurs as soon as the top of the sun appears above the horizon, while sunset occurs when the top of the sun is completely below it. “Because we use a first-up, last-down approach to define the length of a day, rather than tracking when a point on the sun is above the horizon, our day is a few minutes longer than 12 hours,” Capital’s Matthew Cappucci of Gang’s Weather explained in 2020.
The second reason we see more than 12 hours of daylight is because Earth’s atmosphere can refract or bend sunlight. This allows us to see the sun even though it’s technically below the horizon. The size of the fracture depends on the atmospheric pressure and temperature. “[W]when we see the sun sitting on the horizon as a red-orange ball, we see an optical illusion. In fact, it’s completely below the horizon,” Space.com explained in a recent article.
These two factors — how we measure daylength and atmospheric refraction — add several minutes of daylight to the equinox — from 12:60 p.m. near the equator to about 12:20 p.m. in Earth’s polar regions.
At the autumnal equinox we experience the fastest drop in daylight, although the rate of change depends on how far from the equator you live. Near the autumn equinox, Washington loses 2 minutes and 30 seconds of daylight per day, while Miami loses just 90 seconds. At higher latitudes, the loss of light is more dramatic: in Seattle, daylight disappears by almost 3½ minutes each day, and in Anchorage the difference is more than 5½ minutes.
As a call, our first full day of fall will definitely feel it. A strong cold front arriving Thursday will bring clear fall weather to DC 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 are your plans for the rest of the season? While colder weather is inevitable as we head into winter, the decline in equilibrium should still be warmer than normal, as we can expect in our warming climate. Similar to last year, NOAA is again forecasting a warmer-than-average fall for most of the lower 48.