After record heat baked much of the Northern Hemisphere this summer, the arrival of fall and cooler temperatures may come as a relief to many. The change in seasons is marked by the autumnal equinox, which arrives Saturday at 2:50 a.m. Eastern time.
The autumnal (or fall) equinox is the halfway point between the summer and winter solstices and marks the precise moment when the sun appears directly over the Earth’s equator.
The exact time of the equinox varies each year. While the autumnal equinox usually occurs on Sept. 22 or 23, it occasionally falls outside those dates. An early Sept. 21 equinox last happened more than 1,000 years ago, but it will happen again later this century — in 2092 and 2096, according to timeanddate.com. On the other hand, a late Sept. 24 equinox occurred as recently as 1931 but won’t happen again until 2303.
What happens on the equinox?
During the September equinox, the sun’s direct rays cross Earth’s equator into the Southern Hemisphere, where winter is ending and spring is beginning. In the Northern Hemisphere, we’ll continue to lose daylight until the winter solstice, as the sun traces a shorter and lower path across the sky. The reduced sunlight is the main reason trees change color 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.
Equinoxes and seasons happen because the Earth doesn’t orbit the sun completely upright. Because it is tilted on its axis by about 23.5 degrees, the Northern and Southern Hemispheres receive different amounts of sunlight throughout the year. On the spring and autumnal equinoxes, however, both hemispheres receive equal amounts of the sun’s energy, causing day and night to be nearly equal everywhere on Earth.
Day and night are not quite equal
Though “equinox” comes from the Latin words “aequus” (equal) and “nox” (night), the Earth has more than 12 hours of daylight during the equinox.
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Washington has 12 hours 7 minutes of daylight on the equinox (sunrise on Saturday is at 6:56 a.m. and sunset at 7:03 p.m.). However, the “equilux” — the day when sunrise and sunset are 12 hours apart — happens three days later. In most of the United States, the equilux occurs on Sept. 25 or 26. Not until March 16 will the sun again grace our skies for at least 12 hours.
There are two reasons we have more than 12 hours of daylight on the equinox.
One is how we define the term “sunrise” and “sunset” to measure the length of a day. The sun appears as a disk, not a discrete point in the sky like a nighttime star. Sunrise occurs the moment the sun’s upper edge appears on the horizon, while sunset doesn’t happen until the sun’s upper edge completely dips below it. If you’ve ever watched a sunset, you’ve likely noticed that it takes a few minutes for the sun to fully disappear from the sky.
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Moreover, the Earth’s atmosphere can refract, or bend, the sun’s light. This optical illusion allows us to see the sun at sunrise and sunset when it’s technically below the horizon.
Share this articleShareTogether, these two factors — atmospheric refraction and how we measure the length of daytime — add several minutes of daylight to the equinox. Near the equator, the sun is up for 12 hours and 6 minutes, while Earth’s polar regions see more than 12 hours and 20 minutes of daylight.
Rapidly losing daylight
The fall equinox is when we experience the fastest loss of daylight, although the rate of change depends on your latitude, or distance from the equator.
Near the fall equinox, Washington loses 2 minutes and 30 seconds of daylight per day, while Miami loses only 90 seconds. At higher latitudes, the loss of light is more dramatic: Seattle’s daylight vanishes by nearly 3½ minutes each day, and in Anchorage, the difference is more than 5½ minutes.
The aurora peak near the equinoxes
The best season for seeing the aurora is around the spring and fall equinoxes. For more than a century, scientists have observed that auroras usually peak in March and October and tend to decline around the June and December solstices. This seasonal pattern in geomagnetic activity happens because the magnetic fields of the Earth and sun are optimally aligned near the equinoxes.
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The Earth and sun each have a magnetic field with a north and south pole. Geomagnetic storms that cause auroras are stronger and more frequent when the Earth’s and sun’s magnetic fields point in opposite directions — much like two magnets oriented opposite one another.
Near the equinoxes, Earth’s magnetic north and south poles are aligned at right angles to the flowing solar wind. “During these times, the solar wind is effectively stronger, enhancing magnetic storms,” EarthSky explains.
What to expect for the fall temperature outlook
While shorter days and longer nights inevitably bring chillier weather as we head into autumn, some areas of the country will be relatively warmer than others. The National Oceanic and Atmospheric Administration is forecasting above-normal temperatures along the East and West Coasts, the Gulf Coast and around the Great Lakes through December. Meanwhile, much of the Plains and South have an equal chance of seeing temperatures either warmer or cooler than average this year.
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