The equinox occurs at 4:02 p.m. Eastern Time on Friday. Here is a guide to the event, told through eye-catching maps.
1. What happens on the equinox?
At the moment of the equinox, the sun appears straight overhead along Earth’s equator. Because neither hemisphere is tilted toward the sun, all latitudes have roughly 12 hours of daylight and darkness. We see this in the map above, where the distribution of daylight and darkness appears symmetrical in both hemispheres.
The September equinox is one of only two days each year when all points on Earth — apart from the polar regions — see the sun rise due east and set due west along the horizon. From now until the winter solstice in December, the sun will continue to rise and set farther to our south.
Hypothetically, if you spent the equinox at the North Pole, you’d see the sun start to set for the first (and last) time in six months. At such high latitudes, however, the transition from day to night doesn’t happen at the flip of a switch. Even though the sun will stay below the horizon until March, you’d first witness about a month of twilight before true darkness, or polar night, sets in.
2. When is sunset on the equinox?
This map — the first of several by Alaska-based climatologist Brian Brettschneider — shows the time of sunset across North America. In much of the country, the sun now sets before 7:30 p.m. (including in the District, where sunset is at 7:04 p.m. Friday). If you live in a purple-shaded area, such as New York or Chicago, the sun is already down before 7 p.m.
Notice on this map how the color contours are oriented straight up and down instead of slanted like they are on the solstices. That’s because on the equinoxes, the daylight terminator — the shadow separating day and night across Earth — runs parallel to Earth’s lines of longitude.
The vertical orientation of the day-night shadow means that if you were to travel exactly due north or south from any location, the time of sunset wouldn’t change, something that only happens on the spring and fall equinox.
3. How fast are we losing daylight?
It depends where you live, but in most places the days are getting rapidly shorter now. The Northern Hemisphere loses daylight at its fastest pace around the fall equinox, which is why the increased darkness becomes quite noticeable in September.
The map above shows that most parts of the Lower 48 lose about two to three minutes of daylight on the fall equinox. The District loses 2 minutes and 30 seconds of daylight each day, while cities across the northern tier lose close to three minutes or more. In much of Alaska, daylight dwindles by more than six minutes per day!
Add it all up and over the course of a month, most of us lose more than an hour of daylight in September, as shown in this next map:
While the southernmost tier — including Louisiana and Florida — loses less than an hour of daylight, northern states near the Canadian border lose more than 90 minutes of morning and evening light in September. Across the midsection of the Lower 48 — a line stretching from San Francisco to the District — the total amount of daylight lost is about 1 hour and 15 minutes.
4. How much more daylight do we lose from now until December?
We’ve got another three months to go until the shortest day of the year. From the equinox until the winter solstice on Dec. 21, most of us lose about another three hours of sunlight.
How much more daylight you’ll lose again depends on your latitude. Florida and southern Texas lose less than two hours over the next three months, while the northern tier loses closer to four hours. Then there’s Alaska, where the next three months will bring an additional six to 12 hours of darkness, from south to north.
5. Why are there more than 12 hours of daylight on the equinox?
A common misconception is that the equinox means day and night are equal everywhere on Earth. In reality, the sun is up for more than 12 hours, as we see in the map below:
In the District, sunrise and sunset occur at 6:56 a.m. and 7:04 p.m., respectively, resulting in 12 hours and 8 minutes of daylight. The date of the “equilux” — when sunrise and sunset are exactly 12 hours apart — happens around Sept. 25 or 26, as shown in this last map:
So why are there more than 12 hours of daylight on the equinox? It boils down to two reasons.
One is atmospheric refraction. This optical phenomenon bends the sun’s light as it passes through Earth’s atmosphere and causes the sun to appear slightly higher in the sky than it is in reality.
The other reason is how we define sunrise and sunset. The sun appears as a disk, not a single point. Sunrise is defined as the moment the sun’s upper edge appears on the horizon, while sunset doesn’t occur until the sun’s upper edge disappears from the horizon. Together, these factors add about 10 minutes of daylight to the equinox, depending on one’s distance from the equator.
The bottom line is we get an extra three or four days before nighttime gains the upper hand. Even as summerlike warmth dominates the eastern United States for the next week, it won’t be long before temperatures tumble. So if you’re ready for crisp mornings, apple picking and colorful fall foliage, the September equinox is a reminder that autumn is on our doorstep.