Are day and night equal at equinoxes?

September 23 is the equinox. The word means “equal night.” Days and nights are nearly equal now at the equinox, but not quite. Here’s why. Also, we’ve got a new word for you, “equilux.” It’s the word for when day and night are, in fact, equal.

Wide partial arc of setting sun, glowing orange below and yellow above.

Flattened sunset by Helio C. Vital in Rio de Janeiro, Brazil. A flattened sunset is an effect of atmospheric refraction. Refraction also gives us a few more minutes of daylight on the equinox than we would have otherwise.

The upcoming equinox – the Northern Hemisphere’s autumn equinox and Southern Hemisphere’s spring equinox – falls at 07:50 UTC on Monday, September 23, 2019. For North American time zones, that’s September 23 at 3:50 a.m. EDT, 2:50 a.m. CDT, 1:50 a.m. MDT and 12:50 a.m. PDT. Twice a year – on the March and September equinoxes – everyone worldwide supposedly receives 12 hours of day and 12 hours of night. Generally speaking, that’s true. But, precisely speaking, there is more daylight than nighttime on the day of the equinox, an additional eight or so minutes of daylight at mid-temperate latitudes. There are two reasons why we have more than 12 hours of daylight on this day of supposedly equal day and night. They are:

The sun is a disk, not a point

Atmospheric refraction

Keeping reading to understand more about why, and to learn a new word, equilux:

Row of suns descending over dimly silhouetted Arabic-style domes and towers of Taj Mahal.

Abhinav Singhai created this beautiful composite image of the sunset over the Taj Mahal.

The sun is a disk, not a point. Watch any sunset, and you know the sun appears in Earth’s sky as a disk.

It’s not pointlike, as stars are, and yet – by definition – most almanacs define sunrise as when the top of the sun first touches the eastern horizon. They define sunset as when the sun’s trailing limb finally touches the western horizon.

This in itself provides an extra 2 1/2 to three minutes of daylight at mid-temperate latitudes.

Earth on left with arrows to real and apparent position of sun on right.

Atmospheric refraction actually raises the sun about 1/2 degree upward at sunrise and sunset. This advances the sunrise yet delays the sunset, adding several minutes of daylight at each end of the day. Image via Wikipedia.

Atmospheric refraction. The Earth’s atmosphere acts like a lens or prism, uplifting the sun about 1/2 degree from its true geometrical position whenever the sun nears the horizon. Coincidentally, the sun’s angular diameter spans about 1/2 degree, as well.

Therefore, atmospheric refraction advances the sunrise and delays the sunset, adding nearly another six minutes of daylight at mid-temperate latitudes.

Astronomical almanacs usually don’t give sunrise or sunset times to the second. That’s because atmospheric refraction varies somewhat, depending on air temperature, humidity and barometric pressure. Lower temperature, higher humidity and higher barometric pressure all increase atmospheric refraction.

On the day of the equinox, the center of the sun would set about 12 hours after rising – given a level horizon, as at sea, and no atmospheric refraction.

Person sitting on hillside viewing setting sun over the sea.

Contemplating the sunset on the Philippine island of Leyte. Photo by Abie Oquias Baybay.

What is an equilux? Here’s a new word for you, equilux. The word is used to describe the day on which day and night are equal. The equilux happens a few to several days after the autumn equinox, and a few to several days before the spring equinox.

Much as earliest sunrises and latest sunsets vary with latitude, so the exact date of an equilux varies with latitude. That’s in contrast to the equinox itself, which is a whole-Earth event, happening at the same instant worldwide. At and near the equator, there is no equilux whatsoever, because the daylight period is over 12 hours long every day of the year.

Visit timeanddate.com for the approximate date of equal day and night at your latitude

Bottom line: There are two reasons why we have more than 12 hours of daylight on the day of the equinox. First, the sun is a disk, not a point of light. Second, the Earth’s atmosphere refracts (bends) sunlight. These factors add up to provide an additional eight or so minutes of daylight on the day of the equinox at mid-temperate latitudes.

Bruce McClure