Are day and night time equal on the equinox?


Equinoxes at proper. Solstices at left. In every of the pictures, Earth’s rotational axis is perpendicular (straight up and down), with north at high and south at backside. Images by way of Geosync.

More day than night time

The upcoming equinox will come on Wednesday, September 22, 2021, at 19:21 UTC. It’s the Northern Hemisphere’s autumn equinox and Southern Hemisphere’s spring equinox. You generally hear it mentioned that, on the equinoxes, everybody receives about equal daylight and darkness. But there’s actually extra daylight than darkness on the equinox, eight extra minutes or so at mid-temperate latitudes. Two elements clarify why we’ve greater than 12 hours of daylight on this day of supposedly equal day and night time. They are:

1. The sun is a disk, not some extent.

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2. Atmospheric refraction.

Read more about the September 2021 equinox: All you need to know

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Silhouette of a man seated on a beach, watching sunset over an ocean.
Contemplating the sundown on the Philippine island of Leyte. Photo by Abie Oquias Baybay.

The sun is a disk, not some extent

Watch any sundown, and you recognize the sun seems in Earth’s sky as a disk.

It’s not pointlike, as stars are, and but – by definition – most almanacs regard dawn as when the forefront of the sun first touches the japanese horizon. They outline sundown as when the sun’s trailing edge lastly touches the western horizon.

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

Are day and night equal at an equinox. No. Diagram shows direction of refracted sun's position as opposed to sun's true position.
Atmospheric refraction raises the sun about 1/2 degree upward in our sky at each dawn and sundown. This advances the time of precise dawn, whereas delaying the time of precise sundown. The result’s a number of minutes of additional daylight, not simply at an equinox, however each day. Image by way of Wikipedia.

Atmospheric refraction

The Earth’s ambiance acts like a lens or prism, uplifting the sun about 0.5 degrees from its true geometrical place at any time when the sun nears the horizon. Coincidentally, the sun’s angular diameter spans about 0.5 levels, as properly.

In different phrases, while you see the sun on the horizon, it’s really slightly below the horizon geometrically.

What does atmospheric refraction imply for the size of daylight? It advances the dawn and delays the sundown, including practically one other six minutes of daylight at mid-temperate latitudes. Hence, extra daylight than night time on the equinox.

Astronomical almanacs normally don’t give dawn or sundown instances to the second. That’s as a result of atmospheric refraction varies considerably, relying on air temperature, humidity and barometric strain. Lower temperature, greater humidity and better barometric strain all improve atmospheric refraction.

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

Are day and night time equal?

So, no, day and night time should not precisely equal on the equinox.

And right here’s a brand new phrase for you, equilux. The phrase is used to explain the day on which day and night time are equal. The equilux occurs a couple of to a number of days after the autumn equinox, and some to a number of days earlier than the spring equinox.

Much as earliest sunrises and newest sunsets fluctuate with latitude, so the precise date of an equilux varies with latitude. That’s in distinction to the equinox itself, which is a whole-Earth occasion, occurring on the identical on the spot worldwide. At and close to the equator, there isn’t any equilux in anyway, as a result of the daylight interval is over 12 hours lengthy each day of the 12 months.

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

Earth, with axis vertical and sun's rays hitting day side perpendicularly.
Illustrations like this one make it appear as if day and night time must be equal on the equinox. In truth, they aren’t precisely equal.

Bottom line: There’s barely extra day than night time on the day of an equinox. That’s as a result of the sun is a disk, not some extent of sunshine, and since Earth’s ambiance refracts (bends) daylight.

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