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Equinox: Almost Equal Day and Night

Equinox in Latin means equal night, giving the impression that the night and day on the equinox are exactly 12 hours long. And even though this is common wisdom, it isn't entirely accurate.

Sun setting in the background with cloudy sky and trees in the foreground.

The day is just a bit longer than the night on an equinox.

©iStockphoto.com/shanecotee

The day and night are only nearly equal on the September equinox and the March equinox. In fact, most locations on Earth tend to enjoy more daylight hours than nighttime hours on these two days of the year, thanks to the Earth's atmosphere and our definition of sunrise and sunset.

More Than 12 Hours of Sun

Equinox and solstice illustration.
Equinox and solstice illustration.

On the equinox, the tilt of Earth’s axis is perpendicular to the Sun’s rays as in the illustration.

At 13:30 UTC on September 22, 2020, the Sun will appear to cross the Earth’s imaginary celestial equator from the north to the south. This exact moment marks the beginning of the Northern Hemisphere’s astronomical fall/autumn and the Southern Hemisphere’s astronomical spring.

On this day, New York City residents will enjoy a 12 hour and 8 minutes long day. Those in Amsterdam will get three more minutes of daylight with a 12 hours and 11 minutes long day. Even in Longyearbyen, one of the world's northernmost cities, the day will last for 12 hours and 34 minutes.

Moving southwards, Melbourne in Australia will see a 12 hours and 7 minutes long day. Johannesburg inhabitants will also enjoy a similar number of daylight hours.

On the South Pole, the Sun will be up 24 hours. Perfect opportunity to see the Midnight Sun!

On the equator, the day is always a little longer than 12 hours all year round.

Sun's Upper Edge Defines Sunrise

One of the reasons why most locations on Earth do not enjoy exactly 12 hours of daytime and 12 hours of nighttime on the equinoxes is how sunrise and sunset are defined.

If sunrise and sunset were defined as the moment the geometric center of the Sun passes the horizon, then the day and night would be exactly 12 hours long. But that is not the case. Sunrise and Sunset are defined as the exact moment the upper edge of the Sun’s disk touches the eastern and the western horizon, respectively. The time it takes for the Sun to fully set, which can be several minutes, makes the day just a bit longer than the night on the equinoxes.

Approx. date of "Equal Day & Night"

LatitudeMarchSeptember
60° NorthMar 18Sep 25
55° NorthMar 17Sep 25
50° NorthMar 17Sep 25
45° NorthMar 17Sep 25
40° NorthMar 17Sep 26
35° NorthMar 16Sep 26
30° NorthMar 16Sep 27
25° NorthMar 15Sep 27
20° NorthMar 14Sep 28
15° NorthMar 12Sep 30
10° NorthMar 8Oct 4
5° NorthFeb 24Oct 17
EquatorNo equal day and night
5° SouthApr 14Aug 29
10° SouthApr 1Sep 10
15° SouthMar 28Sep 14
20° SouthMar 26Sep 16
25° SouthMar 25Sep 17
30° SouthMar 24Sep 18
35° SouthMar 24Sep 19
40° SouthMar 23Sep 19
45° SouthMar 23Sep 19
50° SouthMar 23Sep 20
55° SouthMar 23Sep 20
60° SouthMar 22Sep 20

Refraction: Light Lingers

Another reason for why the day is longer than 12 hours on an equinox is because the Earth's atmosphere refracts sunlight.

This refraction, or bending of the light, causes the Sun’s upper edge to be visible from Earth several minutes before the edge actually reaches the horizon. The same thing happens at sunset when you can see the Sun for several minutes after it has dipped under the horizon. This causes every day on Earth, including the days of the equinoxes, to be at least 6 minutes longer than it would have been without this refraction.

The extent of refraction depends on atmospheric pressure and temperature. Our calculations in the Sunrise and Sunset Calculator assume the standard atmospheric pressure of 101.325 kPa and temperature of 15° C or 59° F.

Latitude Determines Day Length

Even if day and night aren’t exactly equal on the day of the equinox, there are days when sunlight hours and nighttime hours are both very close to 12 hours. This day is known as the equilux, and its date depends on a location’s latitude and can occur several days to weeks before or after an equinox.

The table shows approximate dates for when day and night are of similar lengths for some latitudes.

Topics: Astronomy, Seasons, March, September, Equinox, Sun