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Equal day and night - but not quite

"Equinox" means "equal night" in Latin. But even if the name suggests it and it’s widely accepted, it isn't entirely true that day and night are exactly the same on the equinox all over the world – only nearly.

Illustration image

The Earth's axis doesn't tilt away from or towards the Sun on the equinox (ill.not to scale).

When is the equinox in my city?

What is an equinox?

On any other day of the year, the Earth's axis tilts a little away from or towards the Sun. But on the two equinoxes, the Earth's axis tilts neither away from nor towards the Sun, like the illustration shows. The equinoxes occur the moment the sun crosses the celestial equator – the imaginary line in the sky above the Earth’s equator. This happens in March and September every year.

More than 12 hours of sunlight

 Approximate date of "Equal Day & Night"
Latitude         March    September
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
No 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

On these two days, the geometric center of the sun is above the horizon for 12 hours, and one might think this would indicate that the length of the day (hours of daylight) would be the same.

However, ‘sunrise’ is defined as the instant when the upper edge of the sun's disk becomes visible above the horizon – not when the center of the sun is visible. In the same sense, ‘sunset’ refers to the moment the upper edge disappears below the horizon. At both instances, the center of the sun is below the horizon, and therefore the equinox day lasts a little longer than 12 hours.

Atmosphere causes optical illusion

Another reason why the Sun is visible longer than 12 hours on an equinox is that the Earth's atmosphere refracts sunlight.

Refraction causes the Sun’s upper edge to be visible from Earth several minutes before the edge actually reaches the horizon. In the evening, we can see the sun for several minutes after it has actually dipped under the horizon. This causes every day on Earth – including the days of the equinoxes – to appear at least 6 minutes longer than it actually is.

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

Latitude determines length of the day

Even if day and night aren’t exactly equal on the day of the equinox, there are days when day and night are both very close to 12 hours.

However, this date depends on the latitude, and can vary by as much as several weeks from place to place. The table shows approximate dates for when day and night are as similar as possible according to latitude.

On the equator, the day and night stay approximately the same length all year round, but the day will always appear longer than 12 hours, due to refraction and the other factors mentioned above.


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