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What Is the Equilux?

Twice a year, day and night reach a perfect balance of 12 hours each, creating a little-known event called the equilux.

Body of water with hills in the background at twilight, with a clear sky on the left and a dark, cloudy sky on the right.

The equilux brings equal hours of day and night.

©iStockphoto.com/Tamar Nadiradze

Equinox—Close, but Not Quite Equal

Many of us think that an even balance of day to night happens during an equinox. After all, the word translates as “equal night.”

So, a little confusion is understandable. But there’s a subtle time difference between an equinox and an equilux.

Approx. equilux dates

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

Equal Light

“Equilux” is drawn from the Latin terms for equal (equi) and light (lux). So how do we find out which dates fit the description and qualify as truly equal day and night?

To measure the day/night split in a 24-hour span, astronomers use common definitions of sunrise and sunset. Simply put, sunrise is defined as when the first bit of the Sun’s disk appears and sunset is when the last bit of the disk vanishes.

Calculating the length of day between those two moments, we find that two dates every year reach equilux in most latitudes.

In the Northern Hemisphere, these happen a few days before the spring equinox (vernal equinox) and a few days after the autumn equinox. South of the equator, it's the other way around.

A location's equilux dates depend on the latitude (see table). Locations on or near the equator never experience equal day and night.

Imagine a Disk

But why aren’t day and night of equal length at an equinox?

To answer that question, it helps to think of the Sun in two different ways—as a disk and as a point.

To pinpoint the days of equilux, the Sun is considered a disk, and we measure daytime from the first appearance of the Sun’s disk to the last bit slipping below the horizon.

A Point at the Center of the Sun

To calculate an equinox, on the other hand, the Sun is thought of as a single point, set in the center of the disk. An equinox occurs when the subsolar point—the spot on the Earth directly beneath the Sun—crosses the equator, equally straddling the Southern and Northern Hemispheres.

On those days, the center point of the Sun indeed rises and sets 12 hours apart.

But since we measure sunrise and sunset by thinking of the Sun as a disk, the top edge of the Sun appears a little earlier and sets a bit later than the center point. This difference creates a few extra minutes of daylight on the date of an equinox at most latitudes.

Bright orange Sun setting behind dark silhouette of forest and river.

Sunrise and sunset are determined by viewing the Sun as a disk, not a singular point


Atmospheric Bending at Work as Well

Another variable is the fact that the Earth’s atmosphere bends light from the Sun, like a lens. This distortion makes the Sun appear higher than its true position on the horizon.

This light bending action, known as atmospheric refraction, means that at most mid-temperate latitudes, sunrise starts earlier and sunset ends later, making the day of an equinox a bit longer than the night.

Plus, the local weather can have an effect on the amount of atmospheric refraction. Temperatures, humidity, and barometric pressure all influence the degree of light bending.

Want to learn more about how our calculators deal with atmospheric bending? Take a look at “Does the Sun Calculator take into account refraction?”