Autumn equinox: Why day and night are not exactly equal

Tuesday 29 September 2026 - 22:10

By: Azzat Manal

Autumn equinox: Why day and night are not exactly equal

The autumn equinox is approaching in the Northern Hemisphere, marking the astronomical start of the season and the beginning of spring in the Southern Hemisphere. This year, the equinox will occur at 12:05 GMT on September 23, when the Sun will be directly above a point in the equatorial Pacific Ocean, around 400 kilometres southeast of Tarawa, the capital of Kiribati.

The word “equinox” comes from Latin and is commonly associated with the idea of equal day and night. However, despite its name, daylight and darkness are not actually equal in duration on the day of the equinox in most places around the world.

The apparent 12-hour division between day and night is complicated by several astronomical and atmospheric effects. If the Sun were simply a point of light and Earth had no atmosphere, the equinox would theoretically divide the Sun’s path into equal periods above and below the horizon. In reality, atmospheric refraction changes what we see.

As sunlight passes through Earth’s atmosphere, the rays are bent, making the Sun appear higher in the sky than it actually is. This effect is particularly noticeable near the horizon. When the Sun appears as an orange or reddish disk during sunrise or sunset, its actual position may already be below the horizon.

The definition of sunrise and sunset also contributes to the difference. Sunrise is recorded when the upper edge of the Sun first becomes visible, while sunset is marked when the last visible part of the upper edge disappears. The Sun’s center is therefore not used as the reference point.

As a result, many locations experience more than 12 hours of daylight around the equinox. In Pittsburgh, for example, sunrise on the equinox occurs at around 7:07 a.m., while sunset is expected at approximately 7:16 p.m., giving the city roughly 12 hours and nine minutes of daylight. Day and night become closer to equal there a few days later, around September 25.

The effect becomes even more striking near the poles. At the North Pole, the Sun currently appears to trace a circular path around the sky, remaining close to the horizon. Although the equinox theoretically marks the moment when the Sun should disappear from view, atmospheric refraction can keep part of its disk visible for longer.

At the North Pole this year, the final point of the Sun’s upper edge is expected to disappear roughly 50 hours and 44 minutes after the autumn equinox. Strong atmospheric refraction near the horizon can also distort the appearance of the solar disk, making it look flattened or oval rather than perfectly round.

The autumn equinox therefore represents an important astronomical transition, but the familiar idea of a day divided into exactly 12 hours of sunlight and 12 hours of darkness is more complicated than it first appears. The atmosphere, the apparent size of the Sun and the way sunrise and sunset are defined all contribute to making daylight slightly longer than nighttime around the equinox.