The centuries-old myth shattered: Day and night are not equal after all

On the day of the autumnal equinox — September 23 — it has been revealed that daylight and nighttime do not actually last exactly 12 hours each.

TodayPress TV reports that experts at the EarthSky portal explained that the main reasons are atmospheric refraction and the fact that the Sun appears as a disk rather than a point. As a result of these factors, people receive approximately eight minutes more daylight than expected.

The idea that daytime and nighttime each last exactly 12 hours during the equinox is widespread. However, astronomers say that at mid-latitudes, daylight always lasts slightly longer.

The first reason is that the Sun appears as a disk rather than a point source of light. In astronomy, sunrise is considered the moment when the upper edge of the celestial body first touches the horizon, while sunset is the moment when the final edge of the disk disappears below the horizon. This adds approximately 2.5 minutes to daylight.

A more significant factor is atmospheric refraction, or the bending of light rays. Earth’s atmosphere acts like a giant lens, making the Sun’s disk appear approximately half a degree higher above the horizon than it actually is. As a result, when we see the Sun on the horizon, it is already geometrically below the Earth’s surface.

This optical effect causes the Sun to appear to rise earlier and set later, adding approximately six minutes to the duration of daylight. Thus, on the day of the equinox, daytime lasts slightly longer than 12 hours.

The date when daytime and nighttime are actually equal is known as the “equilux.” In the Northern Hemisphere, this occurs several days after the autumnal equinox and several days before the vernal equinox.

The exact date of the equilux depends on the geographic latitude of a particular location. For example, at the equator, there is technically no such day, as daylight lasts slightly longer than 12 hours throughout the year.

The main difference between the equinox and the equilux is that the equinox is a specific astronomical moment when the center of the Sun crosses the celestial equator, and this moment is the same for the entire planet.

Thus, the idea that light and darkness are divided into exactly 12 hours each on the day of the autumnal equinox is merely a widespread myth. Atmospheric refraction and the apparent size of the Sun cause daylight to last slightly longer.

It should be noted that the autumnal equinox is an important astronomical date marking the change of seasons.

Giymat Mahir