On Sunday, September 22, 2024, at 15:43, the Sun will cross the Equinox point marking the Autumn Equinox. From that moment until the Spring Equinox on March 20, 2025, the Sun will be above places in the Southern Hemisphere, resulting in less heating in the Northern Hemisphere, which will transition from autumn into its winter period.
However, even though we call that day an equinox, will the durations of day and night actually be equal on Sunday? The answer is no!
Apart from places around the Equator, where day and night have practically equal durations all year round, in other regions, their durations differ on the day of the equinox.
More specifically, in mid-latitudes, like Greece, the duration of the day will be a few minutes longer. The reasons are twofold: the fact that the Sun is not a point but a circle with a diameter of about half a degree, and atmospheric refraction.
– Due to its dimension, the solar disk requires approximately two and a half to three minutes in our mid-latitudes during the equinoxes for the top of the upper arc to reach the horizon from the moment the lower arc touches it.
– Furthermore, due to atmospheric refraction, the Sun appears a few minutes earlier on the horizon at dawn and disappears a similar amount later at sunset. The exact amount is not fixed, as the refraction index of the air depends on temperature, humidity, and atmospheric pressure.
For these two reasons (geometry of the solar disk and atmospheric refraction), the duration of daylight will be at 12:08:49 and not 12:00:00.
An equal duration of day and night, or Equilux, will occur a few days later, on September 25. We always have Equilux a few days after the Autumn Equinox and a few days before the Spring Equinox!
HAVE A GOOD AUTUMN.
Χ.Τ.
Edited by: Zacharias Zacharopoulos