“What kind of equinox is this, when the day is 18 minutes longer than the night?”, asks the emeritus professor of the AUTH Physics Department, Charalambos Varvoglis
The vernal equinoxtoday March 20, 2024, marks the beginning of this year’s spring in Greece and in general in the northern hemisphere.
However, the astronomical term “equinox” often leads to the misunderstanding that this day marks the equal length of day and night.
However, in Greece today the day is longer than the night.
The equal duration occurred a few days earlier, on March 17.
As pointed out by the emeritus professor of the Physics Department of AUTH, Charalambos Varvoglis, if we look for the sunrise and sunset times of this day we will be surprised to see that, for example, the sunrise in Elliniko, Attica is at 6:28 and the sunset at 18:00: 37.
“What kind of equinox is this, when the day is 18 minutes longer than the night? The answer is simple: at the equinox we would indeed have equal lengths of day and night if the Sun were a point (geometry) and if the Earth had no atmosphere (refraction)“, he explains.
In particular, since the Sun is a disc and not a bright point, sunrise and sunset do not occur instantaneously.
Mr. Varvoglis clarifies that the time that elapses between the rising of the edge of the Sun and the rising of its center is very easy to find for places located on the equator, where the Sun rises and sets perpendicular to the horizon, and corresponds to one first minute. But in middle latitudes, such as Greece (it has a latitude of about 40 degrees) this time is longer and is equal to about 1.5 minutes. As the difference in the west is also so much, just because of this phenomenon at the equinox the day in Greece would last 12 hours and 3 minutes and the night 11 hours and 57 minutes, so the difference would be 6 minutes.
Regarding the phenomenon of the refraction of sunlight in the Earth’s atmosphere, Mr. Varvoglis observes that the Earth’s atmosphere refracts the Sun’s rays, so that at sunrise it appears to be higher than it really is.
So due to refraction at the equinox, the day would be 12 minutes longer than the night. As the two effects of geometry and refraction are added, concludes Mr. Varvoglis, the difference of 18 minutes results.
Dr. Tina Nantsou, scientific associate of the Physics Department of EKPAsuggests at the vernal equinox, “to carry out the historical experiment of the ancient mathematician Eratosthenes and with a ruler or rod and our minds to calculate the circumference of the Earth». It is an experiment of the 3rd century BC. with which Eratosthenes managed to calculate the circumference of the Earth in a geometric way.
The method is still used today for educational reasons and the equinoxes and solstices are ideal dates for its implementation.
Source: Skai
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