In Germany and in the entire northern hemisphere, the astronomical spring begins this year in the late evening of March 20th. The calendar day on which the sun moves over the equator is also called the equinox. Can the term keep what it promises?
Claim: Day and night are exactly the same length at the beginning of spring.
Rating: Not quite.
Facts: In the late evening of March 20, spring begins in the northern hemisphere and thus also in Germany – at least according to astronomers. That depends on the position of the sun – and not, like the phenological one, on the weather. On this day, shortly before 10:30 p.m., the sun is exactly vertical over the equator for a moment.
“The same thing happens at the beginning of autumn,” explains astronomer Uwe Wolter from the Hamburg Observatory. “At these two points in the year, the equinoxes, the sun crosses the zenith exactly on the equator.” Day and night are about the same length everywhere in the world – with the exception of the two poles. This can also be derived from the Latin term equinox: aequus means “equal”, nox means “night”. In theory, day and night at the beginning of spring should be exactly twelve hours long. “In reality, however, the day at the equinoxes is a bit longer than the night everywhere in the world,” says astronomer Wolter. Because the equinoxes are calculated without including the earth’s atmosphere, an important component is missing with the air. “The sunlight is refracted in the atmosphere and the sun is lifted a little near the horizon,” explains the astronomer.
What does that mean in practice?
In practical terms, this means that people on Earth can already see the sun in the sky in the morning, even though it is still below the horizon. In the evening it still shines, although it has actually already disappeared behind the horizon. “The length of the day at the equinoxes without an atmosphere would be twelve hours to the second,” says Wolter. “But of course we want to consider our day length including the air.”
There is also another effect: when calculating the equinox, reference is made to the center of the sun. “But of course the center of the sun does not reach the horizon exactly at sunrise or sunset,” explains Wolter. When you look at a sunrise, the upper edge of the sun appears first. And even at sunset, the upper edge of the sun is still visible for a few minutes longer.
If you look at the times for sunrise and sunset, the whole thing can be understood. In Hamburg, for example, the sun rises at 6:23 a.m. on March 20 and sets again at 6:32 p.m. In Munich, the day begins at 6:17 a.m. and it gets dark at 6:25 p.m. And in Cologne, where it gets light at 6:35 a.m., night begins at 6:44 p.m. The bright part of the day is always a few minutes longer.
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