Today, March 20, 2023, at 10:01 am, the spring equinox occurs, the astronomical event that officially marks the end of winter and the beginning of the spring season.
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On this day, the length of day and night are equal: 12 hours of light and 12 hours of darkness. This phenomenon is due to the fact that the Sun's rays, during the equinox, are perfectly perpendicular to the equator, creating a balance between the Northern Hemisphere and the Southern Hemisphere in terms of hours of light and darkness.
Today the length of day and night are equal: 12 hours
Starting today, for the next six months, the northern hemisphere will be the most illuminated. As the National Institute of Astrophysics recalls, the term "equinox" comes from the Latin aequa-nox, which means "equal night", to indicate the parity between hours of light and hours of darkness.
Contrary to popular belief, the spring equinox does not always fall on March 21. In reality, the date oscillates between March 19, 20, and 21, due to the Earth's motion around the Sun and the non-perfectly linear length of the tropical year.
A solar year, in fact, lasts 365,2422 days, almost a quarter of a day more than a calendar year. This difference means that, every four years, a delay of about one day accumulates, which is why the leap year was introduced with the addition of February 29.
This variation affects not only the spring equinox, but also the autumn equinox and the summer and winter solstices, which shift slightly each year. If the year were an exact multiple of the length of a day, the seasons would always begin on the same date, but the fractional nature of the solar cycle makes this regularity impossible.
Article published on March 20, 2025 - 08pm
The spring equinox is a very interesting phenomenon and its explanation is clear, but there are many who do not know that it is not always on March 21st. I would like to know more about how this affects other seasons.
I always thought the equinox was fixed, but now I understand that it varies. It's strange how one day more or less can change the dates of seasons and holidays. Very useful information.
The article explains the equinox well, but some things don't add up for me. Why does a year have fractional days? And how does this affect the equinox? I hope there is more clarity in the future.