Astronomical spring will begin on Friday at 3:46 PM, when the spring equinox occurs, according to HungaroMet Zrt.
They noted that although textbooks usually mark the event as March 21, it can fall on either March 20 or 21. The date “shifts” because a year is not an exact multiple of a day—it lasts precisely 365 days, 5 hours, 48 minutes, and 46 seconds. This is why leap days are necessary, as the difference adds up to nearly 24 hours over four years.
They also highlighted that the Earth’s orbital parameters—such as its eccentricity, the position of the vernal equinox, and the precession of its axis—are not constant, although these cause only minor variations. Over the 400-year cycle of the Gregorian calendar, the timing of the spring equinox shifts by only one or two hours.
The last time the equinox fell on March 21 was in 2011, and in 2048 it will occur on March 19 for the first time.
HungaroMet also pointed out that, despite its name, day and night are not exactly equal in length during the equinox. One reason is atmospheric refraction, which makes celestial bodies appear slightly higher above the horizon than they actually are. This effect is strongest at the horizon, causing the Sun to appear a few minutes earlier at sunrise and set a few minutes later at sunset, slightly lengthening the day.
Another reason is that on the day of the equinox, it is the geometric center of the Sun that spends 12 hours above the horizon. However, because the Sun has a visible size, it is already partly visible before its center rises and remains visible even after its center has set.
