Astronomical spring began on Wednesday, i.e. this morning, at 4:06 a.m. – HungaroMet Zrt. announced on its website, where it drew attention to the fact that textbooks date the event to March 21, but it could fall on the 20th or 21st too.
The date of the equinox “slips” because an earthly year is not an exact integer multiple of the earthly day, to be exact 365 days, 5 hours, 48 minutes, 46 seconds. This is the reason why the leap day had to be introduced, as the difference is almost 24 hours in four years, they explained.
They pointed out that, in addition, the Earth’s orbital parameters – its eccentricity, the location of the vernal equinox, the precession of the rotation axis – are also not constant, but these only cause very small deviations, the time of the spring equinox is only one or two hours during the 400 years of the Gregorian calendar changes.
The vernal equinox last fell on March 21, 2011, and it will happen for the first time on March 19, 2048, they wrote.
HungaroMet Zrt. also explained that during the equinox, day and night are not of equal length, despite the name.
One of the reasons for this is that, due to atmospheric refraction, celestial bodies appear a little higher above the horizon than their true location. This means that the Sun appears on the horizon at sunrise a few minutes earlier than it actually does. In the same way, at sunset, it seems to rest that much later. This phenomenon increases the length of days by a few minutes.
The other reason is that on the day of the equinox, the geometric center of the Sun is above the horizon until noon, but the Sun has an extension: that is, it can be seen even when its center is not yet, or even when it is radiating when it has settled at its center.
For this reason, at this time, or even exactly, there is never an equality of day and night, but on March 16, the length of day and night is almost equal.
(MTI)
