The vernal equinox will occur on March 20 at 19:15 Moscow time, when the Sun will be directly above the equator. But it turns out that not everywhere on Earth on this day is equal to night. Why?
Sunrise for Moscow 2018-20-03 - 6:32, and sunset - 18:42. Longitude of the day: 12 hours 10 minutes 23 seconds. It turns out that on March 20 the day will be 10 minutes longer than the night.
And the equinox, when the day is equal to the night, actually occurs several days before the specified date. In particular, in Moscow the day was almost equal to the night of yesterday, March 18 (12 hours 1 minute).
So why are day and night not equal at the equinox?
First of all, the Sun is a disk, not a point source of light. Therefore, sunrise is the moment when the upper edge of the solar disk rises above the horizon, and sunset is when the same edge drops below the horizon. It takes a few minutes for the solar disk to travel across the horizon (from edge to edge).
The picture is complemented by the optical effect of refraction of sunlight by the atmosphere. The observer sees the Sun beginning to appear above the horizon, although in fact it is still below the horizon. Likewise, at sunset: it has already disappeared, but the observer still sees it.
The combination of these two factors makes the equinox day 10-15 minutes longer than the night, depending on the latitude.
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This is how nature is deceiving!