It's not a whole day. It is the exact moment when the sun crosses the equator and the days and nights are equal in hours in the Northern Hemisphere.
Vernal translates to "new" and "fresh," and equinox derived from the Latin aequus (equal) and nox (night).
Essentially, the period of time each day between sunrise and sunset has been growing slightly longer each day since the winter solstice in December — the day of the year with the least amount of daylight.
After three months, we still see less light than darkness over the course of a day. The vernal equinox marks the turning point when daylight begins to exceed darkness.
Tomorrow, the direct rays of the sun will shine down on the equator, producing the effect of equal day and night (give or take a few minutes).
After the vernal equinox, the direct rays of the sun migrate north of the equator (with hours of daylight steadily growing longer) until they finally arrive at the Tropic of Cancer (latitude 23.5 degrees north).
The migration of the sun’s direct rays comes to a halt on that day; this is as far north as they will go. We call this the summer solstice (solstice is a suspension of the migration of the sun’s direct rays). It is the longest day of the year in terms of hours of daylight.
After the summer solstice, the direct rays proceed to head south and the days begin to grow shorter. It will take another three months, until the autumnal equinox, for the periods of daylight and darkness to reach equilibrium once again. The rays ultimately reach the Tropic of Capricorn (latitude 23.5 degrees south) on the day of the winter solstice and the whole cycle begins again!
On Monday, the Farmers' Almanac published a fact vs. fiction article on the five common myths associated with the spring equinox, written by Amber Kanuckel. Here are some highlights:
Can you stand a raw egg on end?
This myth holds that the sun’s presumed gravitational pull on the Earth, and its position in the sky, means that you can stand an egg up on end during the precise moment of the vernal equinox. Verdict: equinoxes don't make this feat any easier than on any other day.
Is it easier to balance things?
Again, some believe the sun’s gravitational pull on the Earth during the equinox makes it easier to balance objects like brooms. Verdict: as with the myth about balancing eggs, the sun’s gravitational pull doesn’t affect any balancing act you’re trying to pull off.
Do you cast a noontime shadow?
Depends on where you are: conditions have to be incredibly precise for this to happen. When the Sun is at at an angle to you, you always cast a shadow. In order not to cast a shadow, the sun needs to be directly overhead. Because the Sun is situated over the equator at the equinox, you’d have to be standing at the equator precisely at noon on the day of the equinox for this to happen.
For more, visit the Farmers' Almanac website and view Kanuckel's full article.
Source: Farmers' Almanac; "5 Spring Equinox Myths Debunked," by Amber Kanuckel, www.FarmersAlmanac.com, March 18, 2019.