Today is the vernal equinox, what a lot of folks think of as the first day of spring (though given the forecast, people on the U.S. East Coast can be forgiven if they’re rolling their eyes at that thought, assuming their eyeballs aren’t frozen to their eyelids).
There are many definitions of the vernal equinox; it’s when the center of the Sun is directly over the Earth’s equator, or it’s when the Sun’s coordinates on the sky (the celestial equivalent of latitude and longitude) are 0°, 0h. That happens today at 16:57 UTC (12:57 p.m. EDT).
One definition is a bit wonky, though. Equinox means literally “equal night,” the time when night and day are the same length. But that’s not really the case!
Daytime starts at sunrise, which starts when the top of the Sun pokes over the horizon. But nighttime starts when the top of the Sun dips below the horizon, and it disappears. For day and night to be equal length on the equinox, you’d have to measure from when the middle of the Sun rises to when that same point sets! It depends on your latitude (because that changes the angle at which the Sun rises and sets, which in turn changes how long the process takes), but actually daytime is a few minutes longer on the equinox than nighttime.
In fact, it gets worse. Because the Earth has air, it changes the length of daytime too. The air acts a bit like a lens, bending sunlight when the Sun is near the horizon. The light is bent down, but in the end the effect is to make it look like the Sun is above the horizon when technically (if we had no air) it would have already set. The same is true at sunrise, making the Sun rise earlier than it would if we had no air. This makes daytime even longer.
So even the name “equinox” is a lie. And don’t even get me started on calling it the first day of spring.
Finally, of course, seasons are opposite in the Southern Hemisphere, so a better name for this would be the March equinox, since for them it’s a harbinger of colder days as winter sets in.
Therefore, I propose we call it the “March and not Vernal because of the Southern Hemisphere not quite equinox so let’s say it’s the quasinox.”
Rolls right off the tongue, doesn’t it?
And because why not: Here is a time-lapse video showing a year of sunrises, taken by a weather satellite in a geosynchronous orbit. I explained how that all works in an article about the autumnal equinox in 2011. Enjoy.