You might think that you have a handle on this whole basic idea of what the equinox is, but chances are you're not as clear on it as you think. So what is an equinox? Most of the time, people will tell you that it's the 24-hour period when day and night are the same length — but that's not actually technically accurate. Here's what it really means.
Although we typically label a certain day as the equinox — either the autumnal or vernal one, depending which month we're talking about and which hemisphere you're in — in actuality, the equinox is a single moment, specifically the moment that the sun crosses the equator. You see, every day, the planet Earth rotates around its axis — which is like an imaginary line running between the North and South poles — but that axis is actually set at an angle, not up and down. And, crucially, the angle of Earth's axis doesn't change as the planet revolves around the sun. This means that either the Northern or Southern Hemisphere is usually tilted towards the sun, meaning that half of the Earth receives more sunlight — and is also the reason the two hemispheres have opposite seasons. And this tilt also means that, from the perspective of someone on Earth's surface, the angle of the sun appears to change.
The equinox, then, is the moment when the sun crosses the celestial equator, when it switches from shining mostly on the Northern hemisphere to shining mostly on the Southern hemisphere or vice versa. And that happens not just on a day, but during a single instant — or rather, during two instants every year: One in March and another in September.
So, since the equinox is all about the sun being evenly divided between the two hemispheres, doesn't that mean that day and night actually are equal length?
Although the sun is shining directly on both hemispheres on the equinox, that doesn't automatically mean there's an equal amount of daylight. This is due to a phenomenon known as refraction. Earth's atmosphere refracts sunlight, meaning the light from the sun bends, and thus the sun is technically "visible" for a few minutes before and after it's actually directly visible from a given point on Earth. In fact, every day on Earth is about six minutes longer, on average, than it would be if we were just seeing the sun directly, without light being refracted in the atmosphere. Thus, even though the sun is shining equally on the equinox, day and night technically aren't the same length.
So when are day and night equal?
That depends, too, based on where you live. For instance, the upcoming September equinox in 2016 happens on Sept. 22 — but if you live in, say, Miami, day and night are both 12 hours long on Sept. 27. If you live in Chicago, on the other hand, that happens on the 25th. And if you head to the Southern Hemisphere, day and night are roughly equal in Sydney, Australia on Sept. 19.
On the day of the equinox, on the other hand, day and night are almost equal, but the days are about seven or eight minutes longer because of refraction.
Which isn't to say that the whole day isn't significant — the two hemispheres are getting an equal amount of sunlight. It's just not quite equal to the amount of night time we're also experiencing.
So congratulations! You are now probably better informed than most of your friends. Happy equinox, everyone!