Equal Parts, Day and Night, Just Add Egg

It’s official: it’s autumn in the Northern Hemisphere (which is where Astropotamus lives). And as everyone knows, on the equinox, there are equal parts day and night, right? Oh, and that you can balance an egg on its end, too. Surely this is all true?

Shortest post ever: No. Now, let’s take a closer look.

Celestial orbits are simple in one’s mind but complex in reality. Lots of things make our trip around the sun not quite perfect when you look closely at it. Plus, there’s that darned atmosphere. More about that in a moment. Let’s start with what an equinox is.

Latin for equal night (roughly translated), the equinox happens twice a year when the Earth make its tilted, elliptical way ’round the sun and occurs at the exact moment when, if you were standing on the equator, the sun would be exactly directly overhead. Then it’s gone. It happens for a brief moment and it’s over. This happens once in late March and six months later in late September.

There are lots of resources for learning about the equinox and how it’s defined and when it will be (the next time a fall equinox will be on September 21, the day most people think it is, will be 2092). You can go find them yourself. What I want to talk about is why we don’t see equal parts day and night. Oh, and why the equinox doesn’t make eggs stand on end.

“Day” is when the sun is visible with an unobstructed view of the horizon. Today, that’s mostly at sea on a calm day, but use your imagination and pretend there are no trees, buildings, or cell towers. If you stand in your yard (or street or tennis court or flight deck) and look at the eastern horizon in the morning, you will, on a cloudless day, eventually see the sun. But the sun is not a point of light. It’s a huge ball of gas and even from 93,000,000 miles away, it’s so big that it looks like it’s about the size of your pinky fingernail held at arm’s length (so does the moon, by the way, which is why we have total solar and lunar eclipses). So you’ll see a slice of the sun come up and get bigger and bigger, and when it goes down at nighttime in the west, you’ll see it get smaller and smaller until it shrinks to a sliver and then is gone.

So you see the sun for longer than it’s really “daytime” because the sun isn’t a point of light that’s “on” and then “off.” This is complicated by our old friend, the atmosphere. The same reason that the UARS satellite isn’t falling to Earth in a predictable fashion causes us to see the sun before it’s actually up over the horizon. This is called refraction and it’s is why light bends and makes rainbows, among other things. You’re actually able to see the sun before the sun rises, and depending on the size of the atmosphere (believe it or not, it gets bigger during solar activity [this is why UARS is not being predictable]), it can expand and contract, and thus refract more or less.

This is starting to make my head hurt. Luckily, there’s only a few more reasons why we don’t have equal days and nights on the equinoxes. The first is altitude. The higher you are, the sooner you’ll see the sun and the longer you’ll see the sun for on that day. Denver sees more sun than Kansas City, even though they’re about the same latitude, because Denver is a mile higher and can see the sun for a few minutes more each day than Kansas City does.

The last thing we’ll consider is latitude itself. This is how far up or down from the equator you are. Remember – the Earth is not round, it’s egg-shaped. As you get nearer to the poles, the diameter shrinks faster than if you were simply getting close to the “pole” of, say, a basketball. This causes the local curvature of the surface to be lesser or greater depending on where you are, which makes daylight longer or shorter as a result.

The only times that the length of day is equal to the length of night is actually a few days before the spring equinox and a few days after the fall equinox. This is called the equilux and occurs because there’s actually about 14 minutes more of sunlight on the fall equinox, so you have to wait a few days until the day shrinks enough to be equal to the night. The reverse is true in spring, so you have to catch it a few days early. The equilux also depends on what your latitude is and there are no equiluxes on the equator. Man, sometimes the simplest things can be so complex because of such esoteric things as egg-shaped planets.

Kids Balance Eggs
Kids Balance Eggs

Oh, that reminds me. Do eggs stand up on their own on the equinox? Traditionally, this is done on the Spring Equinox, with raw eggs, as a symbol of rebirth and growth and blah blah blah. And yes, you can stand an egg on its end on the equinox. Really. You can! Try it! And then do it a week later. And then a month later. And then two months after that during the solstice. You’ll find it works every day, not just the equinox. You just have to have patience. It’s not weird science and it’s not gravitational alignment, it’s center of gravity and little, teensy-tiny bumps on the egg shell that cause it to be able to stand up on a smooth surface.

And if you’re having trouble getting your egg to stand up, it’s even easier on not-so-smooth surfaces. Failing that, put a little pile of salt down first, get your egg to stand up in it, and then gently blow the excess salt away. Take your picture now and have eggs Benedict later!