Of eggs and equinoxes

¡SkyCaramba! Weekly astronomy blog for the week ending September 21, 2013

The September equinox occurs on the 22nd at 20:45 Universal Time. This is when the sun crosses the equator. It’s moving south. Surely, you’ve already noticed the amount of daylight shortening if you live in the northern hemisphere. Those south of the equator are getting longer days and shorter nights.

There’s a popular legend that only during an equinox can an egg be stood on its end. The usual tale purports that it must be during the March equinox. It make claims about various natural forces aligning to make this feat possible only at that time, however, it falls short of explaining why the same thing supposedly can’t be done during the September equinox.

Well, if you have access to an egg you can find out for yourself. A chicken egg is said to be the best for this experiment. They’re the most widely available and the most cooperative. It may take you a few tries. But if you have a steady enough hand and you try this on a sturdy enough surface, you should be able to make the egg stand on one end or the other. Maybe you’ll even be able to make it stand on either.

Here’s what’s really going on. Egg shells feel very smooth. Some are smoother than others though. If you look close enough, you’ll see many small bumps on the surface. These bumps act as little legs upon which the egg’s weight can rest. When placed just so, the little legs will hold it until some slight disturbance tips the egg. The disturbance could be a breeze or a vibration that travels through the counter top or table you’re doing this on. Eggs from some animals are indeed very smooth and the tiny bumps on their surface don’t lend themselves readily to supporting the rest of the egg.

Notice that the explanation of what’s really going on has nothing to do with the alignment of the earth and sun. You can try this any day of the year and get the same result unless for some reason the place where you try this is all quiet and calm only during an equinox. So then why do people tell the story of the egg balancing only on the equinox? My guess is it’s fun for people to believe they understand and can momentarily harness a force that’s bigger than they are.

In case you just can’t get the egg to stand up, you’re not alone. Most days of the year, you might try to do it if you got the notion but you’d give up right away. If you were under the spell of the legend, you might think it can’t be done on those days. The truth is, you will have no more and no less luck with it any day.