This Saturday, March 20, is the equinox. At 4:37am Central time, 5:37am Eastern, the Sun is going to shine straight down on the the Earth’s equator as our poles point perpendicular to the Sun-Earth line. Now, this means that for those of us up north, we are transitioning from winter to summer. For those of you in the southern hemisphere, your summer is coming to an end. But everyone on that one day gets sunlight, both poles and everything in between.

There are some cool experiments that you can do, knowing that the Sun is over the equator on this day. If you go outside and stab a stick into the ground (because why not), and you measure how tall that stick is and then watch its shadow over time… at the moment that shadow is the shortest, that is when the Sun is as close to straight overhead as it is going to get. The ratio of the actual length of the stick to the length of the shadow can be used to calculate just how far north or south of the equator you are.

If this experiment sounds familiar, it’s because Eratosthenes of Cyrene, which is now Shahat in Libya, was the first person to measure accurately the size of our planet Earth, and he did it by comparing the lack of a shadow as sunlight shined directly into a well and the length of a shadow where he was in Alexandria. This experiment did not happen on the equinox; it actually happened on the summer solstice. And that well was in Swenet, now Aswan, a city on the Nile River.

When he made his calculations, what he was looking at was the difference in angle between where he was in Aswan and where he was in Alexandria. He knew how far apart these two places were in terms of how far a camel could travel in a day. And he was able to figure out the size of our Earth based on how far a camel travels. Since you know our world is a sphere, more or less, and you know the Sun will be directly over the equator on Saturday, you can use that shortest point on your stick to figure out where on the world you happen to be located.

So go out, look up on Saturday; I hope the Sun is out. See how short your shadows get.

More Information

March Equinox – Equal Day and Night, Nearly (timeanddate.com)

Does the equinox sun rise due east and set due west? (EarthSky)

Measuring the Circumference of the Earth (Khan Academy)