# Measure the tilt of the Earth today!

A person without a shadow should keep out of the sun, that is the only safe and rational plan. –Adelbert von Chamisso

A few years ago, there was a rumor going around that the Earth’s axis had shifted, and that we were no longer inclined to the Sun at 23.5°.

Well, guess what? Today, June 21st, like most June 21sts, is the Summer Solstice for the Northern Hemisphere. This means, for everyone (like me) living North of the Tropic of Cancer, this is the one day of the year where the Sun reaches its absolute highest point in the sky.

(If you’re South of the Tropic of Capricorn, this will apply to you on December 21st.)

This is particularly interesting, because not only do you receive the greatest amount of daylight on the Summer Solstice, but because the Sun reaches its highest point in the sky, shadows of completely vertical objects reach their shortest length at (astronomical) noon on this day, as opposed to any other.

Back in ancient times, this idea that different latitudes had shadows of different lengths allowed us to measure the circumference of Earth for the first time.

But we know the size of Earth quite well now, and we’re also pros at measuring latitude and longitude.

This means that we can use our stick and its shadow, combined with our knowledge of our latitude, to measure the tilt of the Earth!

How’s that? It’s pretty simple. All you have to do is measure what length your shadow is from a vertical object over the course of a day.

You’ll find one point — that corresponds to astronomical noon — where the shadow is shorter than all others. If you use a little geometry, you’ll realize you know the length of the shadow and the actual height of the stick, and so can figure out what angle the Sun is at when it reaches its highest point during the solstice.

And that’s it! Just subtract this angle from your latitude and you’ve measured the Earth’s axial tilt! This is probably the easiest way to do it, and it works for everyone North of 23.5°. (Or, it would if we all had a sunny day!) And don’t worry too hard if you missed today; you’ll get some really close answers if you do it any time this week!

So happy Summer Solstice, but for those of you who have Winter Solstice today, I was ready for you six months ago!