Today’s Spectacular Vernacular is sidereal, a heavenly word that is perfect for a Leap Day. Bonus—it has math, science, and astronomy implications.

Before we get started, let’s talk about pronunciation. You may have looked at this word and thought it was side-real—two simple syllables. Good guess, but it’s actually four syllables. Think sigh-deer-e-al. It comes from the Latin sidereus meaning “of the stars” or “of the constellations.”

To explain what it means, though, let’s play with a trick question. As you know, a year on earth is 365 days. It’s, of course, a little more complicated, because it’s actually about 365.25 days which is why every fourth year, we have 366 days.

Annoyingly, a year is actually 365.242190 days, which is why every hundred years we skip the leap year, except every four hundred years we skip the skip and have a leap year. Except… Oh, fine, let’s stick to 365.25 days.

So now for the trick question. The earth revolves around the sun once a year. During that year, how many times does the earth rotate on its axis?

Did you say 365 times? Or 365.25? Or, if you’re being exact, 365.242190?

Wrong. The earth rotates 366 times (or 366.25, or 366.242190).

Whoa. Confusing, right? The reason we think it’s 365 times is because we are looking at rotation from the perspective of the earth looking at the sun. But because the earth is also revolving around the sun, the earth actually rotates slightly more than once every day.

Here’s a simple way to demonstrate what I mean. Take two coins of the same size and lay them side by side. Hold one in place and roll the other around the exterior until it comes back to its starting point. If you focus on the staionary coin, it will appear to have rotated once.

But do it again slowly and watch the moving coin carefully. You will notice that halfway around the circle, it will have actually already rotated once. When you return to the starting point, it will have rotated twice.

That perspective is how we can understand it rotates once more than we expected. Likewise, if you watched the earth rotate from the stars, you would clearly see it rotates 366.25 times a year (one revolution around the sun). That view—the perspective based on the stars—is sidereal.

So we here on earth perceive 365 rotations or solar days. But, from the stars, we realize that we have had 366 rotations or sidereal days. A sidereal day is about 4 minutes shorter than a solar day because the Earth has to turn a little more than 360 degrees for the Sun to return to the same position in the sky due to the Earth’s orbit around the Sun.

When we’re studying stars thousands of light years away, that small difference is critical to understand movement in the universe. Thus, astronomers measure sidereal time rather than solar time.

Now you are armed with a little leap day spectacular vernacular. Test your friends and see if they correctly guess how many times the earth rotates in a year. Be prepared with a pair of coins to overcome their disbelief.

And let me know if it earns you the same stares I receive when I produce such exciting trivia. Yeah, I never claimed to be the life of the party.