You may be running the AC full blast, but today is the first day of fall. That's because regardless of the temperature, we mark the beginning of autumn with the equinox. What is the equinox, and how did we all agree on how we mark the seasons? Edgar B. Herwick III from WGBH's Curiosity Desk explained to WGBH Radio's Arun Rath. This transcript has been edited for clarity.

Arun Rath: What is the equinox? How do we mark it?

Edgar B. Herwick III: Let's look at the word equinox. This basically comes from Latin. "Equi" means equal and "nox" means night. Equal night. The point is that when the equinox happens, essentially all over the planet you have 12 hours of daylight and 12 hours of night. The reason this happens is kind of tough to get your head around, but it's essentially to do with the fact that as Earth is going around the sun, it's tilted — a little more than 23 degrees.

Rath: Like when you see a globe that's mounted, it's always off on that little tilt.

Herwick: Right. So that tilt, if you think about it, is why we have the seasons. So when we're traveling around the sun — when we're in winter time, for example, here in the northern hemisphere, it's because the northern hemisphere is tilted away from the sun. As we circle around the sun and head into summertime, the northern hemisphere is tilted toward the sun. So if you imagine if the Earth was not tilted, and we were spinning around every day, the sun would just go around the equator and we'd have even light in the northern and southern hemisphere. But it's because of the tilt that we have both seasons and two equinoxes each year, because that is the moment when the tilt of the Earth no longer matters relative to the sun.

Rath: You've explained it clearly, but it gets confusing when we think about, okay, they're equal now and the days will get shorter. But it already feels like the days have been getting shorter since the middle of the summer.

Herwick: And they have. We're at equal day and night right now, but when we are moving our way away from the longest day, which is about 15 hours of daylight and nine hours of darkness, that occurs at the summer solstice. So yes, the days have been getting shorter. This is the midpoint where they're even, day and night, and the days will continue to get even shorter, when we get to the winter solstice, when unfortunately we only have about nine hours of daylight and about 15 hours of dark in our 24-hour period. Again, it's all because of that tilt.

Rath: So today, with even day and night, today is the first day of fall.

Herwick: It's the first day of fall here in the northern hemisphere. In the southern hemisphere, it's the first day of spring. They're having the opposite equinox, because they're about to get longer days. This is why it's winter in the southern hemisphere when it's summer in the northern hemisphere and vice versa.

Rath: If you can clear up maybe one last bit of confusion here, we have two names for this season. Why autumn and fall?

Herwick: It's an interesting thing. A lot of times in English when we have two names for the same thing, it has to do with the Norman invasion of England. When you think about words like "kingly" and "royal" or "buy" and "purchase" or "smell" and "odor," these mean the same thing, and largely this is because you had an English-speaking world that had a Germanic base to its language that got conquered by the Normans, and they introduced the French or a sort of Latin-based language and English shifted and adopted all of these kinds of words.

Now in this case, that's not actually quite what happened, which is kind of fascinating. So autumn is the Latin-based term, and this got introduced to English around 1300. Up until that point, they did have a Germanic term for the season, but it wasn't "fall." They called it "harvest." It was about the 1600s, much later, when the word "fall" gets introduced. Language experts think it's basically a poetic turn of phrase that got adopted. Exactly like you would think, we call it fall because of the fall of the leaves. So that's about the 1600s, and that's about the time when "spring" emerges as a term, too. Again, an active term - things "springing" from the ground or the "spring" of the leaves.

When the English came to the new world here in the 1600s, they brought with them both of these terms. Fall and autumn are both terms that existed at that point. Autumn was far, far more popular. Nobody's really sure why, but over the years, fall started to win out here in the United States and it started to fall out of favor in England. So in England, it's far more common to hear the word "autumn," and here in the United States, it has become far more common to hear the word "fall." They know that this has happened by about the mid-1800s, but nobody's exactly sure why.