MARY LOUISE KELLY, HOST:
Welcome to the longest day of the year and the arrival of summer, at least here in the Northern Hemisphere - astronomical summer, that is. This afternoon, at 4:51 Eastern Time, the Earth's North Pole was the most tilted toward the sun that it will be all year - summer solstice. It's a day celebrated in many parts of the world, and this time around, it came earlier than it has in more than two centuries. Jackie Faherty joined me earlier today to explain this unusual timing. She's an astronomer at the American Museum of Natural History in New York. Jackie Faherty, welcome.
JACKIE FAHERTY: I'm happy to be here to explain the phenomenon.
KELLY: When we say this happened earlier this year than it has in some time, how much earlier? Are we talking minutes, seconds, what?
FAHERTY: Yeah. So in 2020, it happened on June 20 at 5:43 p.m. This year, it's happening at 4:51 Eastern Daylight Time, so we're talking about just about 50 minutes here.
KELLY: OK. And why? And I just want to put this in perspective. The last time it was this early, George Washington was president.
FAHERTY: Right. But in four years, it's going to happen even earlier than this year. And this all boils down to the fact that we have a calendar system that does not line up with how long it takes for the Earth to get it all the way around the sun to the exact same position it was in the year prior.
KELLY: Which is why we have leap years to try to correct it.
FAHERTY: That's right. So we have leap years to correct it, and our leap year system is a little bit tricky. Most people know that it's every four years, but there's a couple of exceptions such that it's going to get earlier and earlier until the year 2100, when we're going to have it kind of go back so that the summer solstice always bounces around between June 20 and June 22.
KELLY: Is there anything observable outside as a consequence of this that we can watch for? I don't know - like, length of shadows that are different?
FAHERTY: Well, the word solstice refers to sun stop. And that is because today is the furthest north the sun is going to set on your horizon. And then tomorrow, it's going to be setting further and further south, which means the days are going to get shorter and shorter and shorter, even though it's the summer, and we're all excited. So that is a phenomenon you can definitely do. Mark exactly where you see the sun set tonight, and then do it tomorrow and the day after and the day after. And you'll notice that it's changing its position on the horizon. Through summer, the sun will start to look a little bit different, and the days will get shorter as we move on.
KELLY: You doing anything today to celebrate the solstice?
FAHERTY: I'm going to get a cocktail, and I...
KELLY: (Laughter) May or may not be solstice related but, you know...
FAHERTY: I'll put some orange juice in it, so it's got a sun-like look. I'm in New York City. It's very hot today, and so I'm going to enjoy watching the sunset. I like that it's happening at 4:51 Eastern Daylight Time. That means that it's right around Happy Hour. That's a perfect time to be celebrating the first moment of summer.
KELLY: Jackie Faherty is an astronomer at the American Museum of Natural History. Thank you, and happy summer solstice.
FAHERTY: You're welcome.
(SOUNDBITE OF JONAS BROTHERS SONG, "SUMMER BABY") Transcript provided by NPR, Copyright NPR.
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