On Monday at 22:34 UTC (6:34 pm Eastern U.S. time), the Sun will reach its highest declination in the sky, its farthest point north for the year. That is the moment of the June solstice.

This means that, in the Northern hemisphere, we have the longest day of the year, and the shortest night. If you live your life standing on your head in the Southern hemisphere, it means you have the shortest day, and the longest night.

I enjoy writing about the solstices and equinoctes* when they happen, so you can read all about how and why they occur in past articles. I’ll note that Monday is not the date of the earliest sunrise and latest sunset though; that has to do with the Earth’s orbit being slightly elliptical, so I’ll make a special point of linking to this article last year where I explain why that happens. I’ll also note that some people call this the first day of summer (or winter for those in the south), but I disagree; I tend to think of it as actually the midpoint. You can read about that to your brain’s delight as well.

No, instead of spending time on that here, I’d prefer to point out something rather special that led to me wandering down a rabbit hole Sunday night as I researched it: Not only is the solstice Monday, but the Moon is full on Monday as well. That moment occurred at 11:02 UTC (07:02 Eastern; I’ll note it’ll look full all day and probably even Tuesday as well).

A full Moon on the same day as the June solstice (or the December one, for that matter) is relatively rare. As I thought about this Sunday night, I wondered just how rare it was. My first thought was that it probably happens once every 30 years or so, since the full Moon can occur on any day, and there are 30 in June.

But then I realized it’s not that simple. Sometimes in astronomy two cycles can beat together in unusual ways, throwing off what you might expect. So I dug into it. I found a list of solstice full Moon dates on the Farmer’s Almanac website, and perusing the numbers it appears that we get a full Moon on the June solstice roughly every 19 years or so … or multiples thereof.

Nineteen years? That sounded familiar. It took me a few minutes, but then it clicked: That’s the Metonic cycle! Let me explain.

The Moon goes through a full phase of cycles (from full to new and full again) in about 29.53 days. That all by itself is interesting, and I talk about that in the episode of Crash Course Astronomy on the phases of the Moon:

One Earth year is, on average, about 365.24 days long. But there’s a funny coincidence here: 19 years is 6,939.56 days, and that is almost a perfect multiple of 29.53! Nineteen years is almost exactly 235 lunar phase cycles. That means that when you have a full Moon on a given date, 19 years later it’ll be on that same date once again. That’s what’s called the Metonic cycle. This fact has been known for about 2,500 years, which is pretty amazing.

But looking at the Farmer’s Almanac, you see it doesn’t seem to happen every 19 years. Why not?

This is where I really started to dig deep. I looked at leap years, and fractional leftovers between the lunar phase month (called the synodic month) and the Earth’s year, and on and on. That can account for some of the reason the full Moon doesn’t always fall on the same calendar date every 19 years.

Then I realized something: time zones.

Astronomical sites list the times of the solstices and full Moons in Coordinated Universal Time (UTC, similar to Greenwich time). That makes it easy for everyone, since you can just look up how far off your time zone is from that (for example, right now the East Coast of the U.S. is on Eastern Daylight Time, UTC – 4 hours).

But that can mess up the full Moon June solstice cycle. Why? Because the exact moment of the solstice changes year to year, and can even occur on different days! It can be on June 20, 21, or sometimes even 22. If the solstice occurs on June 21 at 23:59, and the full Moon two minutes later, technically they’re on different days!

Worse, that’s UTC. In the U.S., where it’s four to seven hours earlier than UTC, both would occur on the same calendar day. So it’s possible (and even likely) that one place in the world would see a full Moon on the same calendar day as the solstice, and another part of the world wouldn’t. What a mess!

Look at Monday’s solstice: It occurs at 22:34 UTC. For someone a couple of time zones east of the U.K., that means it happens on June 21. For them, they don’t get a full Moon on the day of the solstice. The exact moments of the solstice and full Moon are independent of time on Earth (the solstice occurs at the same moment for everyone on the planet, for example), but because we bin time up into days, that can throw off the days on which we say those events happened. Weird.

Going back to the Farmer’s Almanac, you may notice that while the solstice full Moon doesn’t happen every 19 years, it does appear to have a cycle of multiples of 19. For example, there was one in 1796, then the next in 1834, a gap of 38 years, 2 x 19. Other such gaps can be found. The gaps happen because the full Moon missed the calendar day of the solstice by some hours. Not only that, but that table is for the U.S. East Coast, so it doesn’t work for the whole world.

The root of this problem is using calendar days, which are arbitrary to some extent. There’s an overall 19-year cycle, but because of time zones it can get thrown off. A better way to do this would be to ask, “How often does a full Moon occur within a day of the solstice?,” or better yet within 12 hours before to 12 hours after the moment of the solstice.

In that case, I’d expect the 19-year Metonic cycle to be more obvious. However, looking that up using calendars for the full Moon (like this one) and the solstices (like this one) is difficult and tedious. The best way would be to run the calculations specifically looking for that, which I thought of too late to ask anyone to do for Monday’s events. I’ll leave that as an exercise for the reader.

Anyway, my point is … well, I guess I don’t have a point except that numbers are fun to play with and the cycles in the sky are neither always obvious nor simple to grasp.

But they’re there, and if a full Moon falling on the solstice is interesting enough to people that they go outside and take a look for themselves, then I’m all for it.

So happy full Moon June solstice! Enjoy it, because the next one won’t be for a while—June 21, 2062, in fact … if you use UTC.

*Equinoctes is the actual plural for equinox. Some dictionaries say that’s a bit old-fashioned, and equinoxes is now used, which is fine by me. Languages change over time. But I rather like the way equinoctes sounds, and I like using it. So I do.