1000 Full Moons

Sunday, January 17, 2021 - 3:13 PM

3 minutes read

Finding out how many Full Moons you could see in your lifetime

Last year's Halloween was unique, the first to have a Full Moon on the holiday since 2001. The next time a Full Moon will take place on Halloween is 2039.

Do you notice something about these years? 2001. 2020. 2039. They're each 19 years apart.

This isn't some coincidence: the Moon – like a lot of things in space – goes through cycles. This cycle, where the Moon appears in the same phase on a particular day of the year, is the Metonic Cycle.

The next Full Moon you see, take note of the date.

For example: the next Full Moon as I write this will happen on January 28th. What this means is that the next time a Full Moon will happen on a January 28th is 19 years from now. Looking at this through Stellarium, sure enough, it checks out:

January 28th, 2021

Screenshot from Stellarium showing the Full Moon on January 28th, 2021.

19 years later, January 28th, 2040

Screenshot of Stellarium showing another Full Moon on January 28th, 2040.

Note: you can see that the Moon is not in the exact same place in the sky, but it's close. This is addressed in the Metonic Cycle Wikipedia page.

This got me thinking: "because the Full Moon happens on Halloween every 19 years, I'll only see a Full Moon on Halloween about 4 times in my life." A small number, yet kinda academic. Then I wondered...

How Many Full Moons Could I See in My Life?

A Full Moon happens every 29.5 days.

There's a few ways to measure this, but let's keep it simple and say the average Canadian lives to be 82 years old.

82 × 365 days = 29,930 days

29,930 ÷ 29.5 = 1,014 moon cycles

So if you live to be the average age of a Canadian citizen, you will get to see 1014 Full Moons. Because there are usually 13 Full Moons in a year, if you reach 81 years old, you'll be in Full Moon 4-Digit Club, having been around for 1000 Full Moons.

Granted, 82 being the average lifespan is a relatively new thing. But I find that age closely matching up to an eye-pleasing "1000" curious.

How Many Have I Already Seen?

This is pretty simple to figure out. Find out the number of days you've been alive, then divide by 29.5, the number of days between Full Moons.

11,874 ÷ 29.5 = 402 Full Moons

Realizing now, as I write this: the Halloween Full Moon of 2020 was my 400th Full Moon.

"See"

Yeah, there's a few flaws in using the word "see":

  1. It's not going to be clear every time there's a Full Moon
  2. You probably don't recall watching the Full Moon as a baby
  3. You probably won't go outside for every Full Moon as old age sets in (or maybe you'll have more reason to?)

You're not going to see every single one, but it'll be up there, shining away. One way to think about it the next time you do see a Full Moon, assuming you live to be 82 years old:

This is 0.1% of all the Full Moons you'll be around for.

Moon rising above a downtown Montréal building, taken with a telescope.