I read that we’re seeing a “blue moon” today, July 31st, but I have no idea what that is. Can you explain?
The traditional explanation of a “blue moon” is that it’s the second full moon of a month, so it’s really all about the intersection of the lunar cycle and the calendar cycle. That it happens on a month with 31 days in it – like July – is no surprise: a second full moon in February when there are only 28 days would be even more unlikely.
Turns out you can calculate the probability of a second full moon happening by simply comparing these cycles too. The full lunar cycle from new moon to crescent to full moon and back (called “waxing” and “waning” if you’re curious) takes 28 days. To be more precise, however, it takes 29.5 days to complete a cycle, though only 27 days, 7 hours, and 43 minutes to complete one full orbit around Earth. This is called the sidereal month, and is measured by our Moon’s position relative to distant “fixed” stars.
So theoretically if we could schedule a full moon on the first of each month, just about every month would end up having a blue moon, right? Well, yes, but…
The lunar cycle is 29.5 days, which means that in a calendar year (approximately) there are 12.37 lunar months, not exactly 12, so which day into the month that each full moon occurs keeps changing. Kind of like how your birthday’s not always on the same day of the week. 🙂
How frequent are blue moons, using this common definition? Here are the blue moons for the next five years:
- 2015: July 2, July 31.
- 2018: January 2, January 31, only in time zones west of UTC+11.
- 2018: March 2, March 31, only in time zones west of UTC+12.
- 2020: October 1, October 31 only in time zones west of UTC+10.
Four times in five years, approximately once every 15 months. That’s pretty rare. In fact, I’d say that these second full moons in a calendar month are, well, once in a blue moon.
Oh, and the moon’s not really going to be blue. If you see pictures showing it with a blue tint, that’s just a filter being used!
A blue moon is when 2 full moon’s happen in the same calendar month.