Friday the 13th Micro Moon

September 13, 2019

The moon won’t officially be full until early morning Saturday, but by most definitions, tonight will be a full moon. It’s also Friday the 13th. This coincidence hasn’t happened since 2006, and won’t happen again for ANOTHER 13 years.

But aside from arbitrary dates curiously aligning with the lunar cycle, there’s something else at work. Since the Moon has an elliptical orbit, there’s a point every month when the moon is at its closest, perigee… and when it’s at its farthest, apogee. The two points vary by several thousand miles, but the moon’s appearance in our sky doesn’t seem to change much to a casual observer. That said, this weekend’s moon has been dubbed by some as a micromoon because it’s a full moon at apogee. The opposite, a full moon at perigee, has been called a supermoon. But again, the difference between a supermoon and a micromoon is pretty hard to notice. A supermoon appears about 14% larger than a micromoon, and about 30% brighter. And just like the Friday the 13th coincidence, this is a chance aligning of the lunar apogee with the full moon phase--two things that happen every month--just not always at the same time.

The best time to view a full moon is at moonrise, which tonight is just before 9pm. That’s because the moon seems to appear so much larger when viewed against objects in the foreground. We know, however, that the size of the moon in the sky at moonrise or moonset is the exact same as when it’s at its highest--it’s just an optical illusion.

So enjoy the full moon tonight. Pet a black cat, and walk under as many ladders as you like.