John Metcalfe was CityLab’s Bay Area bureau chief, covering climate change and the science of cities.
That's because it's about 31,000 miles more distant than 2015's other full moons.
Tonight is the full "Worm Moon," but it might not be that impressive. Heck, you probably shouldn't even bother glancing up. Due to an alignment of phase and distance, the lunar orb could look weirdly shrunken, like a gray, moldy apple forgotten on the counter.
That's because the moon is baring it all at its farthest point from earth—about 31,000 miles more distant than 2015's other full moons. The rocky satellite's orbit is elliptical; when it's at perigee closest to the planet, it can look bigger, and when it wanders to its most-distant apogee it can seem smaller. When it reaches its full phase at apogee some folks call it a "micromoon." That pop-culture term distinguishes it from its opposite partner, the "supermoon," an astrological bugbear supposedly responsible for earthquakes, tsunamis, and volcanic eruptions. (That's rubbish, of course.)
The upshot is that the full moon will be less bright and fat, as depicted in this mock-up from Alan Dyer at Amazing Sky:
Don't you just want to pat the left one condescendingly on the head? But though the differences look stark here, up in the celestial dome it might be more subtle. Explains Tony Phillips at Spaceweather:
Can you tell the difference? Some people say "yes," others "no." There are no rulers floating in the sky to measure lunar diameters. Without a reference, it can be challenging to distinguish an apogee Moon from a perigee Moon. Decide for yourself. Go outside after sunset on March 5th, look east, and enjoy the mini-moonlight.