A blue ‘Hunter’s moon’
According to the Old Farmer’s Almanac, a late-October full moon is often referred to as the “Hunter’s moon,” coming at a time of year when animals would historically be hunted in the months leading up to winter.
During a full moon, the moon occupies the location in the sky exactly opposite the sun. As a result, sunset and moonrise are ongoing virtually simultaneously. In Washington, moonrise Halloween night will be at 6:28 p.m., just about 20 minutes after sunset.
Since the moon will be opposite the sun, it will be visible and dazzle all night long for those planning any socially distant outdoor Halloween festivities.
What is a ‘blue moon’?
Blue moons happen once every two or three years on average. There are multiple definitions for what qualifies as a blue moon. Most consider it the second full moon within a month, challenging to muster because the moon’s cycle has a period of 29.5 days.
The last time a calendar month featured dual full moons was on March 31, 2018. That year also featured a blue moon on Jan. 31. EarthSky reports that the next time a year will boast two blue moons will be in 2037.
Others assert that a blue moon happens when four full moons squeeze into a season, with a season defined as the time between a solstice and an equinox. The third of them is called a seasonal blue moon. The next one of those comes up Aug. 22, 2021.
If you’re looking for something super wild, a calendar blue moon and a seasonal blue moon can take place in the same year. That occurs very infrequently, things having to line up just right with the calendar and the Earth’s orbit about the sun. EarthSky indicates the next such year will be 2048.
Are actual blue moons real?
The cynics out there will point out that blue moons are visually unremarkable, pedestrian and appear the same as just about any other full moon. That is correct. There is nothing astronomically or scientifically unique about a blue moon; it all has to do with our calendar. So no, the moon will not turn blue.
If you really want to see the moon turn blue, root for a giant volcanic eruption. The 1883 eruption of Krakatau spewed enough volcanic aerosol matter into the atmosphere to scatter certain wavelengths of light, leaving the moon tinged an unusual aquamarine at the edge.
Blue moons also accompanied the eruption of El Chichon in Mexico in 1983, Mount Pinotubo in 1991 and Mount St. Helens in 1980. Large enough wildfires can also have the same effect.