There is a Full Moon on Halloween tonight, Saturday, October 31, 2020. But not just any full moon.
That’s because this Full Moon is a Blue Moon because it is the second moon occurring within the same month.
The Halloween Blue Moon begins with a moonrise over the Atlantic Ocean at 6:57 p.m. Eastern Daylight Time on Saturday, October 31, 2020, and sets the following morning at 7:20 a.m. (with a few minutes of variation depending on your exact location in Florida or several minutes along the rest of the U.S. east coast).
This Blue Moon will technically be 99.7% full on October 31.
The Blue Moon can be seen from anywhere on Earth, unless there is local cloud coverage.
Blue Moons occur when there is a second full moon in a calendar month or when a season has four full moons. Full moons are separated by 29.5 days but seasons are 88 to 92 days long – so it is possible to fit four full moons into a single season. This happens just over two-and-a-half years, on average. When there are four full moons in a season, the third full moon is considered a Blue Moon.
This is why the phrase “Once in a Blue Moon” is commonly known to mean something rare and offbeat because of the rare occurrence of a Blue Moon.
The date of a full moon doesn’t affect the full moon’s color. The Full Moon on Saturday, October 31, 2020, will be pearly-gray to most locations on Earth, as usual.
According to NASA, the key to a moon appearing blue is to have lots of particles slightly wider than the wavelength of red light (0.7 micron) and no other sizes present in the air. This is rare, but volcanoes sometimes produce such clouds, as do forest fires.
Humans saw blue moons almost every night when the Krakatoa volcano exploded in 1883 with the force of a 100-megaton nuclear bomb. Plumes of ash rose to the very top of Earth’s atmosphere.
Some of those ash-clouds were filled with particles about 1 micron wide – just the right size to strongly scatter red light while allowing other colors to pass. White moonbeams shining through the ash-clouds emerged blue, and sometimes green.
People also saw blue-colored Moons in 1983 after the eruption of the El Chichon volcano in Mexico. And there are reports of blue Moons caused by Mt. St. Helens in 1980 and Mount Pinatubo in 1991.
According to the Farmer’s Almanac, Native Americans looked to October’s full moon as the signal to gather meat for winter so this full moon was called the Hunter’s Moon.
The “Blood Moon” name is believed to come from the blood associated with hunting or the turning of leaves to red during the Fall.