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October full moon 2020: A 'Blue Minimoon' will haunt Halloween

A full moon shines over Virginia.
A full moon shines over Virginia.
(Image: © NASA/Joel Kowsky)

October will get a second full moon on the 31st, just in time for Halloween. 

The moon becomes officially full at 9:49 a.m. EDT (1349 GMT), according to NASA's SkyCal site. For New York City observers, the moon will set that morning at 7:14 a.m. and rise that evening at 6:13 p.m. local time. The moon sets the morning of Nov. 1 at 7:13 a.m.

Usually called the Hunter's Moon, this one gets to be what's known as a "blue moon" — the second of two full moons that occur in the same calendar month. Calendar months and lunar cycles are not perfectly synced up; while the name "month" comes from the Proto-Germanic word for "moon," the current Western calendar is divided into months of 30 and 31 Earth days, except for February. This means that, every two to three years, the lunar cycle, which is about 28.5 days long, will "line up" and two full moons will happen in a single calendar month. The last time this happened was in March 2018. 

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An older definition of blue moon is the third of four full moons between a solstice (on or around June 21 or Dec. 21) and an equinox (on or around March 21 or Sept. 21). Typically there are three full moons in those periods, which are three months long, but sometimes you get four full moons. The last time this happened was in 2019, with the relevant full moon appearing on May 18. 

Another phenomenon that will accompany Halloween's full moon is lunar apogee. On Oct. 30, the moon will be at its farthest point from Earth in its elliptical orbit. So the full moon will appear a tiny bit smaller than normal, though unless one is especially keen-eyed it's not noticeable. The moon will be 252,522 miles (406,394 kilometers) from Earth on Oct. 30 at 1:46 p.m. EDT (1746 GMT), a bit farther than its average distance of about 238,900 miles (384,500 km). When apogee coincides with a full moon — as this almost does, though not quite — it's called a "minimoon." 

The moon will be in the constellation Cetus, the whale, a faint group of stars that will be mostly washed out by the moon itself. For observers in New York City, the moon will reach a maximal altitude of about 59 degrees above the horizon at midnight. As one moves south, the moon's altitude will increase, until one gets to about the latitude of the Philippines, central Africa or northern South America. For example, an observer in Abuja, Nigeria, will see the moon rise at 6:15 p.m. on Oct. 31, just a few hours after it becomes officially full at 3:49 p.m. local time, and at midnight the moon will be a full 88 degrees above the horizon — nearly at the zenith. (Your clenched fist held at arm's length covers about 10 degrees of sky.)

As one moves farther south, the moon starts to drop in altitude again, but this time it is toward the northern rather than southern horizon. In Melbourne, Australia, the full moon will occur at 1:49 a.m. local time on Nov. 1 — so Aussies won't get an official Blue Moon — with the just-under-full moon rising at 7:29 p.m. on Oct. 31 and reaching a maximum altitude of about 45 degrees at 1:04 a.m. on Nov. 1. 

An interesting phenomenon is that, while the moon will appear to be in the constellation Cetus for Northern Hemisphere skywatchers, it will look as though it is in Aries for antipodeans. That's because the moon is close enough to Earth that separating two observers by a distance approximating Earth's diameter is enough to cause a visible shift in the moon's position against the background stars. In this case, it's less than a degree, but that's just enough to put the moon over the "border" between constellations. 

And for those who ask: The moon never really appears blue unless there's something in Earth's atmosphere filtering out the red light reflected from it. Dust or smoke particles can sometimes scatter blue light and make the moon look bluer, but such phenomena are unconnected with the phase of the moon itself. 

Related: Earth's moon phases, monthly lunar cycles (infographic)

Planets and stars, too!

Mercury will be just visible in the predawn sky from New York City and locations of similar latitude, with the innermost planet rising at 6:28 a.m. EDT (1028 GMT) on Oct. 31. On that day, the sun rises at 7:25 a.m., and Mercury will only be about 6 degrees in altitude by 7:00 a.m., in the constellation Virgo, according to heavens-above.com calculations. The situation doesn't get much better from the Southern Hemisphere; in Cape Town, South Africa, for example, the planet rises at 5:22 a.m. local time on Oct. 31, but the sun rises at 5:46 a.m., since late October is approaching the Southern Hemisphere summer. Mercury will be barely above the horizon before the sun renders it invisible. 

Venus will still be a "morning star" in the constellation Virgo, rising at 4:31 a.m. local time in New York on Oct. 31. Sunrise is not until 7:25 a.m. local time, and the planet will be 31 degrees in altitude by then. Venus is bright enough that it remains visible even as the sky lightens somewhat. 

On the night of the full moon, Mars will rise ahead of the moon, at 4:59 p.m. local time in New York, and it will be to the right of the moon as they travel together across the sky. At sunset on Oct. 31, at 5:52 p.m. New York time, the planet will be 10 degrees above the eastern horizon in the constellation Pisces. The planet will be obvious, because Pisces contains relatively faint stars and Mars shines at magnitude -2.1, which is nearly twice as bright as Sirius, the brightest star in the sky. 

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Jupiter and Saturn will both be in Sagittarius, low in the western half of the sky, by 8:30 p.m. EDT (0030 GMT on Nov. 1). From New York City, the gas giants will be about 16 and 19 degrees in altitude, respectively. Saturn will be to Jupiter's left, with the "teapot" shape of Sagittarius to the right of Jupiter. 

Looking directly up from the two giant planets, observers will see the stars Altair and Vega, which form part of the Summer Triangle. By 11:30 p.m. local time in mid-northern latitudes, the "base" of the Summer triangle, consisting of Altair and Vega, appears almost parallel with the western horizon. At that point in the evening, turning left from the Triangle will lead observers to the south, where the "wet region" of the sky, which is full of water-themed stellar groupings, will be front and center. Cetus and Pisces will be near their high points, and just to the west of Cetus is Aquarius. Moving up — northward — from Cetus, one can see the Great Square of Pegasus, which connects to Andromeda, which is not only the home of the eponymous galaxy (visible as a smudge of light) but also, in Greek mythology, the woman who was saved by Perseus from Cetus the leviathan, and taken away on a flying horse for which Pegasus is named. The Andromeda galaxy is easily visible on moonless nights, but with a full moon it is more difficult to see; it sometimes helps to look away from the moon for a few minutes. 

The bright winter constellations become prominent in late October, with Orion and Taurus above the horizon in the east by midnight along with Gemini and Auriga; the four form a group of relatively bright stars that can be easily found by looking to Orion's belt.  

The October full moon is often called the Hunter's Moon, according to the Old Farmer's Almanac, or the Harvest Moon if it occurs closer to the autumn equinox, which occurs on or around Sept. 21 every year. This year we get two full moons in October, which makes the latter full moon a Hunter's moon. 

Traditional names for the full moon often reflect local environment and history; according to the Ontario Native Literacy Coalition, the Ojibwe people call the eleventh full moon the "Freezing Moon" or Mshkawji Giizis. (This full moon would ordinarily fall in November, but the presence of two full moons in October changes the count). The Cree people call it "Kaskatinowipisim" (Freeze-Up Moon) for similar reasons. In the Pacific Northwest, the Tlingit call the eleventh full moon "Kukahaa Dís" (Scraping Moon), according to the "Tlingit Moon and Tide Teaching Resource" published by the University of Alaska Fairbanks. 

Many Muslim countries will celebrate Muhammad's birthday on Oct. 30, the day before the full moon, as it is traditionally set as the 12th day of the month of Rabīʿal-Awwal. 

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