Blue Moon on Halloween 2020

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For the first time since 2001, a blue moon will occur on October 31. While normally each month sees one full moon, October this year will bring two full moons.

A moon’s lunar phase cycle takes 29.5 days to complete. As the moon revolves around the Earth, the part of the moon that is lit by the sun is not always completely visible to our planet.

If you were to watch the moon each night, you would notice that it appears to change shape. Sometimes only a sliver of the moon is visible, other times the moon is fully illuminated.

A new moon is a moon that appears completely dark when viewed from Earth. A full moon is completely visible when viewed from Earth.

The moon itself produces no light. All light from our solar system comes from the Sun. Moonlight is light reflect off the moon from the Sun.

Light from the Sun illuminates one half of the moon. Half of the moon is in daylight and the other half is in nighttime depending on what side is facing the sun.

How visible the moon is depends on the its position in its orbit around the Earth which in turn affects how much of its face is illuminated by the sun.

The image below illustrates how the illumination of the moon from the Sun changes as the moon orbits around the Earth. There are eight phases of the moon.

Phases of the moon.  Image: NASA, public domain.
Phases of the moon. Image: NASA, public domain.

When the illuminated side of the moon is fully facing the Earth, that is known as a Full Moon. Since the lunar phase takes 29.5 days to complete, we typically only see on moon per calendar month.

However, this year, the first full moon in October occurred on October 1. Since this moon was also the first moon closest to the Fall Equinox, the astronomical start of Fall which occurs on September 21 of 22 each year, the full moon on October 1 is also known as a Harvest Moon.

The second moon of October 2020 will occur on October 31. When a calendar month has two full moons, the second moon is known as a Blue Moon.

October 2020 will see two full moons. Image: NASA, public domain.
October 2020 will see two full moons. Image: NASA, public domain.

The moon doesn’t actually turn a shade of blue or look any different. The term Blue Moon can be traced to the early 1900s where a blue moon meant that four full moons occurred in a season. The term has since evolved to its current meaning. Why the word blue to refer to an extra full moon is not exactly clear.

Blue moons occur every 2-3 years so they aren’t all that common. This is where the phrase, “once in a blue moon” comes from. The phrase means some that occurs very rarely.

Blue moons that occur on Halloween are even rarer. Between 1900 and 2100 (that’s a two-hundred year span), a blue moon will occur on Halloween only 12 times: 1906, 1925, 1944, 1955, 1974, 1993,
2001, 2020, 2039, 2058, 2077, and 2096.

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