Look for a blue — or boo — moon this Halloween | SummitDaily.com
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Look for a blue — or boo — moon this Halloween

Thinking Outside the Classroom with Keystone Science School

Mark Laurin
Keystone Science School adjunct instructor
A full moon seen from Frisco in March 2019.
Tripp Fay / Summit Daily reader

A blue moon is not called that because it appears blue during the night. But that’s not to say it can’t look blue. When the moon appears blue, it is due to the atmosphere being filled with dust or smoke particles of a certain size. And they have to be one size and not a mixture of sizes. Given the fires raging in Colorado and the West, the smoke from these fires could actually give us a blue moon with a blueish tone this year — through it’s rare.    

A blue moon has less to do with its color and more to do with when it occurs. A blue moon is an additional full moon that appears in a subdivision of a year: either the fourth full moon in a season (in this case autumn) or a second full moon in a calendar month.

This year, October has two full moons. Oct. 2 gave us the harvest moon. This is so named as it is the closest to the autumnal equinox, when day and night are of equal length. The harvest moon is also called the full corn moon or the barley moon because the moon pops up over the eastern horizon soon after sunset and throws its bright moonlight over the landscape, prolonging the light available for farmers to bring in crops. Whether the full moon occurs in September or October, the one closest to the autumnal equinox (around Sept. 20 or 21), is the harvest moon. Full moons that occur during autumn are in general brighter and last longer because of their proximity to the equinox. 

The second full moon this month occurs Saturday, Oct. 31, on Halloween! This full moon is called the hunters moon. It is so named because the reflected moonlight enabled hunters to see animals (deer in particular) better in the recently cleared fields and to continue hunting later into the evening.

Why do blue moons happen?

The answer is fairly straightforward. One lunar cycle averages about 29 days. There are about 365 days in a year. Therefore, about 12 lunar cycles are completed each year. There is one full moon each month, with the date of the full moon falling back by nearly one day every month. Each year contains about 11 days more than the number of days in the 12 lunar cycles, and the extra days accumulate. Consequently, every two or three years, there is an extra full moon in the year. That extra full moon must occur in one of the four seasons and so it gives that season four full moons instead of the usual three — hence, a blue moon.

Is it a trick or a treat?

This year’s blue moon is more unique in that it is the first full moon to occur across all time zones around the world since 1944. Even more so, a full moon that rises from the depths on Halloween is not a common occurrence. A full moon on Halloween happens only about every 18 or 19 years with the next ones coming in 2039, 2058 and 2077.

What kinds of tricks, magic or mystery await you this Halloween? One can only imagine. Will the moon actually be blue? What kind of magic is there when Mars stares brightly upon you only to be trailed by a suspiciously full hunters moon? When celestial events like these reveal their mystery and beauty, one can never really know what will come to pass. But one thing is for sure: It will be a treat!

Activity: Learn more about moon phases

Materials needed:

  • 8 Oreo cookies
  • A popsicle stick or other tool for scraping frosting

Slowly twist an Oreo to maximize the amount of frosting on one side when you separate the halves. Use the popsicle stick to create the phases of the moon out of the frosting. Arrange the phases of the moon in order.

Mark Laurin is an adjunct instructor at Keystone Science School.


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