Jimmy Westlake: Blue moons and red
Have you ever seen a blue moon up in the sky? How about a red moon? Well, this year, you’ll have the opportunity to see both, beginning in May with an unusual blue moon.
You might be surprised to learn a blue moon has nothing at all to do with the moon’s color. Let me explain.
As the moon orbits around the Earth, it waxes and wanes through a cycle of phases that repeats itself each month. In fact, our word “month” is derived from the word “moon” because a complete cycle of phases, from full moon to full moon, takes about 30 days. Specifically, the lunar month is 29.53 days long, just shy of 30 days, so that the time of full moon happens about half a day earlier each month. This small difference can add up throughout time so that the date of the full moon slowly works its way forward to the first day of the month. When this occurs, it’s possible to have a full moon early in the month and still fit in a second full moon 29.53 days later. For reasons that are not exactly clear, the second full moon that falls in a calendar month is called a “blue moon.”
The phrase “once in a blue moon” is often used to describe a rare or unusual event. Blue moons are, indeed, uncommon, occurring about seven times out of every 19 years. Originally, the term blue moon was used by calendar makers to denote the 4th full moon in any given season. In recent years, though, the popular meaning has changed to denote the second full moon in any calendar month.
The first full moon in May this year falls on the night of May 1, and since May has 31 days, there is still time for a second full moon 29.53 days later on May 31. This will be a blue moon, although you might not know it just by looking at it.
You’ll have your chance to see an equally unusual red moon this summer. In the wee morning hours of August 28, the full moon will slip into the shadow of the Earth creating a total lunar eclipse. Sunlight filtering through the Earth’s atmosphere will cast an eerie reddish glow onto the darkened moon.
So, this poses an interesting conundrum: What would we call the second full moon in a calendar month that happened to be totally eclipsed? If a blue moon and a red moon happened at the same time, I guess we’d have to call it a purple moon – a most unusual event.
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College’s Alpine Campus. He is an avid astronomer whose photographs and articles have been published on the websites of CNN.com, NASA’s “Astronomy Picture of the Day,” Spaceweather.com, Space.com, Discover.com, and in Sky & Telescope, Astronomy, Night Sky, Discover, and WeatherWise magazines. His “Celestial News” article appears weekly in the local Steamboat Pilot newspaper. He also records a radio spot called the “Cosmic Moment” for the local radio station “The Range” at 107.3 FM.
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