February 5, 2018 1:02 PM

Dr. Casey Watson discusses rare spectacle

A rare phenomenon called a "super blue blood moon" appeared during the morning hours on Jan. 31, however, it was difficult for people in central Illinois to see because of cloudy skies.

A blue moon is when two full moons happen in the same calendar month and lunar eclipses occur when the moon passes into Earth's shadow. Supermoons happen when the moon's perigee — its closest approach to Earth in a single orbit — coincides with a full moon. In this case, the supermoon happened to be the day of the lunar eclipse.

In a recent interview with WAND-TV, Dr. Casey Watson, associate professor of physics and chair of the Physics and Astronomy Department at Millikin University, said, "Unless you have a full moon right at the beginning of the month, it's very rare to have one at the end of the month also. So that's why the 'once in a blue moon' expression arose."    

Casey Watson

Blue Moons happen about once every 2.7 years, because the number of days in a lunation (new moon to new moon) is a bit less than the usual calendar month — 29.53 days as opposed to 31 or 30 days.

"During totality, the only sunlight that can reach the moon must go through Earth's atmosphere," Dr. Watson said. "When it does, only the least scattered wavelength of visible light (red light) efficiently reaches the moon and reflects back to our eyes. As a result, an eclipsed moon looks red and is also known as a blood moon."

NASA leaders say the phenomenon won't happen again until 2037. The next lunar eclipse will be available on Jan. 21, 2019, which will be visible throughout all of the U.S. and will be a supermoon, though it won't be a blue moon.