EAST PROVIDENCE, R.I. (WPRI) — The phrase “once in a blue moon” is a common phrase to describe how rare something is — but some blue moons can be even more rare.
The blue moon this Saturday night is one of those rare ones: a Halloween blue moon. Don’t expect the moon to look any different, however.
First, what is a blue moon? The term has more to do with the calendar we use than it does the color of the moon.
A blue moon is the second full moon of a calendar month. We typically get at least one blue moon every two and a half years.
This month, the first full moon occurred on Oct. 1, which was also called the Harvest Moon. The moon appears in various stages: full moon, last quarter, new moon and then back to another full moon. That cycle takes 29 and a half days to complete, meaning the next full moon this month is on Oct. 31.
The last time a blue moon fell on Halloween night in the United States was 76 years ago in 1944. There was actually another blue moon on Oct. 31, 1955, but that one occurred early in the morning.
The next blue moon on Halloween won’t be until 2039. In fact, there will only be three other blue moons on Halloween this century: 2058, 2077 and 2096.
When the moon comes up over the horizon Saturday evening, it won’t be blue. Instead, it will take on its normal yellow, orange and red colors at moonrise. Those colors are due to the refraction of light in the atmosphere while the moon is low in the sky. As the moon gets higher, the moon will have its normal white/gray look.
Historically, moons have taken on a bluish color. If the atmosphere is filled with volcanic ash or smoke, the moon can appear blue. That would certainly be much more of a rarity than what has been coined a blue moon.
There actually is another kind of blue moon. The third full moon in an astronomical season that contains four full moons is also called a blue moon.
Also of note, we set the clocks back one hour this weekend, at 2 a.m. on Nov. 1, as we move back into Eastern Standard Time.