Spooky Blue Moon a Rare Treat this Halloween

 

This Halloween we get a visit from both the Great Pumpkin and the Blue Moon. Halloween Blue Moons occur 3-4 times on average per century. Illustration: Bob King

Let the Halloween mischief begin. As the little ones fill their trick or treat bags, a bright full moon will light the way. All full moons are special, but this one is unique for a couple reasons. It's the second full moon in the month which makes it a Blue Moon. And it happens on Halloween. Enjoy it because the next one won't light up your pumpkin until 2039. 

There are three types of Blue Moons — the monthly Blue Moon, the seasonal Blue Moon and a moon that actually looks blue. The time between successive full moons is approximately 29.5 days, so we normally get one full moon per month or 12 per year. To squeeze a second full moon (Blue Moon) in there, the first full moon has to occur at the start of the month to make room for a second full moon (Blue Moon) 29.5 days later at month's end. 

October's first full moon occurred on the 1st, leaving enough time for a second one on Halloween. Monthly Blue Moons occur every 2-3 years.

Bare branches frame the full moon on Feb. 18, 2019. Bob King

In contrast, a seasonal full moon is the third full moon in a season that has four full moons. Since a season lasts about three months we normally see three full moons, one per month. The seasonal definition of a Blue Moon predates the monthly one by at least 400 years. The monthly Blue Moon first appeared in a 1946 Sky & Telescope magazine story written by amateur astronomer James Hue Pruett. He misconstrued the seasonal definition to mean a month with two full moons. The story was picked up, spread and has since become the common understanding of a Blue Moon.

Why the third moon in a season of four? That's rooted in the Christian holidays of Lent and Easter. The final full moon of winter always occurs during the month of Lent, while the first full moon of spring happens just before Easter. With the Blue Moon as the third in the series it ensures that the Lenten season and Easter coincide with the correct full moons. 

The moon always looks and feels a little mysterious when it shines through fog. For more information about Blue Moons, visit the Once in a Blue Moon FAQ. Bob King

The next seasonal Blue Moon occurs on August 22, 2021. The next monthly Blue Moon happens on Aug. 31, 2023. Seasonal and monthly Blue Moons can sometimes occur in the same year, but it's quite rare. It last happened in 1934 and will next in 2048. Overall, monthly Blue Moons happen slightly more often than seasonal Blue Moons.

It's possible for a year to have two monthly Blue Moons, but this only happens if no full moon falls in February — called a Black Moon. And that occurs when both January and March are Blue Moon months. The last Double Blue Moon occurred in 2018 and the next will shine in 2037. At this point you're probably wondering if there's such a thing as a Triple Blue Moon. Yes! In 1961 there were two monthly and one seasonal Blue Moon. Sad to say but none of us will be around for the next one in 2143. 

The massive 1883 eruption of Krakatoa, shown in this 1888 lithograph, made the moon look blue at night. Parker & Coward, Britain

Finally, let's look at moons that really look blue. Like some of the calendrical Blue Moons they're rare, which brings us to the origin of the phrase "once in a Blue Moon." If you hear the expression you know it refers to something unusual that doesn't happen very often. 

To make a blue-tinted moon you need either dust from volcanic eruptions, water droplets, ice crystals or even sand grains of just the right size — around 1 micron or one-millionth of a meter. Objects this size will scatter away the warmer colors of the white light that reflects from the moon and tint it blue. 


Have you ever seen such a blue moon? I haven't, but I know how you can fake one. Stare at a candle flame or bonfire for a while and then look up at the moon. For a moment it will appear blue. Our eyes automatically color-balance to make the flame appear close to neutral, so that when we switch from incandescent light to moonlight (which is really reflected sunlight or daylight) it appears blue in contrast. Give it a try this Halloween and let me know if it works!

Tonight the moon and Mars will be just a few degrees apart. The earlier you watch the closer they'll appear. Stellarium

On a final note, I want to bring a nice conjunction to your attention. A sort of pre-Halloween treat. This evening, Oct. 29, the waxing gibbous moon will shine near Mars in the eastern sky. Be sure to catch it. 

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