Rare blue moon will light up Halloween
Q-and-A with Paul Mortfield, astronomer and Seneca professor
Oct. 29, 2020
If 2020 hasn’t been strange enough, a rare blue moon will light up the night sky this Halloween — the first time since 1944. According to Farmers’ Almanac, the next blue moon occurring on Halloween won’t be until 2039.
A blue moon occurs when there is a second full moon during a calendar month. While a blue moon on Halloween is very rare, blue moons in general are also uncommon, occurring only once every two and a half years or so.
“What’s really exciting this year is that we are having a full moon on Halloween,” said Professor Paul Mortfield, who teaches astronomy and natural science at Seneca. “It adds to the spookiness of Halloween, but unfortunately because of COVID, not many people will be out trick-or-treating.”
Mr. Mortfield is an astronomer, computer scientist and a former director of the David Dunlap Observatory in Richmond Hill. He’s a frequent contributor to NASA missions and has hosted educational broadcasts on NASA-TV. Mr. Mortfield has created software for telescopes to photograph fast-moving comets and has discovered four asteroids — one of which is officially named after his wife Karen.
In advance of the spooky show this Saturday, Seneca News caught up with Mr. Mortfield to find out when best to take in the lunar action and whether a blue moon is really blue.
SN: What can you tell us about the blue moon?
PM: The term “blue moon” is just a colloquial phrase denoting its rarity, as it occurs approximately every 32 months. The moon is not actually coloured blue and will look like a regular full moon. The moon orbits the Earth every 29.5 days, so if we get a full moon on the first day of the month, then we can get a second full moon later that month, except February, of course.
SN: But is it possible for the moon to be blue?
PM: On very rare occasions, the moon can have a faint blue tint to it. The cause is smoke, dust and ash particles from immense forest fires or major volcanic eruptions rising up in Earth’s atmosphere. These particles let the blue wavelengths of light through, giving a blue hue to the moon.
SN: Are all full moons the same size?
PM: This particular full moon will appear to be the smallest of the year. The moon’s distance from Earth varies by 25,000 kilometres from its average distance of 385,000 kilometres. Some full moons appear slightly bigger — closer — while other slightly smaller — farther.
SN: Is there something about the moon most people don’t know about that you can tell us?
PM: Yes, the moon is receding from Earth by about four centimetres a year. This is caused by gravitational tidal forces. At its current rate, it will take about four billion years before the moon reaches the distance of around 550,000 kilometres from Earth.
SN: What time should people go outside to see the moon on Halloween?
PM: The full moon will be visible all night, from when it gets dark until dawn, perfect for Halloween.
SN: Besides the moon, is there anything else we should look for in the night sky around this time of the year?
PM: Right now, the brightest planet in the sky is Mars, and it won’t be this close again until 2035. Also, looking to the west after sunset, you can spot Jupiter and Saturn as they will appear to move closer together in the sky over the next two months. However, I am really looking forward to the great annual Geminid meteor shower on Dec. 13. It’ll be wonderful this year since it takes place a day before the new moon, which means there’ll be no moon in the sky to interfere with seeing lots of meteors.
SN: Can you share a couple of tips on how to photograph an “Instagrammable” full moon on a cellphone?
PM: You’ll need a camera app that lets you manually adjust the exposure time, so you don’t overexpose the moon and get a bright blob. You’ll have to experiment with the settings to get it just right.