Supermoon Sunday

Is the December 3 full Moon truly rare and spectacular?

Seronik-Full Moon

Courtesy Gary Seronik

The closest (and, therefore, largest) full Moon of 2017 rises on Sunday evening, December 3. Many will call this a “supermoon” and describe it as “rare” and “spectacular.” But is it really? To understand the ins and outs of a supermoon, it helps to know a little about the Moon’s orbit and cycle of phases.

The Moon’s path around the Earth isn’t perfectly circular—it’s slightly flattened into an ellipse. As a result, sometimes the Moon is closer to us than at other times. When it’s nearest to Earth, the Moon is said to be at “perigee.” Lunar “apogee” occurs when the Moon is farthest away.

Lunar orbit diagram

This diagram exaggerates the eccentricity of the Moon’s orbit for clarity. Variations in the apparent size of the lunar disc arise from changes in the Moon’s distance from Earth.

Independently, as the Moon goes around the Earth, it passes through a complete set of phases—from new, to full, and back to new again—in 29.5 days. If the Moon is full at the same time it reaches perigee, we get what is often called a supermoon.

But there’s a complication. In truth, not everyone is going to be the same distance from the Moon at the same time. How can that be? Chalk it up to geometry. The distance between the Earth and Moon is measured from the centre of the Earth—the “geocentric” distance. However, we actually view the Moon from the surface of a 12,456-kilometre-diameter sphere. This means, at any given moment, some people are roughly 6,200 kilometres closer to the Moon than others. Indeed, the Moon is considerably closer to you when it’s overhead than when it’s rising. That difference can be greater than the difference in distance between full Moons regarded as “super” and some that are not. (For more on this effect, turn to page 34 of the November/December 2017 issue of SkyNews.)

To illustrate the changing size of the lunar disc between perigee (“supermoon”) and apogee (“micromoon”), both of the full Moon images shown here were taken with the same equipment. Photos by Patrick Whelan

And here’s another complication: the time between when the Moon is full and when it’s at perigee varies considerably, making some supermoons more “super” than others. The Moon is officially full at 10:47 a.m., EST, on December 3. Unfortunately, for observers in Canada, the Moon has already set by then. When it rises again on Sunday evening (at around 5:30 p.m., local time), the Moon will already be several hours past full, yet it doesn’t reach perigee until after midnight—at 3:42 a.m. EST, Monday morning (December 4). That’s nearly 17 hours after it was full. So, when the Moon is closest, it’ll actually be a waning gibbous, though to the naked eye it will still appear full.

Although supermoons generate a lot of media attention, the event is really just an interesting coincidence. And because one full Moon looks about the same as the next, the whole supermoon concept is largely an exercise in splitting hairs. That’s why it’s perhaps best to just appreciate the splendour of our nearest celestial neighbour without worrying about whether or not it’s “super.”

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One comment on “Supermoon Sunday
  1. David A. Rodger says:

    I’m not sure how people will perceive a larger full moon than normal tonight, since there’s nothing in the sky to compare it to. If you could take an apogee full moon and place it next to a perigee full moon you’d notice the difference. But since that’s impossible, then only photography will make the comparison real.

    We all perceive that the full moon seems large. But here’s something you can do to prove it isn’t. Extend your arm out so you can see the full moon between your thumb and forefinger. When I do that my thumb and forefinger almost touch. That’s how small the amount of angular space the moon takes up really is.

    And then there’s the famous moon illusion. The moon seems so much larger when it’s rising than when it’s on the meridian. In fact, the moon is further away when it’s on the horizon than when it’s at its zenith. When it’s on the horizon you have to add the radius of the earth.

    To prove this is all an illusion, bend over at the waist and look at the moon through your legs. It will appear its normal size.

    Try it!

    Oh, I forgot. We’re completely socked in with clouds and rain here in southwestern BC, so no one’s going to see any moon, super or otherwise tonight.

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