Podcaster: Richard Drumm
Title: The Supermoon! What is it all about?
Organization: Astrosphere New Media
Link : astrosphere.org
Description: Today in Q&A I’m going to tackle The Supermoon! What is it all about?
Bio: Richard Drumm is President of the Charlottesville Astronomical Society and President of 3D – Drumm Digital Design, a video production company with clients such as Kodak, Xerox and GlaxoSmithKline Pharmaceuticals. He was an observer with the UVa Parallax Program at McCormick Observatory in 1981 & 1982. He has found that his greatest passion in life is public outreach astronomy and he pursues it at every opportunity.
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Transcript:
Welcome to 365 Days of Astronomy’s Q&A series where we answer your questions.
Shine on, shine on supermoon, up in the sky!
Oh. Wait. What?
OK, just stop right there, Richard Drumm The Astronomy Bum, if that’s your real name. Stop right there! What in the name of Carl Sagan are you on about this time!
Well, today’s question is:
What is the supermoon?
That’s actually an interesting question, so let’s start with the basics.
The Moon is in orbit around the Earth (duh!) and all orbits are ellipses.
What’s an ellipse? Well, simply put, an ellipse is what you get when you cut a cylinder on an angle. So slice a pickle or salami on an angle and voila! An ellipse. If you look at the top of a coffee mug or glass from an angle, that is, not right straight down from above, your eye sees an ellipse. It’s sort of a squashed circle. The more squashed it is, the more eccentric it is.
It is possible to have a circular orbit, but even the slightest out-of-roundness, even a millimeter, and it’s an ellipse. So basically all orbits are ellipses of varying degrees of roundness.
The Moon’s orbit, being an ellipse, means that at one point in it’s monthly orbit it’s closest to the Earth for that orbit, and about 2 weeks later it’s farthest away. By how much? Well not much, really, 87% is fairly typical or 55,000 km. A long way to walk, but not that much when weighed against the 410,000 km apogee maximums.
Here is a table of the perigees & apogees for part of 2014.
For those of you listening to the podcast, it’ll be in the metadata show notes attached to the file, and also on the 365 Days of Astronomy website show notes for today’s episode. Just Google 365 Days of Astronomy and you’ll find it.
Date – distance km – phase – % illumination – lunar diameter
Jan 01 – 353,417 – (New) WxC 0.2%
Jan 16 – 410,272 Jan 30 – 353,240 – (New) WnC 0.2%
Feb 12 – 409,963
Feb 27 – 356,360 – WnC 4%
Mar 11 – 409,077
Mar 27 – 361,412 – WnC 14%
Apr 08 – 408,374
Apr 23 – 365,637 – WnC 37% May 06 – 408,338 May 19 – 363,683 – WnG 71%
Jun 03 – 409,094
Jun 15 – 358,412 – WnG 91%
Jun 30 – 409,879
Jul 13 – 354,467 – WnG 98.97%
Jul 28 – 410,782 Aug 10 – 353,238 – (Full) 99.8% Aug 24 – 410,659
Sep 08 – 353,915 – WxG 98.7%
Now, just read along with me and you’ll see a pattern developing. We start the year with the perigee happening at essentially New Moon phase. Jan 1 being a waxing crescent and Jan 30 being a waning crescent. That’s what the WxC and WnC codes mean. WxG means waxing gibbous, by the way.
As the year progresses, the perigees slowly drift out of phase with the New Moon phase and slide backwards toward Full Moon, ultimately reaching it on August 10th’s perigee. This is the supermoon that everybody in main stream media went nuts about. The whole cycle takes a little over a year, about 412 days.
You’ll notice that the perigee numbers fluctuate between 353,000 km and 365,000 km. So some supermoons are more super than others. It was a slightly super-de-dooper supermoon a couple years ago that first seized the imagination of the media and the meme took off like a rocket.
So how did this supermoon idea get started?
The term “supermoon” was apparently coined by astrologer Richard Nolle in 1979 in an article in a magazine titled “Horoscope”. So you can see where they are coming from. His definition of a supermoon was where the new Moon or full Moon phases fall within 90% of the perigee. This is called perigee-syzygy by astronomers.
Though what that 90% means precisely, Nolle doesn’t say. 90% of the angular rotation, 90% of the orbital time, 90% of the distance from Earth, I couldn’t tell you. Doesn’t matter, though. He set a threshold that works for his purposes and I’m cool with that.
His idea, though, is that when perigee happens near full or new phases when tides are highest, that all sorts of natural disasters are more frequent. In reality, tides are a bit higher with a perigean spring tide, which is what you’d have with the supermoon alignment, but it’s a tiny effect, just a couple of inches at most.
Nowadays, however, it’s only the full Moon perigee that is called a supermoon. I suppose it’s because that’s the one you can easily see. If it’s not cloudy, that is…
If people tell you that there was an earthquake, hurricane, tornado, wildfire or other natural disaster within a week of a supermoon and therefore was caused by it, they’re being a bit disingenuous. All things happen in a band of time that starts a week before and ends a week after a full or new Moon. We humans are a storytelling species and a scary story is a compelling one. Just be sure to be skeptical of claims made on the internet. To quote Wikipedia’s article: “No evidence has been found of any correlation between supermoons with major earthquakes.”
And to top it all off, the percentages are minimal. The August 10 supermoon is 14% closer to the center of Earth than when the Moon is farthest away, at apogee. That’s what you get with the numbers for the supermoon closest perigee and the highest apogee, which was 2 weeks before the supermoon. The visual diameter of the Moon is only 13.5% larger by my Starry Night Pro software calculation.
If you consider the perigee numbers alone, the figure drops to about 3.4%. The so-called supermoon is only 3.4% closer than the highest perigee. If you use the average perigee, it gets even smaller.
Think of it this way. You order a 10″ pizza and you get a pizza that ranges from 10″ to 11.35″ over the course of a month. When you get that 11.35″ pizza, are you gonna jump up & down proclaiming it to be a “super pizza”?
Yeah, I didn’t think so either.
Thank you for listening. This has been 365 Days of Astronomy.
365 Days of Astronomy is a community podcast made possible thanks to the contributions of people like you. Please consider donating at 365DaysofAstronomy.org/Donate
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365 Days of Astronomy
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