Supermoon Disappointment

Putting on my astronomer hat, as one whose main research focus involves measuring the distance between the Earth and Moon, I feel compelled to “speak out” about the “supermoon” hype that crops up periodically.

Last night’s full moon was touted to be a “supermoon”—larger than normal.  As a result, many folks made it a point to watch the Moon rise.  I love the fact that people are paying attention to the Moon, getting outside, and enjoying the serene experience of watching the Moon creep over the horizon.  What I don’t like is that the hype leads to an overall sense of disappointment in many.  Is the campaign a net positive, or a net negative?  I don’t know.

In this post, we’ll look at the numbers and see just how special the supermoon is.

Last night, I was at a party and went out to see if the Moon was up yet and if the sky was still clear—just because I like to stay tuned in to such things.  I was delighted to see that the Moon was just creeping over the next hill over, about 200 meters away.  I went inside to alert others at the party about the fantastic moonrise in progress, only then to find out that this was a “supermoon.” I found that there was an overall sense of disappointment among the viewers that the moonrise—while cool in and of itself—did not seem especially cool or amazing relative to other moonrises.  I got home to find a message from another disappointed friend, registering a complaint about not “feeling the joy.”

And they are right to complain.  The Moon gets close to Earth once a month.  Sometimes, this happens to coincide with the full moon. The Moon is in an eccentric (elliptical) orbit that produces a monthly variation of about ±5.5% of its range.  On top of this are solar perturbations adding (or subtracting) another 1% or so.  The net effect is that the Earth-Moon range varies from 356,500 km to 406,700 km all the time (the time-average distance is 385,000 km).  So the closest full moons are about 8% closer than the average.

Maybe a perceptive individual—well-calibrated to the apparent angular size of the rising full moon—could tell that a full moon near perigee (close to earth) is bigger than normal.  I’m not sure that I could, even though I tend to pay attention to these things.  Complicating the perception are psychological effects that make us think that the moon on the horizon is large, compared to how it looks high in the sky (the actual truth is that it’s about 1.6% smaller on the horizon due to the fact that the rotating Earth moves us closer to the Moon as it appears to rise in the sky).

As hard as it is to perceive the large (8%) effect, how special are the supermoon events, anyway?

Well, if we look at the period from 2010 to 2015, we see the Moon going about its usual dance (blue line), making many cycles between near and far (perigee and apogee).  I have placed red dashes on the graph to indicate the day surrounding full moon.  Sometimes these happen to be near the perigee, sometimes near apogee, sometimes just nowhere special.

You can see the “special” full moon episodes as the little red marks near the bottom of the plot.  They happen about every 412 days (the beat period between the perigee-to-perigee period of 27.5545 days and the full-to-full period of 29.5306 days).  So they happen almost every year.  But look at the adjacent months: these are within 0.5% as good.  Even the well-calibrated observer could never be expected to distinguish this month’s full moon size from last month’s or next month’s.  Move along, folks.  Nothing to see here.

Okay, so there is something slightly special about the 2012 and 2013 supermoons just in how well-aligned they are relative to the extreme perigee: note the symmetry in the heights of the red marks on the left and right flanks.  Over the long haul, these get out of whack a bit, before coming back in.  But this is a very subtle point.

Does the full moon ever coincide with the lesser perigees?  Are we in a special period in which the full moon happens to find the closest of the perigees?  Well, no.  The solar perturbation that causes the modulation of perigee distance is itself tied to the lunar phase.  So full moon will always be near the bottom of the envelope.

The full pattern is 18.6 years, and is shown below.  You can see that when full moon happens near perigee, it’s always associated with the most extreme perigee events.  Sometimes the two adjacent months are equally “super”—which makes for an ever-so-slightly diminished effect (back to what makes 2012/2013 slightly better than other years). This effect is most evident around 2003, about halfway back in the 18.6 year cycle.

Nothing terribly unusual going on here.  Just the Moon in its usual dance, getting closer and farther every month—sometimes happening to be at full phase when at perigee.  From year to year, the closest full moon doesn’t vary by more than a few tenths of a percent.  Guess that’s enough for some headlines.  But do they put the “specialness” in quantitative perspective?  And diminish the spectacle, impacting sales?  Why would they ever…?

So my advice is: enjoy a moonrise whenever you get the chance.  There’s plenty to appreciate about the experience without the hype.  I can only hope that the hype has not had a net-negative effect in turning people off from enjoying the natural sky.

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16 thoughts on “Supermoon Disappointment

  1. Tom – excellent post. You can see just how the size changes work through a synodic month with some excellent animations. A good one is at Lunar libration with phase on Wikipedia. It’s marked as a public domain image so you may be able to add it to this post.

  2. Around seven years ago I was sitting in the revolving restaurant in Harbour Tower, Vancouver, BC. It was already dark, but the weather was clear. ESE from us was (still is) Mt. Baker – a big volcano in Washington State. I was watching the volcano since it looked lava was flowing out of it. It was not supposed to happen since warnings would have been all over the news. But it seemed that the flow of lava was getting bigger and bigger. Then I realized that there was (of course) no lavaflow but the (yellowish) Moon was rising just behind the volcano. But the perception was perfect.


      • Ok, let me tell my story then. Several weeks ago I was on holiday in Zanzibar, East Africa, with my girlfriend. One evening we were waiting for dinner on the terrace facing the see and the sky, both dark black because of that blessing which is the absence of light pollution. Then, we saw a little reddish-yellowish spot on the horizon that in the first place I thought it came from a ship. Fishermen, I thought. Soon, the spot became too big to come from a ship unless it were a ship on fire, so we kept on staring at it as it grew, to discover in a short while that it was the moon rising on the horizon over the Indian Ocean. The view was magnificent.
        Great blog, by the way.

  3. ‘Complicating the perception are psychological effects that make us think that the moon on the horizon is large, compared to how it looks high in the sky (the actual truth is that it’s about 1.6% smaller on the horizon due to the fact that the rotating Earth moves us closer to the Moon as it appears to rise in the sky).’

    Doesn’t the moon looks bigger on the horizon because of the magnification caused by the lensing effect of the curvature of the atmosphere? A physical effect, not psychological.

    • This actually has the opposite effect. Differential refraction means the Moon appears scrunched shorter. Refraction makes an object look higher in the sky than it would be without an atmosphere, and gets more significant toward the horizon. The bottom of the rising/setting Moon is therefore boosted upward more than the top, which flattens the disk. Horizontally there is no effect.

      So two physical effects actually make the Moon smaller at the horizon than at zenith: refractive shortening, and increased distance. The Moon is typically about 60 Earth radii from the center of the Earth. At zenith, this becomes 59, so the Moon is larger, while you’re looking at the usual 60 on the horizon. That’s where the 1.6% comes from (becoming 3% in area). The differential refraction is a roughly 15% effect (and in one dimension, so 15% in area).

      The psychological inflation is strong enough to overpower these two physical effects.

    • The effect is one of perception: A rising (or setting) moon will be measured against objects on the horizon, while a moon high in the sky has no immediate references around it and will be “swallowed” by all that sky surrounding it.

  4. Please withdraw my previous comment, I’ve just checked it out on Wiki, it is an optical illusion ! My physics teacher was wrong all those years ago! I’ll try measuring it next time we get a clear sky and full moon.

    • Oops: too late—I saw this retraction after I had already answered. So consider it a service you provided for those who might have had the same impression.

  5. I’m quite pleased with myself that I figured out last night why this weekend’s supermoon also explains why the eclipse in a couple weeks will be annular, not total.

    Tom, I think it’s every astronomer’s civic duty to tell people how to get the most out of an eclipse, and I’m hoping you won’t disappoint on that matter. I’m especially hoping so since I’ll be at the Grand Canyon where the annular phase will last for a bit over three minutes when the sun is ten degrees above the horizon.

    Indeed, I think it just might be your civic duty to come join me and any of your other readers in the Southwest so you can offer said advice in person. Besides, do you have any idea just how long it’ll be before there’s another eclipse that sets inside the Canyon?



  6. I love watching the full moon come up over the horizon. In the suburbs it is a pretty good site but when I lived in the country, away from the city lights, it was awesome as was the entire night sky. The light was often so bright that you could discern colours. But the last two Total Lunar Eclipses in Australia were even more awesome! (I think I’m running out of superlatives)

  7. It might be interesting to show the first graph with the y axis starting at zero to give another impression of the magnitude of the effect.

    I remember in college people got hyped about a lunar eclipse. I enjoyed seeing the moon turn colors, get dark, and nearly disappear. I’m not sure how much effect it had on others. I half-joked to a non-science-major friend that if the Earth got between the Sun and Moon it would cut off the gravity, just like a table cuts off the gravity to keep something on it from falling to the floor, and the Moon would fly away. He knew I wasn’t serious but could only reply “Stop it. I know that’s wrong but I don’t know why!” I wasn’t trying to trick him.

    I think making news about the beauty of nature and science helps at the margin. People who love nature already love it. People who don’t care have other things to do. For those two groups it doesn’t make a difference.

    Some people who kind of like nature and science might take an interest they wouldn’t otherwise. That kind of beauty and the curiosity it excites are what got me into studying nature.


  8. The “supermoon” events do provide for slightly crisper photographs of the moon if you’re using a decent lens, though right? 8% is enough that it seems you’d get significantly more fine details in your photos.

  9. Nice post, but what is the point of talking about the average moon? You might see a full-moon rise a couple of times a year — if one were the apogee, the other a perigee, you’ve got a 14% effect right there.

    But even that is understating it. The moon appears two dimensional to us, and casts light in proportion to it’s apparent area. If we take your how-much-better-than-average approach, we see that the moon is 17% brighter at perigee than average. But if the last time you saw it was at apogee, the thing would be 30% brighter. In this case, you wouldn’t be too far wrong in exclaiming, “That’s a moon and a half!”. Indeed, before I heard about the supermoon carry-on, I had remarked a couple of times in the last 10 days on the apparent brightness of the gibbous moon.

    • Good point. Note that few will remark on the brightness of a rising moon. We are impressed by the brightness when we see the Moon illuminating the night-time landscape. For this, the projected light becomes important. A zenith moon will have twice the impact, in W/m², than when the moon is at 30° elevation, four times as much as when at 15°, etc. So depending on how late it is when one steps out to look at the full moon landscape, an apogee moon can easily appear brighter than a perigee moon. The effect is easily overwhelmed by other factors, in other words.

  10. Personally, I regularly love to explain the ash-grey illumination (that’s what we call it in Holland) of the moon disc several days after New Moon.
    And there’s another effect manifest in those days, making the crescent seem
    larger than the disc: irradiation.

    I’ll let Tom do the honors, but not before having thanked him for this enormously important blog-series, to which I have even pointed some Dutch politicians.

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