The “Supermoon” Observing Challenge

This month's full Moon will appear 30% larger in area than the smallest full Moons. But can you tell just by looking at it? Maybe!

perigee and apogee moon

A comparison of the Moon at its closest to Earth (left) and farthest. The change in distance makes the full Moon look 14% larger at perigee than apogee.
Laurent Laveder

It's "supermoon" season again, with three full moons in a row taking place quite close to lunar perigee — that is, when the Moon is closest to Earth on its somewhat elliptical orbit. Monday's full Moon is the closest of the three, and as pointed out here already it will actually be the closest full Moon since 1948.

This factoid — even promoted by NASA now — is causing considerable media interest that splits two ways: some articles tout it as a sky sensation, while others claim that all full Moons look the same to the human eye, regardless of their distance. A closer look shows that neither statement is correct and that one should enjoy and observe supermoons in general.

It all comes down to relative numbers: (1) the changing distance of the centers of Moon and Earth, (2) your changing distance to the Moon each night as you race around the center of the Earth while standing on its surface, and (3) the differing Moon-Earth distances during various full Moons that occur at perigee. We're fixated on the latter, yet in comparison it's practically irrelevant. For example, the Moon's proximity on November 14th is only 0.008% farther than it was during the previous extra-close minimum in 1948 or 0.02% farther than during the next one in 2034.

These differences of a few dozen kilometers change the outlook of the Moon's disk not one bit: Monday's "supermoon" is exactly as good as the ones of 1948 and 2048. Compared to the ones in 2015 and 2017, it's just 0.1% and 0.02% closer, respectively. Likewise, it's 0.6% and 0.8% closer than the so-called supermoons of October and December 2016. To the eye, there's still no difference whatsoever.

Much more dramatic are the effects due to your position on Earth, an effect that's often overlooked. As the Moon arcs from the horizon to nearly overhead, its distance changes by a few thousand kilometers. This changes the disk's apparent diameter by about 1%: it looks smallest moonrise or moonset and and largest when transiting. And your latitude (and thus the Moon's altitude when highest in the sky) can also make a difference of several thousand kilometers or about 1%.

The Moon's changes its distance each night

The might look close when it's rising or setting over your horizon, but at such times it's actually several thousand kilometers farther away than when it's overhead.
Sky & Telescope diagram

So this "personal" effect dwarfs the differences of the minimum distances of the November supermoon with the full Moons before and after it — let alone the minute differences between the "best" supermoons over time. And yet the difference is still invisible to the eye. But that changes when one compares such perigean full Moons with full Moons near apogee, as was the case in April: then the change in diameter is closer to 14% and the disk's area (and brightness) near 30%.

But Can You See the Difference?

Until six years ago I was blissfully unaware that this effect is actually detectable with the unaided eye and without any measuring devices. Just before the "supermoons" became a media sensation, however, I accidentally noticed that the February 2011 full Moon appeared unusually large in the sky. Checking an ephemeris revealed that it was indeed very close to perigee. On various occasions since, the sequence has been the same: I'll spy an seemingly oversize full Moon, then confirm that it's close to perigee — each time not knowing beforehand where in its orbit the Moon was.

Nowadays, with supermoon articles everywhere, such "blind" experiments are hard to pull off. But those firsthand experiences allow me to state that perigean full (and gibbous) Moons do indeed look bigger and more impressive. It's important to make these observations with the Moon high in the sky, well away from the horizon, foreground buildings, tree branches, and even high-contrast cloud edges. Anything in the Moon's vicinity triggers the mysterious "Moon illusion" to some degree. And seeing a larger-than-normal full Moon in the sky, well away from anything else, is what makes it so remarkable.

Supermoon measuring device

Make a simple Moon measuring device by cutting slots of different widths in an index card. Bob King

It would be fun to do some experiments under controlled conditions. Perhaps you can try Bob King's clever handheld "slot" method. Or try sketching the maria that you glimpse on the lunar surface at different times — does the visibility of small details improve near perigee? Maybe a clever digital designer out there can create a virtual-reality experiment to test our visual perception of different-sized full Moons.

Finally, I wonder whether the changing apparent size of the Moon was noticed centuries or millennia ago. Alas no such observations have been located thus far in the literature.

So we've come full circle then: do not let articles that tell you the changing size of the Moon "cannot be seen" distract you from making observations yourself. Watch the supermoons of November and December, high in the sky, try to memorize "what they were like," then repeat the observation several months later. Some see the effect easily, others struggle, and most just never even try. But if you do see a difference, you have seen a fundamental property of the universe —orbits are usually elliptical — with your own eyes.

13 thoughts on “The “Supermoon” Observing Challenge

  1. Eric G. Canali

    No no no no no ! – – S&T complicit in the “supermoon” rubbish – – oh please, say it ain’t so !! ? I understand that you’re talking to > us < the somewhat seasoned astronomy enthusiasts who understand the context and observational points of interest – but PLEASE, at least don't use that horrible term ("S……M…."), We were all just fine with standard orbital terminology. Facebook-"Astrological" lexicon should not sully these pages! 😉

  2. Frank-ReedNavigation.comFrank-ReedNavigation.com

    Ah yes… You’re back… I do remember well your claims from the last supermoon run. “I can see it! I can see it!” But can you? And how do you know? How do you KNOW that you are seeing a real phenomenon and not something entirely in your head? The problem of “confirmation bias” raises its ugly head…

    You describe an incident some years ago when you felt that the Moon looked unusually big to you. And you checked sources, and, what do you know, it was close to perigee!! This is classic confirmation bias — a well-known problem in science for some centuries now. You think you see something, and then one bit of data seems to match so you now think you have found an objective phenomenon. But you can’t assert that you can detect a signal in noise unless you test the claim under controlled circumstances. Now there’ no question that observers can detect the difference in size of an apogee full moon compared to a perigee full moon when images of those are displayed side-by-side. That much is certain –and irrelevant. There is also no question that the size can be measured with a telescope reticle or with a sextant. But the difficult, controversial, and EXTRAORDINARY claim that you make is that observers, with some observing experience, can detect this when they do not have a comparison and, furthermore, when they do not know that the Moon is at perigee. This extraordinary claim requires “good” evidence (thought I was going to say “extraordinary evidence” didn’t you? but this particular claim isn’t important enough for such care).

    Since we probably cannot conduct this experiment in any reasonable fashion using the real moon. Why not try it with balloons, or maybe illuminated disks, in early twilight sometime. Get observers on different nights to note the apparent sizes of balloons with the exact same physical size when viewed at slightly different distances simulating the apogee-perigee range –and including many in between so that there is not a binary comparison to train the observer– then see if there’s any correlation. Make sure the actual distances are hidden from the investigators so that you have a little double-blinding. Now what do you see??

    You brought up the “augmentation” which I discussed at length the last time this came up. This is the apparent change in angular diameter as the Moon rises from horizon to maximum altitude. It’s something that has been well-understood for centuries, and ocean-going navigators regularly corrected for it when “lunars” were important for longitude 200 years ago. They also, of course, knew to correct for the Moon’s changing distance, which they did with a parameter known (even to modern celestial navigators) as the “horizontal parallax” (a number which peaks at 61.5 this Monday). It is interesting to note that despite that long era of some 75 years when navigators found observations of the Moon so critical, there’s no record (that anyone has found) of navigators or the astronomers and mathematicians who derived their techniques discussing how “obvious” it was when the Moon was closer and larger. That’s because it is not obvious. You see it through confirmation bias. If you know it’s there, you are primed to spot it. This is not a scientific observation. Meanwhile that group of authorities knew and discussed the famous “moon illusion” which overwhelms any casual observations of the Moon. The Moon looks huge when near the horizon, despite the fact that it is objectively somewhat smaller, and even 200 years ago astronomers understood that it was “all in our heads”.

    The supermoon label burst onto the social media scene in March of 2011 thanks to a coincidental article on space.com connecting earthquakes to lunar perigee (an astrological connection with next to zero scientific merit, except for extremely small earthquakes). When the earthquake/tsunami catastrophe struck Japan, the mush-brained internet re-posted and forwarded, as it does so well, and in no time the phrase “supermoon” was everywhere. This is not observable. It’s social media mania arising from the confusion between the “moon illusion” and the confirmation bias of the internet bubbling away about this non-event.

    Finally, another popular astronomical claim is that a “supermoon” is observable because it’s “so much brighter”: a perigee full moon is 30% brighter than an apogee full moon. This is true, but this is right at the limit of normal visual detection. Thanks to a numerical coincidence, 30% difference in brightness corresponds to about 0.3 visual magnitudes. That’s not much! Only experienced variable star observers can reliably detect a brightness difference of 0.3 magnitudes, and that’s with comparison stars close-by. Further, as the Moon is climbing from the horizon, its brightness increases by 0.3 magnitudes when it climbs from 16 to 24 degrees above the horizon and another 0.3 rising from 24 to 45, and yet these are not observable changes. We can see the increased brightness with no more reliability than we can see the modest increase in angular size. These are real phenomena when measured with accurate tools, but they are not phenomena that human vision is able to detect.

    Frank Reed
    ReedNavigation.com
    Conanicut Island USA

    1. Skyweek

      I did a ‘blind’ experiment, repeatedly, and could not come up with any factors other than the perigee causing the bigger-than-usual impression – where is there a confirmation bias? On the other hand simply stating that thou cannot perceive a disk in the sky changing its area by 30% over some months sounds like an ‘extraordinary claim’ to me, given what we know about what human senses can (sometimes) achive. How much “experience” is needed to make the perigee effect ‘click’ is an interesting question: perhaps unexpected sightings after long gaps (with which I’m “blessed” by our German weather) actually increase the ‘signal’; that would explain why some seasoned amateur astronomers are adamant they don’t see a thing while occasional Nature watchers tell me they do. As you and I point out, one could try clever systematic experiments with realistic fake moons, but they are not easy (or cheap) and could be subject to other biases we don’t have a handle on – see the ongoing debate about the exact mechanism behind the horizon effect. May the dreaded supermoon hype raise interest in the community to actually come up with viable protocols: we may actually learn something new about human vision …

    2. Richard SauderRichard Sauder

      To Frank Reed’
      It is a breath of fresh air to read someone’s coherent explanation of a phenomenon that is frequently taken out of context and reported on in error!
      Thanks a million!
      Dick Sauder

    3. Martin Lewicki

      Confirmation bias is not necessarily negative. Amateur astronomers indulge in it regularly – and probably even you Sir! I recall S&T pages in the past where amateurs would consult astro photos of deep sky objects and planets to identify features at threshold of detection to confirm their existence visually. Once you know it is there the interpretive neurons (“software”) in the brain can then tease out the existence of these features. I have for example glimpsed Proxima Centauri at the limit of detection in a 90mm telescope in light pollutes skies – only because I knew where it was exactly located. Without this knowledge I would have certainly missed it. While there are no records I know of naked eye visual crater sightings on the moon, once one knows they are there and exact location, craters such as Clavius and Maginus can be confirmed on the lunar terminator a the optimum lunar phase with good eyesight. This I achieved back in 1998 (See S&T February 1998 page 12). So I think it is entirely possible for the assiduous observer to detect the difference in size and luminosity of the full moon at extreme of perigee and apogee given the positive confirmation bias that there is a detectable difference. The lack of pre telescopic historical records of observations this difference and also naked eye crater sightings is due to dis-confirmation bias. There was no reason to believe there was any difference in apparent lunar size and brightness or naked eye crater sighting that craters existed in the first place so no one thought of looking for it. I agree with some sentiments here that hard-line admonishment of observers and public with apriori theoretic refutations is counter productive. It kills wonder and may even stifle discovery.

      Martin Lewicki

    4. Michael-MartinMichael-Martin

      Dear Frank
      We can always depend on your insight
      ( For Me ) in making sense out of non- sense when it comes out of, for obvious reasons, it being “A Slow News Day” over there at CNN. I believe everyone would agree with me that Social Media is having a field day with this Lunar Event, and I even saw Facebook asking people to send in there photos of the SuperMoon ( Sorry Brothers I had to say it ) which prompted everyone with an IPhone to take a photo of the Moon. I believe The Kardashians are having a special broadcast for this event.
      Now that I have that off my chest, I have a question for Frank and all the other AstroVets. When we speak of the Earth-Moon perigee With the Eliptical Orbit of the Moon, can perigee occur during any Lunar Phase or can it only occur during a Full Moon. Now if my theory is correct, and I hope someone ( Frank ) can prove me wrong, can perigee only happen during a “SuperMoon” because the Eartg might exert a gentle Tidal Tug or a Gravitational Pull on the Moon so that Perigee can be achieved.

      Thanks All and Clear Skies
      Michael G. Martin

      1. Mike-McCabe

        Dear Mike,
        Perigee can and does happen during all phases of the moon (a phase is only the illumination of the moon, it’s always the same sphere no matter how much of it is lit) and the so-called supermoon happens only when perigee and the full phase coincide.

  3. PHolder

    Quote: This factoid — even promoted by NASA now — is causing considerable media interest …

    It’s a pet peeve of mine… please check the definition of “factoid” as I doubt it means what you think it does.

    From Merriam-Webster ( http://www.merriam-webster.com/dictionary/factoid ) :
    Full Definition of factoid
    1 : an invented fact believed to be true because it appears in print
    2 : a briefly stated and usually trivial fact

    The 1st means gives the flavour that factoids are not in fact factual… but misuse of the word has led to the 2nd meaning. I think in almost every case of misuse, the correct word is fact or the phrase “simple fact”.

    See also: peruse… and unthaw… other words that have been so misused as to have received changed definitions…

  4. RichInLesta

    All this rabid concern over facts! I’m 63 and still believe in Santa Claus and the Easter Bunny. There’s so much “stuff” going around in the world-at-large that a bit of whimsy every now that then is a very welcomed diversion. If all this “media hype” gets people out looking at the Moon, then I say GREAT! Think of all the kids who may go on to become scientists or astronauts and will look back on events such as this, and remember it as the turning point in their lives…the time they good hooked on astronomy. To me, that’s much more relevant that quibbling over the details of facts.

  5. Zubenelgenubi 61

    I have mixed feelings about the “Supermoon.” On the one hand, anything that publicizes our hobby is good, but on the other hand the effect is so slight that it is hardly noticeable, and apt to disappoint people who are expecting to see a giant Moon covering half the sky. An even bigger pet peeve of mine is when general interest web sites (NOT S & T) publicize minor meteor showers. People don’t understand that the ZHR rate is for experienced observers under ideal conditions, with the radiant overhead, and no moonlight. If you live close to an urban center like most people do, and a bright Moon is out, you might not see a single meteor in an hour’s worth of observing for this year’s Leonids, for example. Even the Geminids are so badly timed this year that I would not recommend that a newbie waste their time freezing on them.

    When people are disappointed by being misled by unwarranted publicity for minor events, they then are apt not to pay attention about the truly spectacular ones. The “Supermoon” is in the marginal category- it is certainly easy for anyone to see, but most people will probably just shrug their shoulders.

  6. Physicist

    This idea of the “Super Moon” being 30% brighter is pure nonsense. In order for the Moon to be 30% brighter, it would have to move approximately 15 million miles closer to the Sun, so that it collected 30% more light from the Sun or the Sun would have to start cranking out 30% more light. Neither of those things happened. Since the “Super Moon” is 14% closer to the Earth, the Earth is now collecting 30% more illumination ( light rays ) from the Moon, on the Earth. We have become a bigger target for the light emanating from the Moon. The physicists call this Irradiance – usually measured in units of watts per square meter. The Irradiance on the Earth has increased by 30%. The Brightness of the Earth, due to Moon Light, has increased by 30%. The physical “Brightness” of the Moon has not changed at all. Quit saying the Moon is now 30% brighter. It’s Not !

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