Supermoon? Or Just Another Perigee Syzygy?

The moon’s orbit is not circular. According to Wikipedia, the moon is closest at around 357,000 kilometers and farthest at 406,000: a difference of something like 13%. That’s a freakishly egg-shaped orbit compared to the earth’s orbit around the sun, for instance. And it moves between these extremes every month.

Tonight, the perigee (the close approach) corresponds with a full moon (a syzygy — when the earth, moon, and sun are all in a line). What does that mean? A brighter-than-average full moon! If you were around for the last “supermoon” in 2011, you’ll have heard that it was the closest/brightest since 1992, or something. Well, this one’s brighter.

But don’t freak out if the clouds are hanging in the sky wherever you live; there’s a perigee full moon every 411.8 days, and there’s going to be one next year too. Unless you’re taking repeated photographs with the same lens, you won’t be able to detect the size difference with your current wetware anyway, due to the aptly named moon illusion. You already perceive the moon varying in size by a factor of 1.5 when it’s on the horizon versus hanging overhead, so an extra few percent is going to be lost in the noise floor. And the difference between a hazy and clear night will easily swamp the difference in brightness.

superm_nAs usual, XKCD sums it up nicely. The “supermoon” is a perigee when the moon happens to be full. It’s a fairly frequent event, by celestial standards, and it’s underwhelming. If you want to see something really freaky, keep your eyes peeled for the total eclipse of the sun in August 2017.

Don’t get us wrong, we think that the moon is super! And there’s nothing wrong with going outside to have a peek at it. Just please, during this year’s perigee syzygy, spare us the hyperbole.

“Supermoon” versus “micromoon” comparison image courtesy [Stephan Sciarpetti].

27 thoughts on “Supermoon? Or Just Another Perigee Syzygy?

  1. Stop it! It isn’t brighter! For the Moon to be brighter the Sun will have to be brighter. It just subtends a greater angle.

    If it was brighter from being closer to the Earth, then it would get brighter as space travelers approach it until they vaporize. But it doesn’t work that way. The Moon doesn’t receive more illumination from the Sun just because you are closer. The science reporting media is crammed full of these misconceptions (as well as ND Tyson in his Cosmos remake, In the first episode even).

      1. That is not brightness. The increase from getting closer is exactly cancelled by the increase in apparent size of the Moon. The exposure in a camera for a photo of the Moon will stay exactly the same, just as for the eye.

        1. You’re both right! :)

          Kratz is thinking of flux passing through an area on the earth — maybe his pupils or something. More light hits the earth. (I guess I was thinking this too, on reflection. (Tee-hee!))

          Regnirps is thinking of the amount of flux coming through an imaginary slice of the sky: there’s constant brightness per radian of moon.

          So yeah — point taken. Any given spot on the moon appears just as bright, it’s just that there’s more of these spots.

      1. No. If it appears brighter, what happens if you are 1/2 the distance? Then 1/16 the distance? On the surface? Consider the variation in the distance between the Earth and Mars. Does the exposure time through a telescope change with distance?

        1. Of you are 1/2 the distance, your retina will recieve about 4 times as much light. R^2 and all. If you are on the surface, the light would be equivalent to day time on earth, since the moon is roughly the same distance from the Sun.

          1. 4 times as much light spread over 4 times the surface area of the retina. Brightness is unchanged, just like film exposure. At all distances, the exposure for the Moon is a constant.

    1. That site has the same error abut brightness. Give them a pencil and paper and ask them to prove it with some diagrams. If it were true you would have infinite free energy and perpetual motion.

      1. Look at it this way. If it were true that it gets brighter, then when you see someone standing in sunlight, as you walk toward them they will get continuously brighter. From some of the arguments put forward here, if they are 64 meters away, and you walk to within 1 meter, they will be over 4000 times brighter. That does not fit my experience.

          1. Sort of. Objects like the Moon don’t have any radiance, except in the far infra-red, and it is not evenly distributed on the surface. Magnitude is defined by an ideal stellar black body. Planets and moons in this discussion, on the other hand, are reflectors. Totally different problem.

          2. Good plan, change the argument to radiance when apparent magnitude specifically includes observed brightness whether reflected or not. Have you thought of running for public office?

  2. My wife and I took a walk together to see the moon. It was so super that we both hugged and wept.

    And I was inwardly cursing that i didn’t have a method on me to measure the absolute brightness and just put an end to any discussion on if it was brighter than average, or not.

  3. Let’s keep calling it a supermoon and hyping the size, because so far that brilliant marketing has convinced people to go outside and spend a moment looking at this beautiful thing that happens all the time! Heck, it even motivated a bunch of us up a mountain last night, and the more excuses there are for things like that the better :)

    1. Yeah, right? It was spectacularly clear here last night, and my 2 year old son was like “moon, moon, moon!” I wasn’t gonna rain on his parade.

      (Of course, he did that a couple nights ago too. It’s his first conscious winter, so the whole night sky thing is pretty new.)

      Can’t wait to drive up into the mountains for a good meteor shower.

  4. I did a Google search and there don’t seem to be many stories about super moon nonsense before about 2011.

    Now it seems there’s some nonsense about the moon several times each year.

    I can only conclude that someone realized moon nonsense would be a handy source of click bait for the web and time-filling b-roll on TV. Plus “send your photos of the super moon” audience participation.

    Or maybe it’s all a plot by Big Moon.

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