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Supermoon Nonsense

There seems to be a growing excitement about the “Supermoon” that is due to occur on 19 March 2011, when the Moon will be at its closest to Earth in this orbit, and closer than it has been at any time since 1992.

Moon

Moon – not Super

The Moon orbits the Earth in an elliptical orbit, i.e. it is not perfectly circular, and so in each orbit there is a closest approach, called “perigee” and a furthest approach, called “apogee”.

At this month’s perigee the Moon will be 356,577km away from Earth, and will indeed be at its closest in almost 20 years [This is WRONG: see Update 2 below!). But how close is it compared with other perigees?

Let’s start by comparing it to the Moon’s average distance from the Earth, which is ~385,000km. This perigee will be ~8% closer to the Earth than average. OK, that’s a bit closer, but not significantly so.

What about comparing it to the Moon’s average perigee distance, which is ~364,000km. So this “Supermoon” will be ~2% closer to the Earth than it is most months at perigee. Wow!

So what will this mean to you? Nothing at all. The Moon will be a few percent bigger in the sky, but your eye won’t really be able to tell the difference. It will also be a few percent brighter, but your eye will compensate for this too, so altogether this “Supermoon” will look exactly the same as it always does when it’s full.

As to all of those soothsayers claiming that there will be earthquakes and tidal waves. There very well might be, but they’ll be nothing at all to do with the Moon.

UPDATE: I predict that lots of people will report having seen a huge Moon on or around 19 March

UPDATE 2: Thanks to “justcurious” for the comment that inspired this calculation, and to Steve Bell at the UK Hydrographic Office for providing the calculations below:

The Moon orbits the Earth once every 27.321 days (called the sidereal period), but as the Earth is orbiting the Sun at the same time, the Moon’s phases appear to repeat every 29.530 days (called the synodic period, which is the time we use to derive the month).

So the first part of the answer is: the Moon is full every 29.530 days.

The Moon’s orbit is elliptical (a squashed circle) and so you would expect a perigee once every 27.321 days. However the elliptical path around which the Moon orbits the Earth precesses (that is it is not fixed with the perigee occuring at the same part of each orbit; the place where perigee occurs moves, or pressesses) with a period of 8.8504 years, so that perigee doesn’t occur once every 27.321 days but rather once every 27.554 days (called the anomalistic period).

To calculate the frequency of perigee full Moons you need to use the equation:

1/P(perigee&full) = 1/P(perigee) – 1/P(full)

where P(perigee) is the anomalistic period = 27.554 days, and
where P(full) is the synodic period = 29.530 days

and when you put those figures in you find that a full Moon will occur at perigee once every 411.776 days (i.e. P(perigee&full)=411.776, or just less than once per year.

All the articles that cite this as the closest full Moon in 18.6 years are wrong; there was a full Moon at perigee 411.784 days ago, on Feb 28th 2010 when the full Moon occurred at 1700UT and perigee occurred just 19 hours before at 2200UT on Feb 27th 2010.

The next so-called Supermoon will occur on May 6th 2012, when the full Moon will occur at 0400UT, with perigee at the same time.

  1. Bubbawake
    March 10, 2011 at 00:42 | #1

    Bubba says: Look up Ingo Swan and see the moon in a different light! Why is the moon so perfectly positioned that we only ever see one side when every other moon in our solar system rotates around fully on its axis? Does this not seem a little too convenient? Not to mention that the moon is in the most perfect orbit to create a perfect elcipse of the sun! Does this not also seem convenient? I would suggest that you take another look at the moon you might be surprised at what you might uncover!

    • March 10, 2011 at 19:54 | #2

      Darkskyman says: I looked up Ingo Swann and it seems that he is a quack. His wikipedia page lists his interests as:
      remote viewing
      psychokinesis
      ufology

      All total bunkum.

      “Why is the moon so perfectly positioned that we only ever see one side when every other moon in our solar system rotates around fully on its axis?”

      OK several things to point out here:

      1. the Moon, along with every other Moon in our solar system, rotates around fully on its axis. All planets, stars, moons, asteroids etc do.
      2. the Moon does, as you say, present the same face to us all the time, due to something called tidal locking. The gravitational pull of the Earth stretches the Moon’s surface so that there is a bulge on the side facing us, and another on the side directly away from us. The Moon was spinning about its axis much faster in the past than it does now, due to these bulges. Every time the Moon spun and a bulge turned away from the Earth, the Earth’s gravity pulled it back a little, slowing the spin of the Moon, acting essentially like a brake. This continued for millions of years until the Moon was slowed enough so that the bulges (and therefore the same face) stayed pointing straight at us (and on the other side directly away from us). Nothing mysterious and nothing unusual either, given point 3. below…
      3. Most moons in the solar system (as well as the planet Mercury) are similarly tidally locked. It is very much the norm for a large moon orbiting near a planet. For example both of Mars’s moons are locked this way, as our most of Jupiter’s, Saturn’s, Uranus’s and Neptune’s, plus Pluto’s only moon

      So, “does this not seem a little too convenient?”

      No.

      “Not to mention that the moon is in the most perfect orbit to create a perfect eclipse of the sun! Does this not also seem convenient?”

      Cool, yes. Lucky, yes. “Convenient?” No.

      I would suggest that the Moon is fascinating enough in reality without having to conjure up nonsenses about it, as Ingo Swann has done.

    • March 12, 2011 at 13:52 | #3

      “the moon is in the most perfect orbit to create a perfect elcipse of the sun!”

      If it were in a perfect orbit, it

      1) Would be circular and there wouldn’t be annular solar eclipses. I was on the centerline for one of those, and am still disappointed.

      2) It wouldn’t be tilted 5° vs. the Earth’s orbital plane. If it shared the same plane we’d have an eclipse every new and full moon.

      3) The moon wouldn’t be spiraling away from Earth. At some point several hundred million years from now all solar eclipses will be annular.

    • Aaron
      March 19, 2011 at 04:37 | #5

      There are things called schools… in some of them, like colleges and universities, they teach a subject called Astronomy!

      I recommend taking one. A lunar eclipse can happen on ever planet in our solar system with an orbiting satellite. Many “moons”, or satellites, rotate in varying degrees (or lack said rotation), and may more orbit their parent bodies in much more “perfect” orbits than does our moon.

      Check out a library, you might be surprised what you might uncover!

      • Aaron
        March 19, 2011 at 04:40 | #6

        I apologize for the typos…
        ever=every
        may=many

  2. March 11, 2011 at 03:37 | #7

    Any news on whether werewolf attacks are expected to increase during the Supermoon period?

  3. Riya
    March 11, 2011 at 09:18 | #8

    It has started! Super moon time – the earthaquakes, Tsunami 8.9 occuring in Japan and alerting in many other places, coastal lines. My God Horror.

    • March 11, 2011 at 11:02 | #9

      Did you know that every earthquake in the past 100 years has occurred within a week of either the new or the full Moon…?

      • Steve
        May 6, 2012 at 08:13 | #10

        Yeah dumbass. Moon cycle ~30 days. New or full moon occurs ~ every 15 days. Within a week means a week before or after. That is 14 days. So your surprised that earthquakes occur on any of 28 days out of 30? Not so surprising.

      • May 6, 2012 at 11:37 | #11

        Um, that’s the point I was trying to make. Maybe my subtle humour was lost on you…

  4. Riya
    March 11, 2011 at 12:33 | #12

    but many say supermoon causes destructions. Isn’t it true? Then how suddenly Tsunami occured ?

    • March 11, 2011 at 13:21 | #13

      The Tsunami was caused by an earthquake, which was caused by the motion of the Earth’s tectonic plates.

      This is from a National Geographic article on whether the Moon causes Earthquakes:

      Is the observed correlation between the moon’s position in its 18.6-year cycle (or any other lunar phase) and earthquake activity a coincidence or something more? That question… is best answered by the U.S. Geological Survey.

      “There’s no evidence to support that,” said John Bellini, a geophysicist with the survey’s National Earthquake Information Center in Golden, Colorado. “There were some studies in the past that tried to link lunar effects to seismicity [the relative frequency and distribution of earthquakes] and there was nothing found.”

  5. March 11, 2011 at 21:11 | #14

    Great post, Mr. Owens. Everyone I’ve seen trying to claim an end of the world type scenario reference Richard Nolle, CPA. Certified Professional Astrologer, even.

    So, if anyone is concerning themselves with the March 19th “Supermoon”…

    Don’t panic.

  6. Patrick Aucoin
    March 12, 2011 at 02:10 | #15

    I am curious and may be misinformed (there does seem to alot of that)but I do have a couple of questions. Is it just a co-incidence that 2 of the seven most powerful earthquakes in history occured just months apart? And the one above about “”that every earthquake in the past 100 years has occurred within a week of either the new or the full Moon…?””

    • March 12, 2011 at 07:45 | #16

      Hi Patrick, in answer to your first question, my understanding is that, yes, it is just a coincidence. Earthquakes are pretty unpredictable and, while not random, the occurrence of one major one so soon after another is unrelated. If you look at a timeline of major quakes then some will occur just after others, and some will be spread out. I should add that I’m no seismologist, and that you should probably listen to them first…

      The second question you asked was about my assertion that “every earthquake in the past 100 years has occurred within a week of either the new or the full Moon”. That was meant to be flippant but it needs a little explanation. The Moon goes through one full cycle of phases in 29 days, or four weeks. Therefore the time between the new moon and the full moon is two weeks. Therefore *everything* happens within one week of a new or a full moon, as you’re never more than one week away from either of them. I was trying to point out how people pedaling misinformation about the moon’s influence on earthquakes (very minor if any) can use facts that sound science-y but are actually just white noise.

  7. josie Orrel-Pearse
    March 12, 2011 at 08:38 | #17

    So, OK, what I’m hearing is that the Earth, over millions of years, can affect the moon but any suggestion that the moon can affect the earth is ‘bunkum’… Is the relationship really that lopsided?

    • March 12, 2011 at 13:33 | #18

      Not at all. The moon influences the Earth in a whole range of ways: it causes tides (and the tides during the so-called supermoon will be very high, and warnings have been made about that); it slows down the spin of the Earth, by about one second every 18 months, until, in billions of years time, the day on Earth will be 47 days long; it pulls the crust of the Earth a few centimetres, just like it pulls the water to cause tides.

      But it does not cause massive earthquakes. That’s the bunkum.

  8. Riya
    March 14, 2011 at 06:45 | #19

    Thank you Steve.
    Nice article :-)

  9. Martin White
    March 14, 2011 at 19:02 | #20

    Look, the law of universal gravitation:

    (mass Earth) * (mass Moon)
    F = G * ——————————-
    (Center of Mass Distance)^2

    When balanced with the distance normal to distance @ 8% closer shows the gravity exerted by the Moon at the height of perigee will be 118% of the normal distance gravitational pull.

    118% of a force that is strong enough to affect our tides and the cause a bulge in our equator.

    Now you can fool other people who don’t have degrees in math, physics, and biology, but I do and I can tell you, with 5 days left till full perigee and thousands of microquakes up and down the San Andreas, I am scared to death! Indonesia was a similar confluence and we’re seeing an uptick in geothermal activity in Hawaii, Indonesia, Japan, Russia, and Yellowstone. Correlation is not causation but sometimes Causation IS Causation!

    • March 15, 2011 at 10:54 | #21

      You say “Now you can fool other people who don’t have degrees in math, physics, and biology…”. I’m not trying to fool anyone, I’m just trying to counter some of the pseudoscience that’s spreading around the internet about this nonsense.

      That said, the equation you cite is not the equation you need to use. We’re talking here about tidal forces, the equation for which is the derivative of the equation you cite, and so tidal forces drop with the distance cubed, or

      delta-F = 2GMm(delta-r) / d^3

      where delta-F is the tidal force experienced by a body of mass m and of size delta-r at distance d from a body of mass M.

      Given that all factors are identical at all positions of the Moon in its orbit, except for d, then

      delta-F(supermoon)/delta-F(average) = (d(average)/d(supermoon))^3

      = (385000/356000)^3 = (1.08)^3 = 1.25

      So in fact the tidal force of the supermoon is about 25% stronger than the average tidal force that the Moon exerts.

      This will result in higher tides, but WILL NOT result in large earthquakes. The tidal forces exerted by the Moon on the Earth’s crust are negligible compared to tectonic forces. 125% of a negligible force is still negligible.

      Finally, you end by saying “Correlation is not causation but sometimes Causation IS Causation!” but in this case there is not even a correlation! Otherwise there would have been significant earthquakes every 18.6 years, which has not happened, and the only people currently claiming that that is the case are ill-informed astrologers.

      At best there is a very weak correlation between lunar phase and minor shallow quakes, but even this is not well established.

      So why is this supermoon different from all the others that have happened? Simple – the Internet, the most efficient way of spreading misinformation that the world has ever seen, and which was barely around 18.6 years ago at the time of the last supermoon in 1992…

  10. Mykill Von Doom
    March 14, 2011 at 19:09 | #22

    Thanks for the facts and figures! Reposting them (reworded) in various places along with some stuff from skepdic.com to quell the chatter of the uninformed.

    • March 15, 2011 at 20:36 | #23

      You’re welcome; please send a link to anywhere you report this, and I’ll stick the link on my blog.

  11. justcurious
    March 18, 2011 at 01:23 | #24

    Hi! What a great website! I have a question. How often do perigee moons occur? Thanks!

  12. justcurious
    March 18, 2011 at 01:38 | #25

    PS: I misspoke. What I want to know is, how often do perigee moons of this proximity occur.

    • March 21, 2011 at 11:09 | #26

      The Moon orbits the Earth once every 27.321 days (called the sidereal period), but as the Earth is orbiting the Sun at the same time, the Moon’s phases appear to repeat every 29.530 days (called the synodic period, which is the time we use to derive the month).

      So the first part of the answer is: the Moon is full every 29.530 days.

      The Moon’s orbit is elliptical (a squashed circle) and so you would expect a perigee once every 27.321 days. However the elliptical path around which the Moon orbits the Earth precesses (that is it is not fixed with the perigee occuring at the same part of each orbit; the place where perigee occurs moves, or pressesses) with a period of 8.8504 years, so that perigee doesn’t occur once every 27.321 days but rather once every 27.554 days (called the anomalistic period).

      To calculate the frequency of perigee full Moons you need to use the equation:

      1/P(perigee&full) = 1/P(perigee) – 1/P(full)

      where P(perigee) is the anomalistic period = 27.554 days, and
      where P(full) is the synodic period = 29.530 days

      and when you put those figures in you find that a full Moon will occur at perigee once every 411.776 days (i.e. P(perigee&full)=411.784, or just less than once per year.

      All the articles that cite this as the closest full Moon in 18.6 years are wrong; there was a full Moon at perigee 411.784 days ago, on Feb 28th 2010 when the full Moon occurred at 1700UT and perigee occurred just 19 hours before at 2200UT on Feb 27th 2010.

      The next so-called Supermoon will occur on May 6th 2012, when the full Moon will occur at 0400UT, with perigee at the same time.

  13. justcurious
    March 21, 2011 at 18:27 | #27

    Very cool! Thank you so much!!

  14. May 24, 2011 at 19:33 | #28

    thats something we all should know!!!!!

  15. Steve
    May 6, 2012 at 08:20 | #29

    And another thing. 411 days is more than a year. Not “just less” than a year as the article says. Oh and 19 hours apart does not constitute a coensidence of full moon and perigee.

    • May 6, 2012 at 11:38 | #30

      It doesn’t say that 411 days is “just less than a year”, but rather that “once every 411 days” is “just less than once a year”. Maybe you should read things more carefully before sounding off.

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