The science behind the Moon’s close approach on 20 March 2011

A simulation of the rising nearly full Moon on the evening of 19 March 2011.

A simulation of the rising nearly full Moon on the evening of 19 March 2011. The large Moon provides a good opportunity to become familiar with its features. On the left is the Sea of Crises, to its right is the Sea of Tranquillity where Neil Armstrong and Buzz Aldrin landed in July 1969 and the compact sea on the far right is the Sea of Moisture. Image from Stellarium software

What’s happening? On the morning of Sunday 20 March 2011 the time of the full Moon will almost coincide with the time of the Moon reaching its closest point to Earth (perigee) in its monthly orbit. Full Moon is at 5:10 am AEDT while perigee is only an hour later at 6:10 am AEDT. Furthermore at this perigee the Moon will be a little closer than usual to Earth.

Should we expect consequences on Earth such as earthquakes? As discussed by Geoff Wyatt in Moonageddon is almost upon us, no disasters are expected as a result. Sadly, earthquakes such as the recent ones in New Zealand and Japan can and do happen, but they do not have an astronomical cause.

Why don’t the times of full Moon and perigee coincide each month? Both full Moons and perigees repeat at intervals of one month, so if they coincide in March you would imagine that they would coincide at subsequent months. They do NOT as there are different definitions of a month. The interval from one full Moon to another is a synodic month that takes 29.53 days on average while the interval from the time of one perigee to the next is an anomalistic month that takes 27.53 days on average. The first of these months, the synodic one, takes longer as it involves not just the Moon circling the Earth, but both circling the Sun.

Why do we have apogees (the Moon at its furthest point) and perigees? The Moon moves around the Earth in an oval-shaped (elliptical) path that can bring it approximately 5.5% closer or take it 5.5% further from the centre of the Earth than its average distance of 385 000 km.

Why is the Moon closer at its March 2011 perigee than at most others? If the Moon circled the Earth without feeling other gravitational pulls than its path would be that of a pure ellipse. However, the Sun also has a strong effect, making the prediction of the path of the Moon highly complex. Many astronomers of the past have dedicated their entire professional careers to studying the orbit of the Moon and developing complicated formulae to predict its motion. More recently, astronomers analyse and predict the Moon’s path on the basis of tables from NASA’s Jet Propulsion Laboratory that are calculated by numerical integration.

Will there be exceptionally high tides on 20 March 2011? At new Moon or at full Moon the Sun and Moon are in alignment so that their tidal effects add up resulting in higher tides than usual. As on this occasion not only is there a full Moon, but the Moon is at perigee we would expect an unusually high tide. Yes, the tidal predictions provided by the Bureau of Meteorology for Sydney Harbour indicate a high tide of 1.87 metres on 21 March 2011 at 10:41 pm AEDT. This is reasonably, but not exceptionally, high. For example, in the following month April 2011 the evening high tide on the day after full Moon is expected to reach 1.98 metres. A possible reason is that there are other terms in the prediction of tides such as the elevation of the Sun and the Moon. On 20 March 2011 the Earth is nearly at the autumn equinox and at that time of the year neither the Moon nor the Sun has a high elevation. If Moonageddon were to happen in December or June, then the tides would be higher.

Thus the Moon’s relatively close approach can be enjoyed without any fear. Ideally, you could watch the large Moon in the western sky just before dawn, say from 5:30 am. However, if like most people you prefer to sleep in on a Sunday morning, have a look at the almost full Moon as it is rising on the evening before, Saturday 19 March. There could be romantic consequences, but for those this blog takes no responsibility!

11 responses to “The science behind the Moon’s close approach on 20 March 2011

  • I’ve one serious problem with this article.

    This is to do with the lag of high tide to the closest approach of the moon, which is different by about 2.5 hours after the Moon crossing the meridian. In truth, the moon drags the tides behind it, as the gravitational force exhibits lags by a well defined time. So, for example, when the moon crosses the local meridian, the time of high tide is about 2.5 hours later. Logically, lunar perigee has the same lag effect.

    As for saying “no disasters are expected as a result.” cannot be proven. By science, you can say there is “no likelihood of a disaster expected as a result.”

    Also, the truth is with the errors of some earthquake happening. By this I mean that the coincidence of a resulting earthquake does not immediately cause the event. The event does not happens at the exact time, but is delayed because it weakens the fault line so that the brake of the friction between the opposing plate happen earlier than the natural point of failure. I.e. The moon’s proximity has a small but important influence of the braking of the fault. (Frankly, you really need a material scientist who has some expertise in the breakage point of interacting moving rocks along plate boundaries. These behaviours are no straight forward and are also mathematically are based on the rules of chaos theory.)

    Were it true we could predict earthquakes, believe me, governments and seismologist would be using it to predict future quakes. As they are not, says much about the imprecision of the science. (Note this does not imply that the science is imprecise, as the evidence is only based on observation not speculation.)

    One final point. What is the difference between a “SuperMoon” and a normal perigee? The difference is not 5.5% but more like 0.2%. The question really in the argument of the moon influencing earthquakes is the alleged supermoon against ordinary perigees. (If this were true, then the correlation would be at EVERY perigee, not just the closest perigees.

    In the end, I have only one question. Is it the perigee that is the problem here or the variance of the gravitational force exerted by the moon during the lunar month that is correlated with the Earth?

    Finally. The influence of the Earth by tides is not only o the ocean, but the crust of the Earth itself. The usual thing missing in these discussions is always the daily so-called solid tides as well.

    While “moonageddon” is of course an misguided illusion of the beliefs of non-science, the issue with science is we dumb-down the whole truth to such an extent that the obvious holes in the simplification of the actual theory are easily exploited. Without explaining the whole essential story of the effects of gravitation from Moon on the Earth, means the science is being weakened and/or compromised.

    Note: Higher “king tides” are produced as the Earth is closest to the Sun around January 4th or 5th. The sun influences the tides by about 25%. The moon by 75%. Calculating high or low tides tides are calculated by the variances (and the maxima and minima of those variances) of the combined two forces. Tides are greatly influence by the slope of the topography terrain, its density, harmonics, and even the resistance of the material flow around the shoreline or continent. An algorithm of the tide is complex, and is based on work by the US Naval Observatory.

    (Those interested in ocean tides should be aware of a freeware program called JTides at http://www.arachnoid.com/JTides/index.html [I’m in no way associated with this software. I just bring the attention of others to it.] Much can be learned by the program, especially when you correlated to the Moon by astronomical data.)

    • Hello Andrew. Thank you again for your interesting and detailed comments. I am not sure that I agree with all of them though! Take the first comment about the lag between the Moon crossing the meridian and the following high tide. This is called the lunitidal interval and for Sydney the Australian Hydrographic Service http://www.hydro.gov.au/prodserv/tides/lunitidal-intervals.htm lists 8 hours 15 minutes, though they note that the concept is now obsolete. On the night of 20 March 2011 the Moon crossed the meridian at 0:45 am AEDT so we would have expected the next high tide at 9:00 am. According to the much more detailed tidal predictions for Sydney from the National Tidal Centre of the Bureau of Meteorology listed in the 2011 Australian Sky Guide http://www.powerhousemuseum.com/publications/publications_item.php?id=246 the time was 9:27 am.

      • Thanks for the additional information on the tides, though I do think we might be talking a little at cross purposes.

        My more simple understand was based on the general assumption that the moon creates two equipotential tidal bulges (a 3D prolate spheroid) with the earths’s oceans — one facing the moon one 180 degrees opposite. Effectively, the tidal bulge remains locked between the moon and the Earth. As the earth rotates on its axis in 24 hours, some location experience two high and two low tides daily.
        When the earth rotates, if the conditions did not have obstacles like continents, varying ocean depth, etc.; then the moon crossing the meridian would correspond roughly to the time of high tide.
        Yet the circulation is not flowing and ebbing evenly, as the water has to rise and fall taking into account that are a myriad of other factors totally possible in the hundreds.* When taking all these into account causes a lag time, which varies significantly from location to location.
        Using the average value, is often simply referred sometimes as the simple ‘semi-diurnal tide’ (it is far more complicated than this), gives the average or mean lag time value of 2.5 hours. (I’d expect in the middle of the Pacific Ocean with little interference by landmasses, the ‘behaviour’ using lunitidal intervals might be more constant – thus predictable.)

        As your reference says; The ‘lunitidal interval’ is applicable “at places where the semidiurnal component of the tide is dominant.” (Could that also infer less ‘interference; from significant parameters like land and continental shelves, means predictions improve?)

        Most places I believe have so-called ‘mixed tides’, varying significantly on the local circumstances influencing tidal flow. (i.e. Ross Island 12h 22m, while the smallest is around −22 minutes at Yam Island. The least I’ve found for the ‘lunitidal interval’ is 2.5 hours BEFORE the moon reaches the meridian, in Norfolk

        If we are referring to “Moonageddon” one would suspect these effects would be taken into account. As they are not even considered, my point was to say usefully predicting earthquakes is just random chance.

        Thanks for you information here.


        * This is more complicated due to the Sun and the small varying distance of mostly the Moon, and a little it by the Sun. It explains why Fourier analysis of the observed tides over long period of time are so important by hydrologists and geoscientists. (From Sydney Observatory’s point of view, I think even H.C. Russell’s in 1870s and 1880s did some basic tidal work look for periodicity in Sydney Harbour. He used some ‘invented’ device, but I’d have to recall the specifics.)

        Note: I have no knowledge on the lag times in the solid tides, but I suspect they would be minimal. Variances I’d think are mostly from the rock densities… but that another story.

  • ISA to Launch Lunar Probe
    March 10, 2011 Daily Press
    The International Space Association (ISA) will launch a probe today to rendezvous with the moon on it’s closest approach to Earth since 2003. “It’s going to be really exciting,” says Mark Fowley, Chief of Mission Operations at the ISA. “We’ve been waiting for the moon to pass closely enough for optimal viewing.” Fowley says.
    The lunar probe, like the recent probe sent by Nasa to study Comet 9P/Tempel, will impact with the lunar surface, sending up a plume of material for ISA to study using light spectronomy. We’ll finally know what type of cheese the moon is made of” said Fowley, “we think it’s Bleu or Roquefort, but this will determine what it really is.”

  • Hi Nick,

    Very good article mate, well explained.

    Just note the small error (typo really) in the 2nd last par: “… but the Moon is at perihelion we would expect an unusually high tide.

    Should read: ” …but the Moon is at perigee we would expect an unusually high tide.


    Les D

        • Hello Lilac. As mentioned in the above post the tides will be high, but not exceptionally so and there will be higher tides later in the year. Maybe in a few decades when sea levels are higher then a similar event could cause problems.

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