Supertides are real!

“But less intelligible still was the flood that was caused by forty days’ rain, and forty nights’. For here on the moors there were some years when it rained for two hundred days and two hundred nights, almost without fairing; but there was never any Flood.” –Halldór Laxness

Once every 18 years, a French Abbey — Mount St.-Michel — becomes inaccessible, as the English Channel rises to such levels that the causeway that normally reaches it becomes engulfed by the surrounding waters.

Image credit: Associated Press.

Image credit: Associated Press.

You might think this is due to the tides, where the Earth, Moon and Sun align, but then shouldn’t this happen twice a month, during Spring Tides?

Image credit: © 2002 By Keith Cooley, via

Image credit: © 2002 By Keith Cooley, via

As it turns out, the effects are much more subtle, and involve the Moon’s elliptical orbit and the equinoxes as well, but when they all align, once every 18 years, a supertide is the result, and Mount St.-Michel becomes an island!


  1. #1 PJ
    Perth, west Oz
    March 26, 2015

    Great article, but ……. doesn’t help catching fish down here.

  2. #2 Wiehan Rudolph
    Johannesburg, Gauteng, ZA
    March 26, 2015

    Quite a sight. Would you say it is possible for us to deduce that an extreme opposite of this could occur in other regions? If the tides are high at one point, it should be low on another point, shouldn’t it?

    UP Student number: 15086187

  3. #3 Andreas
    March 26, 2015

    If the distance to the sun is 400 times that to the moon, shouldn’t the mass of the sun “only” be 400^2=160k times the moon to cause the same gravitational effect because of the inverse square law? I definitely miss something here, because as you said a “sun tide” is only 40% as strong as a “moon tide”, but what is it?

  4. #4 Eric Lund
    March 26, 2015

    @Andreas: The magnitude of the tide goes as 1/R^3, not 1/R^2 (where R is the distance between centers of mass). This is because the tides result from the force on the near side (with respect to the body causing the tide) being larger than the force on the far side. If you do the math, you will find that the terms proportional to 1/R^2 cancel, so 1/R^3 is the leading order.

  5. #5 Noel
    March 26, 2015

    Thanks for this. I was in Costa Rica last week and the tides were bigger than the last time I was there – water came into the on-the-beach restaurant I was at, and it never did that last year! I thought it was the “spring tide” affect, but I actually wondered if might have something to do with the equinox and/or the eclipse.

  6. #6 Toby Goodwin
    Kilmarnock, UK
    March 27, 2015

    Great article! Since we’re getting into Solar-Terrestrial Physics (and Lunar too!)… I’ve often wondered about the effects of tidal forces on the atmosphere (weather!) and Earth’s mantle (earthquakes / volcanoes?)

    Has this been studied at all?

  7. #7 Stan Roelker
    Genoa City Wisconsin
    March 28, 2015

    To me you are the “Carl Sagan” of the Internet! You show me the marvels of the Universe we are part of-from sub atomic particles to giant stars; and you explain it in down to earth language. Keep up the good work.

  8. #8 Tiane' Schmidt
    Pretoria, South Africa
    March 29, 2015

    This is incredible! Does earth warming have an effect on this? Such as if sea level rises due to the melting of icebergs, will it affect this process causing the water to rise higher? When will this happen again? 15008861

  9. #9 Dean Carlisle
    Nelspruit, South Africa
    March 29, 2015

    This apparent “supertide” is a remarkable phenomenon. Having read the blog it appears that in this particular example of a “supertide” almost no damage is caused as the locals are aware of its coming. I could however not stop myself from thinking about the effects such a “supertide” would have on other parts of the world. millions could be lost to damages resulting from these phenomenons and I believe a more in-depth study should be undertaken to assess the possibilities of such an event happening in other areas.

  10. #10 TN Mphateng
    Pretoria, South Africa
    March 30, 2015

    Wiehan, I think it is possible to deduce that the opposite could occur in other regions. Judging by the third image, maximum tides occur when the sun, the earth and the moon are aligned; during full moons; and when the moon is closest to the earth. So, for a region on earth that is furthest form the moon, and also during the first and last quarters of the moon, the tides should be very low.

  11. #11 Ntokozo Dlamini
    Pretoria,South Africa
    March 30, 2015

    Wow,that is very fascinating stuff.Also,like Dean I am very curious to know if such tides have devastating effects on the inhabitants of such surrounding areas.And more especially,are they (always) predictable so as to know which of these areas are no-go-areas during which time(s) of the year?

    Thank you for such revelations.

  12. #12 Ruan de Witt
    Pretoria South Africa
    March 31, 2015

    This is extremely fascinating, especially considering that this is an event that occurs all the time, with the occasional supertide that is mentioned. This truly explains and gives a prime example of Newton’s Universal Law of Gravity and how it affects our planet’s tides. Thank you for sharing with us.

  13. #13 Olga
    March 31, 2015

    Very interesting article and truly a spectacular photo of the “lowest low tide”. I was also wondering about global warming and the effect of the melting ice caps on these super tides. Tiane’, the super tides will happen in 2033 again. 15037780

  14. #14 Lize Claassen 15112595
    April 1, 2015

    The article states that the French Abbey, Mount St. Michel becomes inaccessible once every 18 years I understand. In the picture I realised that there are houses on the island part. So how do these people get access to their food? Or do they travel by boat to the mainland? The island would then only be inaccessible by foot or car. Not completely inaccessible.

  15. #15 Lize Claassen 15112595
    April 1, 2015

    Out of the article I understand that Mount St. Michel becomes inaccessible every 18 years. There are houses on that island. How do they get access to food and life supplies? They could use boats. The island is then only inaccessible by foot or car.

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