Moons and tides and stuff like that

Picture of low tide

Low tide at Hervey Bay

I think everybody knows that the moon has an awful lot to do with the height of the ocean’s tide, and so it’s self evident that the highest tides would coincide with the full moon. But hang on a minute. There are two tides each day. Why would that be?

It has to do with gravitational attraction. I wrote about that when I discussed how much you would weigh on an exoplanet. We have established that there is gravitational attraction between the Earth and the Moon. That’s why the Moon orbits Earth. Water, being a fluid, is able to respond to this attraction better than solids, such as mountains. Now while this neatly explains why we have a high tide when the Moon is visible, why would there be a second high tide twelve hours later? The Moon isn’t in the sky, it’s on the other side of the Earth. By rights there should be a low tide, as all the water is attracted to the Moon down there (author points down at her feet).

This does not happen because, as noted in the previous discussion, the power of gravity decreases over distance. The Moon is about 384,000km from the Earth. The opposite side of the Earth is 40,000km further away (the approximate diameter of the Earth) at 424,000km. The water on the opposite side of the Earth to the Moon is attracted less (due to the distance) than the water closest to the Moon, as shown in this simple diagram.Diagram of earth, moon and tidal forces

We have two high tides facing the Moon, and two low tides at the sides. Why Spring tides and Neap tides? For that, we have to consider the sun. The very fact that Earth orbits the Sun illustrates the power of gravitational attraction. When the Sun and the Moon are in the same side of the Earth, as at New Moon, the gravitational attraction of the Sun on the world’s oceans is added to that of the Moon, and we have unusually high tides and low tides. At Full Moon, the Sun is at the opposite side of the Earth from the Moon, so the two bodies might seem to be pulling against each other. Remember, though, the Moon and the Sun both produce two bulges, so the two forces still operate to increase the tide. It stands to reason that if the Sun was to the right of the Earth in the diagram, the forces of the Sun and Moon would tend to cancel each other out. But not completely. That’s because the Sun produces a lesser bulge on the far side of the Earth. It is larger than the Moon, has a far greater gravitational pull, but the relative difference in the distance between the Sun and one side of the Earth, as opposed to the other, is much smaller, so the lesser bulge is less pronounced.

And there you were, thinking this was simple. It is, really, I suppose. But I bet you needed to concentrate.

I love this stuff.

Now go away and work out what the tides would be like on a world with large oceans, and three moons of varying diameter, in three different orbits.

8 thoughts on “Moons and tides and stuff like that

  1. mformichelli

    I grew up being told about the moon and the tides, and never imagined it would be so complex- but nature is beautiful like that. Also, I found out something about the moon last year that blew my mind. Some scientists think it helps to stabilize Earth’s orbit and prevents us from doing a slow-motion roll-over in relation to the Sun (over millions of years). Wild!

    1. Greta van der Rol

      Ain’t science grand? I don’t think I heard that about the Moon, but I wouldn’t be surprised. After all, it’s really more a dual planet configuration, since our moon is so large in proportion to its primary.

  2. Steven J Pemberton

    You also have to consider local topography – the shape of the coastline, the direction it’s facing, the depth of the water and any obstacles further out to sea. The world in my Barefoot Healer series has two moons, and I tried to figure out what sort of tides they’d have before concluding that as long as I made it sound vaguely plausible and self-consistent, I could get away with almost anything.

    Tide prediction isn’t an exact science, even today. You have to measure the tide height over a few months and derive a system of equations from the measurements. People used to build machines like these to calculate the future values of the equations – http://en.wikipedia.org/wiki/Tide-predicting_machine They were sophisticated enough that during the Second World War, some of their predictions were classified…

  3. Paul Trembling

    The fun thing – from an SF point of view – would be trying to work out what having two moons would do to a planet’s tides! Assuming, of course, that they’re decent sized moons. I doubt it Phobos & Deimos would do much for tides on Mars, if Mars had any seas! Actually, I can’t think off hand of any SF story that has factored in multiple moon tide effects, though I’m sure that someone must have done. It would make for some interesting plot possibilities, with occasional ‘Mega-tides’ when everything comes into conjunction!

  4. juliabarrett

    It is complicated, isn’t it. And we recently had a King Tide, highest I’ve seen in 20 years.

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