Moons and tides and stuff like that
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.
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.
Posted on 26 February 2013, in Science fact and tagged gravitational attraction, gravity, high tide, high tides and low tides, Isaac Asimov, low tide, moon orbits, ocean, spring tides and neap tides, tides. Bookmark the permalink. 8 Comments.