We all learned about the Tides in primary school.  The explanation was always a bit murky, but we knew that the moon was involved.  Sometimes the Sun got involved as well, and then we have spring tides. It was vague, it was oversimplified, and one got the impression that the teachers weren’t entirely sure how it worked either.  Diagrams like the one below only added to the confusion.

It might be of some comfort to know that for most of human history, theories connecting the moon to the tides were considered superstitious nonsense (this at a time when the moon was usually held responsible for the success or failure of crops, people’s moods, people’s sanity, and whether or not anybody was likely to turn into a werewolf!)

The idea of tides first enters the western historical record somewhere around 325 BC when the Greek explorer Pytheas sailed to the Atlantic ocean and noticed something very new and interesting:  The water level on the shores of the Atlantic rose and fell twice a day.  Now the Mediterranean sea does not really experience tides – it is almost landlocked and by the time enough water surges through the Straits of Gibraltar to start affecting the level, the tides have already changed, and they have to start flowing back.  He dutifully recorded the levels at high and low tides, as well as the times of day when they happened, and noticed some important details.  First, the tides appeared twice a day (two high tides and two low tides), and secondly that the levels of the tides varied over the course of about a month.  In fact, Spring Tide coincided with Full moon and New moon, while Neap tides coincided with First and Third quarter (More on the phases of the moon here).  Finally, he noticed that one high tide per day happened when the moon was highest in the sky, and the other when the moon was not visible at all.  He suggested that since the tides synchronised so well with the Moon, perhaps the Moon was causing the tides.

For the next two thousand years, hardly anybody took his idea seriously.  The problem was nobody could imagine how it could be possible.  If the moon had some sort of attractive force which drew the water to it, thus causing high tide when the moon was high in the sky, then surely you would have ONE high tide per day, and the low tide would be when the moon was lowest?  How to explain the second high tide?

The answer came in 1685 when Isaac Newton announced his theory of Universal Gravitation.  For the first time it was understood that the same mysterious force which glued us to the ground and made apples fall from trees was also responsible for the movements of the planets in the sky.  It was what kept earth in orbit around the Sun, and it was what kept the Moon in orbit around the earth (Contrary to popular belief, these ideas were well understood and accepted by then.  Nobody knew how it worked, of course, but decades of carefully precise observations by men like Tycho Brahe, Johannes Kepler and Galileo Galilei had demonstrated these things beyond any doubt).

Newton’s theory of universal gravitation says that every object in the universe (down to the atoms that make up your body) exerts a small force on every other object in the universe. The closer two objects are to each other, and the more massive they are, the stronger the force between them. More interestingly, the distance affects the force according to the “inverse square rule”, which simply means that if you double the distance, the force becomes four times weaker (triple the distance, and the force is nine times weaker. Ten times as far, and the force is one hundred times weaker).

If we look at the Earth and the Moon, we see that they are (on average) about 384,400 km apart. The Earth has a radius of roughly 6400 km. This means that the side of the earth closest to the moon is about 378,000 km away, and the side furthest is about 390,800 km away. If we do the maths (by Newton’s theory), we find that there is about a seven percent difference in the gravitational forces from the Moon to the near side of Earth and from the Moon to the far side of Earth. In other words, the moon tugs quite hard on those parts of Earth which are closest to it, less hard on the middle of the Earth, and less hard still on the furthest parts of Earth. The entire planet is pulled unevenly and stretches! Of course, the Earth is made mostly of pretty solid granite and iron, so doesn’t stretch very much, but the oceans are just water and they distort quite easily.

The result of all this is that waters of the earth form two humps, one directly underneath the Moon, and one directly opposite (as in the diagram at the top of the page). This distortion is fixed and doesn’t move (relative to the Earth-Moon system, of course). As Pytheas, sitting on his Atlantic shore, watched the water levels rise and fall, he was carried around by the earth’s rotation, and moved through first one hump, then the shallow region, then the next hump. From his point of view, the waters moved up and down the shore. Smaller bodies of water, like landlocked seas and lakes, take their water along with them, and so are unable to flow up and down into these bulges, so they do not experience tides.

But what about Spring and Neap tides? Well, the Sun is some 27,000,000 times the mass of the moon, and 392 times further away. It’s gravitational effect on the earth is 176 times stronger than the Moon’s (which is why we orbit the Sun and not the Moon!). Obviously it must also cause tides, separate to those from the Moon, but the difference between the distances from the Sun to the earth’s nearest and furthest points is proportionately very small. Accordingly, the Sun’s tides are actually much less than the Moon’s. When the Sun and Moon line up, at Full and New Moon, their tidal effects add up which causes Spring Tide. When they are at ninety degrees to each other (first and third quarter), they work to cancel each other, and we have Neap Tide.

And there it is. With a bit of background, and some basic knowledge on the nature of gravity, tides become easy to understand. It goes without saying that every other body in the Solar System also exerts a tidal effect on the Earth, but their relatively small sizes and immense distances make these effects unnoticeably small.