Tides are weirder than you think

Tides are weirder than you think

Our world relies on the sea more than ever: 80% of goods traded worldwide move by ship. Today’s mariners take it for granted that they can get an accurate chart of the tides for any location on Earth. This would not have been possible without the work of countless scientists through history. The first workable solution, Lord Kelvin’s tide-predicting machines, came in the 1870s. They were complex masses of gears and pulleys that were the most advanced mechanical computers of their day:

Why this complexity? Ancient Greek philosophers had already deduced that the Moon caused the tides. Isaac Newton determined the Moon’s gravity was responsible in the 1680s, and Pierre-Simon Laplace produced a more refined theory in the 1770s incorporating the Earth’s rotation and landmasses. But Laplace’s equations were very challenging to actually solve for any given location – hence the machines. Let’s go through the different layers that make tides so complicated.

Layer 1: Tides come from the Moon’s gravity

You might remember that the Moon’s gravity causes tides from science class. Those Greek philosophers realized this because high tide gets later by about 50 minutes each day, just as the Moon rises 50 minutes later each day. The connection makes sense, but how does it work?

Think of two satellites orbiting the Earth. The closer satellite feels more gravity than the farther satellite, so it orbits faster. This is why Mercury, the closest planet to the Sun, is also the fastest.

The satellites start lined up… …but they don’t stay that way

If we tied the satellites together with a very strong cable, we could force them to orbit together. This would stretch the cable out taut. This stretching is the “tidal force”.

Tying the satellites together puts tension on the cable. This is the tidal force.

The Moon’s gravity has the same effect on the Earth as they both orbit their common center of mass. Instead of the cable stretching taut, the Earth stretches out. Water stretches more easily than land does, so it bulges up on the sides of the Earth closest to and farthest from the Moon.

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