Moon and Tides

For land creatures, life first began in the tidal pools. Life started its assault on land by establishing a staging area on the inter-tidal zones that served as transition to an eventual life out of the water. In a sense, we owe our existence to the tides.

The gravitational pull of the Moon is the main cause of the tides. Two tidal bulges in the Earth’s oceans are produced, and if you draw a line through the peaks of these bulges, it will point in the general direction of the Moon. The tidal bulge closest to the Moon is drawn towards the Moon because the strength of its gravitational attraction is greatest at that point; this is known as the direct tide.

The Moon’s gravity is weakest at its furthest point from the Moon on the opposite side of the Earth, and centrifugal forces produced by the rotation of the Moon and Earth around the system’s barycenter (their common center of gravity) give rise to an opposite tidal bulge.

As the Earth rotates underneath the two tidal bulges, an alternating rise and fall in the level of the sea with respect to the land is produced, creating the inter-tidal zones. Most of the Earth’s coastlines experience a tidal cycle of two high tides (one direct tide and one opposite tide) and two low tides every day.

The magnitude of the tides experienced on different coastlines will vary: some places have very unequal high tides, while others experience just one high tide a day. If the Moon were stationary in the sky, the tidal cycle would last precisely 24 hours, but because the Moon moves in its orbit, culminating in the sky on average 50 minutes later each day, the tidal cycle is about 24 hours and 50 minutes long.

The gravitation attraction of the Sun also tugs at the oceans, and the tides caused by the Sun amount to about half the height of the Moon’s tidal bulges. When the Moon is near new or full phase, the Sun, Moon and Earth are aligned. When this happens, the Sun’s gravitational attraction complements that of the Moon, producing the most pronounced tidal bulges called spring tides. These highest and lowest tides of the month occur every two weeks, around new and full Moon. When the Moon and Sun are at right angles to each other, the tides tug against each other resulting in very little variation in tide levels. These are called neap tides, and they occur during the first- and last-quarter lunar phases.

Both Earth and Moon have considerable angular momentum (a product of their masses and velocities). There is a law of conservation of angular momentum which states that in a system like that of the Earth and Moon, the total momentum must remain constant. Friction between the ocean tides and land masses causes the Earth to lose angular momentum, and, as its rotation slows, the length of the terrestrial day gradually increases. The rate of change in the Earth’s secular rotation actually amounts to just 2.3 milliseconds per century (23 seconds every million years). It is large enough to be measured against atomic clocks, but seems so inconsequential to us. It just means that a billion years from now, our descendants — if the human species survives that long — will have days that last for 30 hours.

Another consequence of the friction between the Earth’s crust and the ocean tides is that the Moon’s angular momentum increases. As the Moon’s angular velocity increases, so does its distance from the Earth. The Moon is actually receding from the Earth by about 3.8 centimeters per year (38 kilometers every million years) — a figure verified precisely with laser beam reflectors placed on the Moon’s surface almost forty years ago by Apollo astronauts. A billion years from now, our descendants will see a much smaller moon — 38,000 kilometers farther away.

And they will have lower tides.