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By Root 767 0

gravity actually adds to the Earth’s gravitational pull and squashes everything! This is the origin of the tides; because water is easier to stretch than the rock that forms the ocean floor, the water in the oceans bulges outwards relative to the ground beneath the Moon and on the opposite side of Earth to the Moon. The difference in water heights is only a few metres, but can be much higher depending on the shape of the shoreline. It’s worth mentioning that there are also tides in the rocks of Earth’s surface; gravity doesn’t just affect water! But rocks are very rigid, and so don’t stretch much. The surface of Earth does, however, rise and fall by a few centimetres due to tidal effects. As Earth rotates beneath the tidal bulge raised in the oceans, the distorted water surface sweeps past the shorelines and we experience two high and low tides per day.

Next time someone starts trying to tell you that we are made of water and therefore the Moon must have an influence on us, you will now be justified in having a strange, blank and perhaps slightly pitying expression on your face for two reasons. One is that because the tides are a differential effect (that is to say they depend on the change in the strength of the Moon’s gravity across the diameter of Earth), the tidal effect on you is utterly insignificant and makes no difference to you at all because the difference in the Moon’s gravitational force across something the size of your body is negligible. Secondly, it has got nothing at all to do with water in any case!

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Gravity is always a two-way street – just as the Moon raises tides on Earth, so Earth must cause tides to sweep across the surface of the Moon.

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The relationship between the Earth and the Moon is not just a one-way street; just as the Moon’s gravity has transformed our planet, so in turn Earth has transformed its neighbour.

Throughout human history, half of the Moon’s surface remained hidden from view, and it wasn’t until 1959, when the Soviet Luna 3 probe photographed the far side of the Moon for the first time, that we caught our first glimpse of this hidden landscape. Nine years later, the astronauts on board Apollo 8 became the first humans to leave Earth’s orbit and the first human beings to directly observe the far side of the Moon with their own eyes. The reason only one side of the Moon faces Earth, appearing frozen in time and unchanging in the seemingly ever-moving night sky, is down to the tidal effects.

Billions of years ago, the view of our satellite from Earth would have been very different. In its childhood, the Moon rotated much faster, and both sides of its surface would have been visible from Earth. From the moment of its birth, the Moon felt the tug of Earth’s gravity – a force that would have been even greater than it is today because the Moon was also closer to Earth.

LUNAR GRAVITY DIFFERENTIAL FIELD

The lunar gravity differential field at Earth’s surface is known as the tide-generating force. This is the primary mechanism that drives tidal action and explains two equipotential tidal bulges, accounting for two daily high waters.

THE EFFECT OF TIDAL LOCKING ON THE EARTH AND MOON

As the Earth–Moon system moves towards being perfectly tidally locked, the Moon is gradually drifting away from Earth.

A glance at Newton’s Law of Universal Gravitation will tell you that gravity is always a two-way street – just as the Moon raises tides on Earth, so Earth must cause tides to sweep across the surface of the Moon. These tides are not in water, of course, but in the solid rock of the lunar surface. In an amazing piece of planetary heavy lifting, the Moon’s crust would have been distorted by up to 7 metres (22 feet)!

This giant tidal bulge sweeping across the Moon had an interesting effect. As the Moon turned beneath the giant parent planet hanging in the lunar sky, the rock tide was dragged across its surface, but the rising of the tide isn’t instantaneous; it takes time for the surface of the Moon to respond to the pull of the Earth. During that time, the Moon