Understanding the Moon’s influence on the Earth means going well beyond the Moon’s role in causing the tides, although these are the most obvious short-term manifestations. Essentially, understanding the Moon’s influence on Earth means asking the question, what would the Earth be like without the Moon?
The Moon currently orbits the Earth about once every 29.5 days, at a distance of roughly 250,000 miles, although the distance is gradually increasing. The current theory – albeit only a theory – is that the Moon actually formed very early on in the solar system, from debris thrown into orbit by a collision between the Earth and a Mars-sized planet hypothetically named Theia. That impact might have been responsible for knocking the Earth slightly off-kilter on its axis – a tilt which is responsible for the passing of seasons today.
As the Moon circles the Earth (and as Earth rotates at the centre of that orbit), the Moon’s gravity exerts a sizeable influence on the Earth’s surface. It drags a noticeable bulge across the surface of the rotating Earth, which we experience daily in the form of ocean tides. Essentially, the tide is caused by water being dragged or pulled by the Moon. The same force may exert a more subtle influence on the motion of geological plates below the Earth’s surface, particularly early on when it was much closer to our planet and could have contributed to heating of the Earth’s surface and interior. It certainly exerts such a force on the surface of the Moon, where the Apollo astronauts discovered that the forces of gravity can cause tremors known as moonquakes, strong enough to be picked up on seismic sensors.
Other effects on the surface and atmosphere of the Earth may also occur. The tides may have an impact on ocean currents, which in turn play a very large role in global weather cycles such as El Nino-La Nina. The Moon also helps regulate Earth’s axial tilt. Planets without a large moon, like Mars, probably wobble substantially more under the influence of the other planets, namely Jupiter. If so, this would cause considerably greater variation in climate and temperature than we experience here on Earth, where most climate change tends to be very gradual and long-term.
Even life itself bears the marks of the influence of the Moon. Most species, including humans, have evolved in ways that allow them to take advantage of (or least reduce the disadvantages of) the day-night cycle. But virtually all creatures active at night (with the exception of bats, which use echolocation) rely on specially adapted eyesight. Without moonlight, even these adaptations would have little function. In 2009, Scientific American even asked whether there would be life on Earth without the Moon, and concluded that if (as seems very possible) life first emerged in the tidal zones, there might not be.
Earth occupies a privileged position in the solar system, between Venus (possibly too close to the Sun to support life) and Mars (possibly too far from the Sun to support life). However, its habitability is also determined in part by the Moon. It is not entirely a coincidence that the only inner planet with a very large moon is also the only inner planet with life on its surface.