How does the Moon Influence the Earth? TIDES
Perhaps the most overt effect the Moon has on the Earth is the tide. The tides are the regular rhythmic (periodic) rising and falling of the ocean surfaces. The tides are quite different from tidal surges, which are waves caused by storms or earthquakes. The tides are quite regular and predictable. In fact, if you lived near an ocean you could look up the high and low tide times in a local newspaper. The tides are caused primarily by the gravitational attraction of the Moon and Sun and the rotation of the Earth.
We already know that the Moon and Earth revolve around each other, and the Earth-Moon system revolves around the Sun. We also know that the Moon exerts a gravitational attraction on the Earth. So how does this cause the tides? Well, if the Moon is pulling on the Earth, what do you think would move more, the land or the oceans? The oceans, of course. The oceans closest to the Moon at any given time are "pulled" toward the Moon. Thus, the tide is high on the part of the Earth that is closest to Moon. Also, if the oceans closest to the Moon are being pulled out, the oceans at approximately right angles that "bulge out" decrease in their height slightly. This is where the "low tides" are.
There is also another interesting thing that occurs. The tide is also high on the side of the Earth that is directly opposite from where the Moon is. Why would the tide be high there? Well, as the Earth rotates, the Earth exerts another force on the oceans. This is the force that you may experience on a ride at the amusement park that spins and pins you to the outer wall of the ride. This force is call a "centrifugal force." Centrifugal force is always acting on the oceans, but because the Moon's gravitational force gets weaker as the distance to the Moon increases, the oceans on the far side of the Earth can expand outwardly.
Recall that the Earth rotates once a day. That means if you were standing on the coast, you should see two high tides and two low tides in a day. Actually, on average the time between high and low tide is 12 hours and 25 minutes. The cycle for high and low tide get later by about 50 minutes every 24 hours. [2]
The Sun's gravitational attraction also causes tidal action, but the Sun's attraction is much smaller than that of the Moon because it is so much farther away. When the Sun and Moon are aligned, the Sun's and Moon's gravitational forces act together to create higher high tides and lower low tides than usual. These tides are called "Spring Tides." Conversely, when the Sun and Moon are at right angles to each other, the gravitational forces tend to counteract each other and the tides are more subdued than usual. This situation is called a "Neap Tide."
