This difference in forces between the near edge, the center, and the far edge defines what we call tidal forces. This means - after a little math - that the force of gravity of the Moon on the far side of the Earth is about 3.2% weaker at the far edge of the Earth than it is at the center of the Earth, and about 3.4% stronger at the edge of Earth nearest the Moon than it is at the center. But one edge of the Earth will always be closer to the Moon by 6,370 km (the radius of the Earth), and the opposite edge will always be farther from the Moon by the same amount. When we speak about this distance, however, we are talking about the distance from the center of the Earth to the center of the Moon. The Earth is quite far from the Moon, at an average distance of 384,400 km. If you were to look at everything in our Solar System and ask what affects the Earth the most, gravitationally, you'd think to look at two things: the Moon, because it's massive and it's very, very close to us, and the Sun, because it's extremely massive, even though it's quite far away. If you were to take a look at our Solar System, and you were to move the Earth out to where Pluto is, you'd find that the force of gravity from the Sun on the Earth would be an astounding 1,600 times weaker than it is today, as Pluto is 40 times as far away as Earth is from the Sun! The farther away you are from something, the weaker gravity's pull is on you. The reason we have any tides at all are twofold: the Earth is pretty big and gravity cares how far away you are. Variations in the height of the water level are typically on the order of three meters (maybe ten feet) each day, depending on a couple of factors, which I'll go into below. ![]() This goes in a cycle twice per day, with the ocean level reaching its highest point twice daily (high tide), having the water recede over a period of six hours until it reaches its lowest level (low tide), and then having the water level rise again over a period of another six hours until it reaches the next high tide. As you all know, when the tide comes in at the ocean, the water level appears to rise (and can do so significantly), while at low tide, the water level appears to drop. Last week, our longtime reader Pamela asked if I could explain how the tides work. When you get into a tight place and everything goes against you, till it seems as though you could not hang on a minute longer, never give up then, for that is just the place and time that the tide will turn.
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