The strength of lunar gravity is related to the moon's unchanging mass and the distance between the moon and the Earth. As the moon follows its elliptical orbit around the Earth, the distance between the two celestial objects changes. The moon's gravitational pull is strongest when it's closest to the Earth.
Effects of the Moon's Gravity
The moon's gravity reaches all areas of Earth, but the pull only noticeably affects large bodies of water, resulting in tides. The moon's gravitational pull is strongest at the sub-lunar point where the moon is closest to Earth. Lunar gravity causes water to bulge at this point, creating high tides; it also pulls water from other areas, creating low tides. Confusingly, the effect also occurs on the opposite, super-lunar side of the Earth where the moon is furthest away. This happens because the gravitational pull is stronger everywhere else, so the water at the super-lunar point is literally left behind to swell up.
Distance Affects Lunar Gravity
The moon's "perigee" is the point in its orbit where it is nearest to the Earth. At this point, the moon's gravitational pull is the strongest, which results in greater tide variation than normal. This variation creates slightly higher high tides and slightly lower low tides. Conversely, the moon's "apogee" is the point where it is furthest from the Earth, which results in slightly lower tide variation than normal.
Adding the Sun's Gravity
The moon's proximity to the Earth causes it to exert a stronger gravitational pull than the sun. However, the sun's effect is magnified at certain times of the year when the Earth's elliptical orbit brings it closer to the sun. During this time, the alignment of the Earth, moon and sun creates spring tides that result in greater tidal variation. The most significant spring tides occur when the moon is at its perigee, resulting in perigean spring tides that occur three or four times per year. However, even under these conditions, high tides typically don't change enough to cause worrisome effects.
The Effects of Earth's Gravity on the Moon
Although the moon affects the Earth with its gravitational field, the Earth's gravitational field exerts an effect on the moon that is 80 times stronger. This massive gravitational pull caused the surface of the moon to bulge toward Earth, similar to how the moon causes large bodies of water on Earth to bulge. Because the Earth and moon once spun at different rates, the bulge on the moon was constantly rotated away from the Earth. However, the Earth's gravity pulled at this bulge, creating significant friction that eventually slowed the moon into a synchronous orbit, which means the moon's rotation and orbital time are the same as Earth's. This effect is called "tidal locking, " and it explains why the same side of the moon always faces the Earth.