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The Moon’s gravitational pull on Earth varies because of its fixed mass and changing distance from our planet. As the Moon travels its elliptical orbit, the Earth–Moon separation oscillates, making the pull strongest when they are closest.
The strength of a gravitational force follows Newton’s law: it is proportional to the masses involved and inversely proportional to the square of the distance between them. Since the Moon’s mass is constant, the only factor that changes is distance. The Moon reaches its minimum distance—called perigee—about 356,500 km from Earth, where its influence peaks.
The Moon’s pull creates the familiar high and low tides by bulging oceans toward the Moon. The effect is most pronounced at the sub‑lunar point, the spot on Earth where the Moon sits directly overhead. A complementary bulge forms on the opposite side, the super‑lunar point, because the gravitational pull is slightly weaker there, allowing water to rise.
During perigee, tidal ranges widen: high tides rise a little higher and low tides sink a little deeper. The opposite occurs at apogee, the Moon’s farthest point (~406,700 km), producing a modestly reduced tidal range.
Although the Sun’s mass is vastly greater, the Sun’s pull on Earth is weaker than the Moon’s because it is so far away. However, when the Sun, Earth, and Moon line up—either during a new or full Moon—solar gravity combines with lunar gravity to generate spring tides. The most pronounced spring tides, called perigean spring tides, occur when the Moon is at perigee and the Earth is near perihelion, roughly three or four times a year.
Earth exerts a gravitational pull on the Moon roughly 80 times stronger than the Moon’s pull on Earth. Over billions of years, this tidal interaction caused the Moon’s rotation to slow until it matched its orbital period—a state known as tidal locking. That’s why we always see the same lunar hemisphere from our side.
In summary, the Moon’s gravitational influence is at its peak when it is closest to Earth—at perigee—especially during perigean spring tides when the Sun also reinforces the pull. This interplay shapes the tides that we observe daily.
