A tidal force is the effect of an object’s gravitational force on another, nearby object, such as a planet or satellite. This force can influence the object’s shape, orbit, and other characteristics. The most familiar effect of the tidal force is the creation of the high and low tides in the Earth’s oceans. These are created by the gravitational effect of the moon. Other effects are readily observable on the Earth, moon, and other objects in the solar system.
In the 17th century, English scientist Isaac Newton described many of the effects of gravity on the Earth, the moon, and other cosmic bodies. He published his calculations in the groundbreaking treatise Principia Mathematica in 1687. This included a discussion of the tidal force. The word tide refers to the distortion in a body created by another body’s gravity. It is most often used to describe the periodic rise and fall of the Earth’s oceans on its shorelines.
The tidal force causes a planetary body to bulge slightly in the direction of the outside gravitational force. In the case of the Earth’s shape, this effect is so small as to be hardly noticeable. The water in the oceans, however, is so strongly affected that it surges along the shoreline closest to the moon. This is known as high tide. The water recedes on shores not under this influence, an effect called low tide.
Another effect of the moon’s tidal force is to pull the Earth itself slightly away from the oceans on the opposite side of the planet. Consequently, the oceans experience two high tides in a 24-hour period: one when the moon is directly overhead and another 12 hours later, when it is over the opposite side of the globe. When the sun’s weaker gravitational pull combines with that of the moon, more extreme tides, called spring tides, occur. At other times, the sun partially cancels the moon’s tidal force; the resulting weaker tide is known as neap tide.
The tidal forces have other observable effects. The tidal force of the Earth on the moon results in the same side of the moon always facing the Earth as both bodies rotate in tandem. Objects unable to withstand the stresses of tidal forces can be destroyed by them. This occurred in 1992, when Comet Shoemaker-Levy 9 passed too close to the planet Jupiter. The comet broke into fragments, which then spectacularly collided with the gigantic planet on another pass in 1994.