THE POWER OF TIDES

THE PUSH AND PULL OF MOONS WITH THEIR HOST PLANETS

- Curtis Manley

Imagine a planet that spins so fast a day lasts not 24 hours but only six. Daytime is brief, and nighttime is only long enough for a nap. The planet's moon rushes across the sky so close it looks 10 times larger than our familiar Moon!

No, this is not one of the Earth-like planets astronomers have discovered around distant stars. It is Earth itself, 4.5 billion years ago, when the Moon was newly formed and orbiting less than 20,000 miles (32,200 kilometers) from Earth.

Since then, the power of the tides has slowed Earth's spin and pushed the Moon's orbit more than 200,000 miles (322,000 km) farther away.

Other tidal forces have affected many of the solar system's moons. They have controlled their orbits, slowed their spins, melted their interiors, and, in the case of Jupiter's moon Io, driven continuous volcanism.

Rise and Fall of Oceans-and Land

Tides on Earth are caused by the gravitational pulls of both the Moon and Sun. The Moon, though, is much closer, and its pull is twice that of the Sun. Tides on Earth affect more than just the oceans. Though you never notice it, twice each day the solid rock under your feet rises and falls about 15 inches (38 centimeters).

The main tidal bulge caused by the Moon's gravity should point directly toward the Moon-but it doesn't. Earth's fast rotation keeps the tidal bulge ahead of the Moon, so the gravity of the bulge pulls the Moon faster in its path around Earth. Because of this extra pull, the Moon spirals away from Earth-about 1.5 inches (3.8 cm) each year-to find a stable orbit.