Astronomy — Tides
- We feel Earth’s gravity drawing us downward with a force we refer to as our weight, but that is not the only gravity acting on us. The Moon is less massive and is farther away than the center of the Earth is, but its gravity measurably affects Earth. The side of Earth facing the Moon is about 4000 miles closer to the Moon than the center of Earth, and the Moon’s gravity pulls on the near side of Earth more strongly than the Earth’s center. Though we think of our planet as solid, it is not perfectly rigid, so the Moon’s gravity draws the rocky surface of the near side up into a bulge a few centimeters high. We don’t notice the mountains and plains rising and falling by a few centimeters.
- We do, however, notice the Moon’s influence over the oceans. Seawater is fluid and responds to the small force of the Moon’s gravity by flowing into a bulge of water on the side of Earth facing the Moon. There is also a bulge on the side away from the Moon, which develops because the Moon pulls more strongly on Earth’s center than on the far side. Thus the Moon pulls earth away from the oceans, which flow into a bulge on the far side.
- We can see dramatic evidence of this effect if we watch the ocean shore for a few hours. Though Earth rotates on its axis, the tidal bulges remain fixed along the Earth-Moon line. As the turning Earth carries us into a tidal bulge, the ocean water deepens and the tide crawls up the beach. Later, when Earth carries us out of the bulge, the water becomes shallower and the tide falls. Because there are tow bulges on opposite sides of Earth, the tides rise and fall twice a day.
- The Sun, too, produces tidal bulges on Earth. At new moon and at full moon, the Moon and Sun produce tidal bulges that add together and produce extreme tidal changes — high tide is very high and low tide is very low. Such tides are called spring tides, even though they occur at every new and full moon and not just in the spring. Neap tides occur at first quarter moon and third quarter moon, when the Moon and Sun pull at right angles to each other. Then the tides caused by the Sun reduce the tides caused by the Moon, and the rise and fall of the ocean is less extreme than usual.
- Tidal forces can have surprising effects. The friction of the ocean waters with the seabeds slows the rotation of the Earth by 0.001 seconds per day per century. Fossils of marine animals confirm that only 400 million years ago, Earth’s day was 22 hours long. In addition, Earth’s gravitational field exerts tidal forces on the Moon, and, although there are no bodies of water on the Moon, friction within the flexing rock has slowed the Moon’s rotation to the point that it now keeps the same face toward Earth at all times.
- Tidal forces can also affect orbital rotation. Friction with the ocean beds drags the tidal bulges eastward out of a direct Earth-Moon line. These tidal bulges contain a large amount of mass, and their gravitational field pulls the Moon forward in its orbit. As a result, the Moon’s orbit is growing larger, and it is receding from Earth at about 3 cm per year, an effect that astronomers can measure by bouncing laser beams off reflectors left on the lunar surface by the Apollo astronauts.
- These and other tidal effects are important in many areas of astronomy. Tidal forces can pull gas away from stars, rip galaxies apart, and melt the interiors of satellites orbiting near massive planets.
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