Astronomy Ch04.2: Key Concept: Understanding Tides

Part B Most people are familiar with the rise and fall of ocean tides, but do tides also affect land? - Yes, though land rises and falls by a much smaller amount than the oceans.

Tides affect the entire Earth, but they are much more noticeable for the oceans because water flows so much more easily than land. Still, the land rises and falls measurably (by about 1 centimeter) with the tides.

Part F You have found that tides on Earth are determined primarily by the position of the Moon, with the Sun playing only a secondary role. Why does the Moon play a greater role in causing tides than the Sun? - Because the gravitational attraction between Earth and the Moon varies more across Earth than does the gravitational attraction between Earth and the Sun

The Sun exerts a stronger gravitational force on Earth, which is why Earth orbits the Sun. However, tides are caused by the variation in the gravitational attraction across Earth. Even though the gravitational attraction between Earth and the Moon is smaller than the attraction between Earth and the Sun, the Moon's much closer distance makes this attraction vary more across Earth. That is why tides are due primarily to the Moon, with only a secondary effect from the Sun.

Part A As shown in the animation, Earth has two tidal bulges at all times. Approximately where are these bulges located? - One faces the Moon, and one faces opposite the Moon.

The tidal bulges face toward and away from the Moon, because they are caused primarily by the gravitational attraction between Earth and the Moon. (Friction explains why the bulges are slightly ahead of the Earth-Moon line, rather than directly on the Earth-Moon line; but we'll ignore that detail for now.)

One tidal bulge faces toward the Moon because that is where the gravitational attraction between Earth and the Moon is strongest. Which of the following best explains why there is also a second tidal bulge? - The second tidal bulge arises because gravity weakens with distance, essentially stretching Earth along the Earth-Moon line.

Tides are created by gravity, and the tidal force is caused by the fact that gravity weakens with distance. Therefore, the parts of Earth that are closer to the Moon feel a stronger gravitational attraction to the Moon, and the parts of Earth that are farther away feel a weaker gravitational attraction to the Moon. This varying gravitational attraction essentially stretches Earth along the Earth-Moon line, creating tidal bulges on both sides.

Part E As you watch the animation, notice that the size of the tidal bulges varies with the Moon's phase, which depends on its orbital position relative to the Sun. Which of the following statement(s) accurately describe(s) this variation? - Low tides are lowest at both full moon and new moon. - High tides are highest at both full moon and new moon.

As the animation shows, the tidal bulges are largest and the tidal minima are smallest at full moon and new moon, because those are the times when the tidal forces due to the Sun and Moon are aligned (and therefore add to one another). Therefore, high tides are higher and low tides are lower at these times, which are called spring tides. (In contrast, we have neap tides at first- and third-quarter moons, when high tides are not as high and low tides are not as low.)

Part C Any particular location on Earth experiences __________. - two high tides and two low tides each day

The animation shows that any location on Earth passes through both tidal bulges and both tidal minima (the places where the tides are smallest) each day, which means two high tides and two low tides. Again, recall that this is true both of land and oceans, though tides are more noticeable in the oceans because water flows so much more readily than land.