Chapter 14 Section 5 What are the motions of the Moon? Objectives: Describe and identify two motions of the Moon. Explain how the Moon’s motions affect the tides.

Key Terms apogee: point at which the Moon is farthest from the Earth perigee: point at which the Moon is closest to Earth

Key Terms spring tide: tide that is higher or lower than a normal tide neap tide: tide that is not as high or as low as a normal tide

Rotation and Revolution Earth and its Moon travel together around the Sun. In addition, the Moon orbits Earth. Its journey around Earth takes about a month. The Moon also rotates on its axis.

Rotation and Revolution The Moon rotates more slowly on its axis than Earth does. It takes the Moon 27 ⅓ days to rotate once on its axis. The Moon revolves around Earth at a speed of about 3,500 km/h. It takes the Moon 27 ⅓ days to make one complete revolution around Earth. This is the same time it takes the Moon to rotate. All of these motions are constantly changing the relative positions of the Sun, Earth, and Moon and how the Moon appears to us in the sky.

CALCULATE How many kilometers does the Moon travel around Earth in one day?

Apogee and Perigee When the Moon is at apogee, the distance from the Moon to Earth varies from about 405,000 to 407,000 km. When the Moon is at perigee, the distance from the Moon to Earth varies from about 360,000 to 370,000 km. This change in distance has an effect on Earth’s tides, among other things.

2. INFER Why do the distances between Earth and the Moon change?

Moon’s Gravity and Tides At certain times in the Moon’s orbit of Earth, the Sun and the Moon are lined up in their orbits. The combined gravitational pull of the Sun and the Moon results in higher high tides and lower low tides on Earth. Therefore, the daily tidal range is greatest at these times. During these periods, which occur in a regular cycle twice a month, the tides become what are called spring tides.

Moon’s Gravity and Tides At certain times of the year, in between the spring tides, the Sun and the Moon pull at right angles to each other in relation to Earth. Because the Sun and the Moon pull at right angles to each other, their gravity forces do not combine or act together on Earth’s oceans. Instead, they pull against each other. As a result, the daily tidal range is small. Tides that occur during this time are called neap tides.

3. INFER How often do spring and neap tides occur?

Moonrise Like the Sun, the Moon appears to rise in the east and set in the west. As the Moon revolves around Earth, Earth must go through more than one rotation to “catch up” with the Moon. Earth must rotate 24 hours and 50 minutes to bring the Moon back into view. As a result, the Moon comes into view at moonrise about 50 minutes later each day.

4. EXPLAIN Why does the Moon rise about 50 minutes later each day?

Classwork CHECKING CONCEPTS How long does it take the Moon to make one complete revolution around Earth? What causes the tides? Why does the Moon not always rise at the same time?

Classwork THINKING CRITICALLY 4. EXPLAIN: Why does the Moon appear to rise in the east and set in the west? 5. ANALYZE: How many times does the Moon rotate during one revolution around Earth? 6. CALCULATE: If the Moon rose at 6:45 P.M. on Tuesday, at about what time will it rise on Thursday?