Chapter 3: Problem 38

You are watching a lunar eclipse from some place on the Earth's night side. Will you see the Moon enter the Earth's shadow from the east or from the west? Explain your reasoning.

Short Answer

Expert verified

The observer on Earth would see the Moon enter the Earth's shadow from the west. This is because the Earth's shadow appears to move from west to east, due to the Earth's faster rotation and it's position between the Sun and the Moon during a lunar eclipse.

Step by step solution

Understand Relative Motions

Understand that the Earth rotates on its axis from west to east, but the Moon also orbits the Earth from west to east. However, the Earth's rotation is faster than the Moon's orbit.

Visualize the Moon's apparent motion

Visualize that because the Earth's rotation is faster than the Moon's orbit, the Moon appears to rise from the east and set in the west. This is due to the observer's (on earth) perspective.

Identify the Shadow's Direction

During a lunar eclipse, the Earth is between the Sun and the Moon, blocking the Sun’s light. Therefore, the Earth’s shadow will appear to move across the Moon’s face from the west to the east, due to the perspective of the observer on Earth. It is the Earth's shadow that is moving (from the observer's viewpoint), not the Moon.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Earth's Rotation

The Earth rotates on its axis, creating day and night for us. The direction of rotation is from west to east. This movement is quite rapid, completing one full turn roughly every 24 hours.

This rotation has a direct effect on how we perceive celestial events. For example, when observing a lunar eclipse, the swift rotation of the Earth affects our perception of the Moon's movement across the sky. If you were to stand still and watch the sky for a couple of hours, you'd see the Moon and stars rising in the east and setting in the west. This apparent motion is partly due to the Earth's own spin.

Moon's Orbit

The Moon orbits Earth in the same direction that the Earth rotates, which is west to east. Unlike the Earth's rotation, the Moon's orbit is much slower. It takes about 27 days for the Moon to complete one orbit around the Earth.

As a result of this slower movement, compared to Earth's rotation, we usually see the Moon rising in the east. During a lunar eclipse, this fact can seem counterintuitive because the Earth's shadow appears to move in the opposite direction across the moon's surface from what we might expect if we only consider the Moon's orbit.

Relative Motion in Astronomy

Relative motion is a key concept in astronomy that describes how the movement of celestial bodies is perceived differently depending on the observer's position. In the case of a lunar eclipse, multiple motions are happening at the same time: Earth is rotating and the Moon is orbiting Earth.

Because Earth rotates faster than the Moon orbits, we see the Moon traveling across the sky eastward, while at the same time, the shadow during a lunar eclipse will pass westward. This can be confusing, but it's a result of observing two separate movements from a rotating platform—Earth.

Phases of a Lunar Eclipse

A lunar eclipse occurs when the Earth passes between the Sun and the Moon, casting a shadow on the Moon. There are several phases in a lunar eclipse. First, there's the penumbral eclipse, when the Moon passes through the outer part of Earth's shadow and only a subtle dimming may be visible.

Next, the partial eclipse phase occurs as part of the Moon starts to darken. Then there's the total eclipse, where the entire Moon is within the darkest part of Earth's shadow, causing it to take on a reddish color due to refracted light from Earth's atmosphere. Lastly, the phases occur in reverse order as the Moon moves out of Earth's shadow.

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You are watching a lunar eclipse from some place on the Earth's night side. Will you see the Moon enter the Earth's shadow from the east or from the west? Explain your reasoning.

Short Answer

Step by step solution

Understand Relative Motions

Visualize the Moon's apparent motion

Identify the Shadow's Direction

Key Concepts

Earth's Rotation

Moon's Orbit

Relative Motion in Astronomy

Phases of a Lunar Eclipse

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