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Chapter 2: Problem 25

Assume that the Moon's orbit is circular. Imagine that you are standing on the side of the Moon that faces Earth. a. How would Earth appear to move in the sky as the Moon made one revolution around Earth? b. How would the "phases of Earth" appear to you compared to the phases of the Moon as seen from Earth?

Short Answer

Expert verified
Earth appears almost stationary in the Moon's sky, slightly moving due to libration. Earth phases are complementary to Moon phases; full Earth coincides with new Moon and vice versa.

Step by step solution

01

Understanding Earth's Apparent Motion

Since the Moon's orbit around Earth takes approximately 27.3 days and the same side of the Moon always faces Earth (due to synchronous rotation), Earth will appear to stay in a fixed position in the Moon's sky. However, due to the slight wobble called 'libration', Earth will move slightly back and forth.
02

Earth's Phases on the Moon

Just as the Earth’s viewing of the Moon goes through phases (new moon, first quarter, full moon, last quarter), the Moon’s viewing of Earth will also go through phases. When the Moon is at a new moon phase to Earth observers (the Moon is between the Earth and the Sun), an observer on the Moon would see a 'full Earth'. Conversely, when the Moon is full to Earth observers, an observer on the Moon would see a 'new Earth'.

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Key Concepts

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

synchronous rotation
The Moon orbits Earth in what is known as 'synchronous rotation.' This means that the same side of the Moon always faces Earth.
Synchronous rotation occurs because the Moon takes the same amount of time to rotate on its axis as it does to orbit Earth—about 27.3 days. Due to this, an observer on the Moon would always see Earth in the exact same spot in the sky. From Earth, we always see the same side of the Moon.
Imagine standing on the Moon’s surface and looking up to see Earth. No matter where you are on the visible half of the Moon, Earth would seem to hold a constant position—a result of synchronous rotation.
libration
Even though the Moon has synchronous rotation, Earth doesn’t appear perfectly stationary from its surface. This is due to a phenomenon known as 'libration.'
Libration causes the Moon to 'wobble' slightly as it orbits Earth. This wobble arises because of three factors:
  • Elliptical orbit: The Moon’s orbit around Earth is not a perfect circle but an ellipse.
  • Axial tilt: The Moon's axial tilt relative to its orbital plane causes changes in viewing angles.
  • Orbital tilt: The Moon’s orbit is slightly tilted relative to Earth's equator.

Due to libration, an observer on the Moon would see Earth move slightly back and forth and up and down, creating a small oscillation in its position.
phases of the Moon
The 'phases of the Moon' are the different shapes the Moon appears to have over a month. They include stages such as new moon, first quarter, full moon, and last quarter.
The phases occur because of the relative positions of the Moon, Earth, and Sun.
When the Moon is between Earth and the Sun, we see a new moon because the side facing Earth isn't lit. A full moon happens when Earth is between the Moon and the Sun, so the fully lit side faces us.
On the Moon, the Earth also goes through phases:
  • When observers on Earth see a new moon, someone on the Moon sees a full Earth.
  • When Earth observers see a full moon, Moon observers see a new Earth.

This cyclical pattern of phases emphasizes the interconnected dance between Earth, the Moon, and the Sun.

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