Astronomy Ch02.10: Key Concept: Lunar Phases I

Part A Start at the beginning of the Moon Orbit animation, which shows the Sun and part of Earth's orbit. If we wanted to show the Moon's orbit on the scale of this starting frame, we would need to draw a circle that __________. - is smaller than one of the green dashes that show Earth's orbit

The Moon's orbit around Earth is so small compared to Earth's orbit around the Sun that we cannot see it on the scale of the opening frame in the animation. We need the animation to zoom in so we can see the Moon's orbit.

Part B As you play the Moon Orbit animation, notice how the arrows representing the Sun's rays are at first spread apart, but become parallel as the animation zooms in to the Moon's orbit. Why do the Sun's rays become parallel at the end of the animation? - The Sun is so far away compared to the size of the Moon's orbit that all the rays reaching the orbit come from essentially the same direction

As you found in Part A, the entire orbit of the Moon is smaller than one of the green dashes shown at the beginning of the animation. Therefore, although the Sun emits light into space in all directions, all of the rays that reach the Moon's orbit are coming from essentially the same direction.

Part E Look again at the Cause of Lunar Phases animation. To understand phases of the Moon, you must imagine the view from Earth (in the center of the animation) looking out to the Moon as it orbits. At what position(s) in its orbit would we see all of the Moon's daylight side and none of its night side? - at the point to the far left

Note that this is what we call full moon; the Moon looks full in this position because the face viewed from Earth is entirely illuminated by sunlight.

Part G Look again at the Cause of Lunar Phases animation. As the Moon orbits, notice how the astronaut's head always points toward Earth. What do we see from Earth that occurs for the same reason? (Be sure you are viewing the animation with the rotating Moon, not the hypothetical, non-rotating view.) - we always see the same face of the moon

The astronaut shows that the Moon rotates exactly once for each orbit around Earth, which is an example of what we call synchronous rotation.

Part D Although this Cause of Lunar Phases animation is very useful for learning about phases of the Moon, it is inaccurate in some ways. Which of the following correctly identify inaccuracies with this animation? Check all that apply. - the Moon is too big compared to its orbit - the astronaut is too big compared to the Moon - Earth is too big compared to Earth - the Moon is too big compared to Earth

The sizes of Earth, the Moon, and the astronaut are all greatly exaggerated relative to the Moon's orbit. The Moon is also too big compared to Earth, and the astronaut is way too big compared to the Moon. These exaggerations are necessary so that the animation can successfully show its key features, which focus on the way sunlight hits Earth and the Moon and on the rotation of Earth and the Moon.

Part C Play the Moon Orbit animation through to the end, where you see the Moon orbiting Earth at correctly scaled sizes. Notice that Earth's daylight side (white) is on the right in this view, and its night side (gray) is on the left. Where along its orbit would the Moon also have its daylight side on the right and its night side on the left? - everywhere along its orbit

The sunlight is coming from the right, so it must illuminate the right side of the Moon, no matter where it is located in its orbit.

Part F Look again at the Cause of Lunar Phases animation. At what position(s) in its orbit would we see exactly half of the Moon's daylight face and exactly half of its night face? - at the two points at the top and bottom

The top and bottom points represent the first- and third-quarter positions, when the Moon is one-quarter or three-quarters (respectively) of the way through its cycle of phases. As viewed from Earth, we see half of the Moon's daylight side and half of its night side at these times.