Scholars Online Astronomy - Chapter 3: The Motions of the Moon

Homework

• Reading Preparation
  • Study Guide/Reading Notes
• Key Formulae to Know
• WebLecture
• Study Activity
• Chat Preparation Activities
• Chapter Quiz
• Lab Work

Reading Preparation

Reading: Astronomy, Chapter 3: Eclipses and the Motion of the Moon

Study Notes: notes on your assigned reading from the text

• Section 3.1 — Lunar Phases. Be sure that you know the difference between emitted light and reflected light. Technically, anything that puts out more energy than it can absorb and redirect from external sources emits light. Jupiter is the only object in the solar system that does this besides the Sun, so astronomers might consider it a protostar. You should understand the terms for the lunar phases and when and where the Moon is for each phase. Spend some time with a light for the Sun, a partner for the Moon, and yourself as the Earth and practice predicting where the Moon will be relative to the Sun, and the times of day that you can see it, for each of the major phases.

  Remember that you have two motions to worry about: the observer's position on the Earth as the Earth rotates on its axis, and the Moon's position relative to its source of light, the Sun, and to the Earth. Be sure that you understand the difference between the synodic month and the sidereal month and why it occurs.

• Section 3.2 — Lunar Orientation to Earth. Again you may want to use an accomplice to visualize this properly. The Moon keeps a face to Earth, so it has to rotate once on its axis as it moves around the Earth. One of the very difficult things about astronomy is that it demands that we visualize our position in space even as we move, so these exercises, the SN planetarium, and actual observations will help you get more comfortable envisioning the actual motions. Even if it means you have to bribe siblings with cookies, take the time to go through modeling the lunar motions!
• Section 3.3 — Eclipses occur when the Moon is on the nodes of its orbit. The "nodes" are the points where the Moon's orbit crosses the solar ecliptic. When the Moon is on a straight line to the Sun, its position in its orbit intersects with the Sun's position on the ecliptic. If the Moon is in another part of its orbit relative to the nodes, it will cross above or below the Sun's position, and no eclipse can occur. Study figure 3-6 and make sure that you understand how the Moon's position in its orbits relative to its nodes.
• Section 3.4 — Lunar eclipses occur when the Moon passes through the Earth's shadow, which has two parts. From the umbra, no part of the Sun can be seen; the Earth blocks it out entirely. From the penumbra, some part of the Sun can be seen; the Moon experiences a "partial solar eclipse". Because of the Earth's atmosphere, some red wavelength Sunlight is bent into the umbra, and the Moon receives this filtered red light.
• Section 3.5 — Solar eclipses occur when the Moon passes between the Sun and the Earth, so that from some part of the Earth, at least part of the Sun's light is blocked by the Moon. If the entire face is blocked, the eclipse is total; if only part of the face is blocked, the eclipse is partial. If the Moon is at apogee (far from the Earth), its disk is too small to cover the entire solar disk, and we get an annular eclipse.
• Section 3.6 — Distances based on Eclipses. The pattern of eclipses is driven by the periods of the Moon's orbit about the Earth, the rotation of the nodes, and the rotation of the apsides (the points of apogee and perigee). If you put the three periods together, they repeat every 18 years, 11 days, and 8 hours. This means that the same type of eclipse occurs during with that period -- other eclipses occur within the period, obviously. Can you find pairs of eclipses with the Saros cycle period in figure 3-13 that have the same pattern, shifted 1/3 of the globe west? A second point for this section: how did Aristarchus figure the distances to the Moon and Sun? Why was he incorrect?

Key Formulae to Know

Web Lecture

Read the following weblecture before chat: Lunar Eclipses

Study Activity