Notes on Moon Phases and Eclipses

ERTH 105 OL S22: Voyage Through the Solar System - Spring 2022 Overview

Moon Phases

  • Definition of Moon Phases
    • Lunar phases are defined by the Moon's 27.3-day orbit around Earth.

Explanation of Phases of the Moon

  • Illumination of the Moon
    • Half of the Moon is illuminated by the Sun, while the other half remains dark.
    • The appearance of the Moon changes as it orbits Earth, resulting in a variety of visible phases.
  • Illustrative Diagram
    • Light from the Sun can be shown interacting with the Moon at different times, indicating various phases.

Moon Phase Cycle

  • Cycle Length
    • The complete cycle of Moon phases occurs approximately every 29.5 days.
  • Waxing Phases
    • Visible primarily in the afternoon/evening.
    • Appears to get "fuller" as it moves closer to the full moon phase, rising later each day.
  • Waning Phases
    • Visible in late night/morning.
    • Appears to get "less full" as it approaches the new moon phase, setting later each day.

Thought Questions

  • Example Question
    • Scenario: It's 9 a.m. and you see a Moon with half its face bright and half dark.
    • Options:
    • First quarter
    • Waxing gibbous
    • Third quarter
    • Half moon

Synchronous Rotation of the Moon

  • Definition
    • Synchronous rotation refers to the Moon's rotation aligning perfectly with its orbit, allowing only one side to be visible from Earth.

Eclipses Explained

  • Mechanism of Eclipses
    • Eclipses occur when the Earth and Moon cast shadows and one passes through the shadow of the other.
  • Types of Lunar Eclipses
    • Total Lunar Eclipse: The Moon passes entirely through the Earth's umbra.
    • Partial Lunar Eclipse: Only part of the Moon passes through the Earth's umbra.
    • Penumbral Lunar Eclipse: The Moon passes through the Earth's penumbra only.
  • Conditions for Lunar Eclipses
    • Occur only at full moons.

Types of Solar Eclipses

  • Summary of Solar Eclipses
    • Total Solar Eclipse: Occurs within a small central region where the Moon completely obscures the Sun.
    • Partial Solar Eclipse: Occurs in the lighter area surrounding the totality.
    • Annular Solar Eclipse: Occurs when the Moon's umbral shadow does not reach Earth, resulting in a ring-like appearance of the Sun.
  • Illustrative Explanation
    • Diagrams illustrate the path of the Moon's shadow on Earth during solar eclipses. Only a small region on Earth will experience totality at any one time.

Conditions for Solar Eclipses

  • Occurrence of Solar Eclipses
    • Can only occur during new moons.

Eclipses Frequency and Angular Relationship

  • Reason for Infrequent Eclipses
    • The Moon's orbit is tilted at 5° to the ecliptic plane, which causes eclipses to only occur during specific seasons (approximately two times a year).
  • Eclipse Seasons
    • A lunar eclipse at new moon and a solar eclipse at full moon can occur during these seasons.

Conditions for Eclipses

  • Necessary Conditions
    • For a lunar eclipse: Must be a full moon.
    • For a solar eclipse: Must be a new moon.
    • The Moon must be close to one of the two orbital nodes.

Predicting Eclipses

  • Saros Cycle
    • Eclipses recur according to the 18-year, 11 1/3-day saros cycle, with variations in type (partial, total) and location.

Summary of Key Learnings

  • Phases of the Moon
    • The shifting visibility of the Moon is due to the illumination by the Sun and the positions of Earth, Moon, and Sun.
  • Eclipses
    • Types: Lunar and Solar Eclipses explained, with relevant conditions and occurrences.
    • Lunar Eclipse: Earth's shadow on the Moon, occurring during full moons.
    • Solar Eclipse: Moon's shadow on Earth, occurring during new moons.
    • The inclination of the Moon's orbit results in periodic eclipse seasons each year.