week 2

Course Information

  • Course Code: ERTH 105 OL S22
  • Course Title: Voyage Through the Solar System
  • Semester: Spring 2022

Moon Phases

  • Date Reference: January 2014

Understanding Lunar Phases

  • Primary Concept:
    • Lunar phases are the visual consequences of the Moon's 27.3-day orbit around Earth.

Characteristics of the Moon Phases

  • Illumination:
    • Half of the Moon is illuminated by the Sun, and the other half is in shadow.
    • As the Moon orbits Earth, we view a changing combination of the illuminated and dark sides.

Phases of the Moon Explained

  • Lunar Cycle:
    • The Moon goes through a 29.5-day cycle encompassing the various phases seen from Earth, including waxing and waning phases.
    • Waxing:
      • Moon appears to grow larger; visible in the afternoon/evening, rises later each day.
    • Waning:
      • Moon appears to shrink; visible in late night/morning, sets later each day.

Thought Questions

  • Scenario:
    • At 9 a.m., a half-illuminated moon is observed.
    • Choices for the moon phase:
    • First Quarter
    • Waxing Gibbous
    • Third Quarter
    • Half Moon

Synchronous Rotation of the Moon

  • Concept:
    • The Moon exhibits synchronous rotation.
    • It rotates exactly once for each orbit around Earth, resulting in only one side being visible from Earth.

Eclipses

  • Causes of Eclipses:
    • Eclipses occur when the Earth and Moon cast shadows, leading to an eclipse when one body passes through the other’s shadow.
Types of Lunar Eclipses
  • Total Lunar Eclipse:
    • Occurs when the Moon passes entirely through Earth's umbra (the darkest part of the shadow).
  • Partial Lunar Eclipse:
    • Occurs when only a part of the Moon passes through the umbra.
  • Penumbral Lunar Eclipse:
    • The Moon passes through the penumbra (the lighter part of the shadow).
Conditions for Lunar Eclipses
  • Occurrence Timing:
    • Lunar eclipses can only happen during a full moon.
    • Types include penumbral, partial, or total lunar eclipses.
Types of Solar Eclipses
  • Total Solar Eclipse:
    • Occurs in the small central region where the Moon's umbra falls on Earth.
  • 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.
Conditions for Solar Eclipses
  • Occurrence Timing:
    • Solar eclipses can only occur during a new moon.
    • Types include partial, total, or annular solar eclipses.

Why Eclipses Are Not Frequent

  • Orbital Tilt:
    • The Moon’s orbit is tilted 5° relative to the ecliptic plane.
    • Results in variations and occurrences of eclipses typically during two periods each year:
    • Lunar eclipse at the new moon.
    • Solar eclipse at the full moon.

Summary of Eclipse Occurrence

  • Conditions for Eclipses:
    • For a lunar eclipse:
    • Must be a full moon.
    • For a solar eclipse:
    • Must be a new moon.
    • Node Requirement:
    • The Moon must be near one of the two points in its orbit where it crosses the ecliptic plane (its nodes).

Predicting Eclipses

  • Eclipses recur following an 18-year, 11 1/3-day saros cycle.
  • Variations in eclipse types (like partial or total) and their locations on Earth may vary within this cycle.

Key Learnings

  • Lunar Phases:
    • Result from the positioning of the Sun, Moon, and Earth, with half the Moon illuminated and half in shadow.
  • Eclipses:
    • Lunar Eclipse: Earth's shadow on the Moon.
    • Solar Eclipse: Moon's shadow on Earth.
    • The tilt of the Moon's orbit influences the timing of these eclipses.