ERTH 105: Lunar Phases and Eclipses

ERTH 105 OL S22: Voyage Through the Solar System - Study Notes

Course Overview

  • Course Title: Voyage Through the Solar System
  • Semester: Spring 2022

Lunar Phases

  • Duration and Cause of Phases
    • Lunar phases are a consequence of the Moon's 27.3-day orbit around Earth.

Characteristics of Moon Phases

  • Illumination

    • Half of the Moon is illuminated by the Sun and half is dark.
    • We perceive a changing combination of the illuminated and dark faces of the Moon as it orbits Earth.

  • Cycle Description

    • The Moon goes through a 29.5-day cycle relative to its phases:
    • Waxing Phase:
      • The Moon is visible in the afternoon and evening.
      • The illuminated portion appears to grow "fuller" each day as it rises later.
    • Waning Phase:
      • The Moon is visible in late night and morning.
      • The illuminated portion appears to decrease or gets "less full" as it sets later.

Thought Questions on Phases of the Moon

  • Example Question:
    • If it's 9 a.m. and you see a moon with half its face bright and half dark, what phase is it?
    • Possible answers include:
      • First Quarter
      • Waxing Gibbous
      • Third Quarter
      • Half Moon

Synchronous Rotation

  • Description:
    • The Moon exhibits synchronous rotation, meaning:
    • It rotates exactly once with each orbit around Earth.
    • This phenomenon results in only one side of the Moon being visible from Earth.

Eclipses

  • Cause of Eclipses:
    • Eclipses occur due to the Earth and Moon casting shadows.
    • When either celestial body passes through the other's shadow, an eclipse occurs.
Types of Lunar Eclipses

  • Categories:

    • Total Lunar Eclipse:
    • The Moon passes entirely through Earth's umbra (the darkest part of the shadow).
    • Partial Lunar Eclipse:
    • Only part of the Moon passes through Earth's umbra.
    • Penumbral Lunar Eclipse:
    • The Moon passes through the penumbra (the lighter part of the shadow).

  • Requirement for Lunar Eclipses:

    • Lunar eclipses can only occur during a full moon.
    • Types of lunar eclipses include penumbral, partial, or total.
Types of Solar Eclipses

  • Categories:

    • Total Solar Eclipse:
    • Occurs when the Moon completely covers the Sun.
    • Only a small central region will experience totality.
    • Partial Solar Eclipse:
    • Occurs in areas surrounding the region of totality where the Moon only partially covers the Sun.
    • Annular Solar Eclipse:
    • Occurs when the Moon's umbral shadow does not reach Earth, producing a ring-like appearance of the Sun.

  • Requirement for Solar Eclipses:

    • Solar eclipses can only occur during a new moon.
    • Types of solar eclipses include partial, total, or annular.

Conditions for Eclipses

  • Eclipse Occurrence Factors:
    • Not all new and full moons result in an eclipse due to the tilt of the Moon's orbit (5° to the ecliptic plane).
    • Approximately two eclipse seasons occur each year, during which:
    • A lunar eclipse occurs at the new moon.
    • A solar eclipse occurs at the full moon.

Summary of Eclipse Requirements

  • Two conditions must be satisfied to have an eclipse:
    • For Lunar Eclipse:
    • It must be a full moon.
    • For Solar Eclipse:
    • It must be a new moon.
    • Additionally, the Moon must be at or near one of the nodes of its orbit where it crosses the ecliptic plane.

Predicting Eclipses

  • Cycle of Eclipses:
    • Eclipses recur in an 18-year, 11 1/3-day cycle known as the saros cycle.
    • The type of eclipse (e.g., partial, total) and the precise location of the eclipse on Earth may vary within this cycle.

Key Takeaways

  • Moon Phases:
    • Caused by the interplay of sunlight and the Moon's position relative to Earth.
  • Eclipses:
    • Occur due to the shadows cast by the Moon and Earth, with specific conditions for their occurrence depending on the alignment of the Earth, Moon, and Sun.