Study Notes: Voyage Through the Solar System - Moon Phases and Eclipses

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

Moon Phases

• Date Reference: January 2014 Calendar displaying Moon Phases.

Understanding Moon Phases

• Cause of Lunar Phases:
  • The phases of the Moon result from the Moon's orbit around Earth, which takes approximately 27.3 days.

Characteristics of the Moon's Appearance

• Illumination:
  • Half of the Moon is illuminated by the Sun while the other half remains dark.
  • As the Moon orbits Earth, we observe various combinations of the illuminated and dark portions.

Phases Cycle

• Cycle Duration:

  • The Moon completes a full cycle of phases approximately every 29.5 days.

• Waxing Phase:

  • Occurs when the Moon is visible during the afternoon and evening.
  • The Moon appears to become "fuller" and rises later each subsequent day.

• Waning Phase:

  • Occurs when the Moon is visible late at night and into the morning.
  • The Moon appears to become "less full" and sets later each day.

Interactive Thought Question

• Observation Scenario: You observe the Moon at 9 a.m. with half its face brightly illuminated and half dark.
  • Possible phases:
  • First Quarter
  • Waxing Gibbous
  • Third Quarter
  • Half Moon

Synchronous Rotation of the Moon

• Observational Evidence:
  • Earth can only see one side of the Moon due to synchronous rotation.
  • Definition: The Moon rotates once on its axis for every orbit it completes around Earth, meaning one hemisphere constantly faces Earth.

Eclipses Explained

• Causes of Eclipses:
  • Eclipses result when the Earth and Moon cast shadows due to their respective positions.
  • An eclipse occurs when either celestial body passes through the shadow of the other.

Types of Lunar Eclipses

• Total Lunar Eclipse:

  • The Moon passes entirely through Earth's umbra (innermost shadow).

• Partial Lunar Eclipse:

  • Only part of the Moon passes through the umbra.

• Penumbral Lunar Eclipse:

  • The Moon passes through the penumbra (outer shadow) only.

• Occurrence:

  • Lunar eclipses can only happen during a full moon.

Types of Solar Eclipses

• Total Solar Eclipse:

  • Occurs when the Moon entirely covers the Sun within a small central region on Earth.

• Partial Solar Eclipse:

  • Occurs when only part of the Sun is obscured outside the area of totality.

• Annular Solar Eclipse:

  • Takes place when the Moon's umbral shadow does not reach Earth, visible in a small central region.

• Diagrams:

  • Visualization of Moon's shadow on Earth indicating the different types of shadow - umbra and penumbra.

Conditions for Eclipses to Occur

• Solar Eclipses:

  • Occur only at new moon.

• Lunar Eclipses:

  • Occur only at full moon.

• Eclipse Seasons:

  • Due to the Moon's orbit being tilted at 5° to the ecliptic plane.
  • Approximately two eclipse seasons occur each year, allowing for lunar eclipses at new moon and solar eclipses at full moon.

Summary of Eclipse Conditions

• Eclipse Requirements:
  • For Lunar Eclipses: Must be full moon and the Moon must pass through Earth's shadow.
  • For Solar Eclipses: Must be new moon and the Moon must be positioned at or near its orbital nodes (points where it crosses the ecliptic plane).

Predicting Eclipses

• Saros Cycle:
  • Eclipses recur in an 18-year, 11 1/3-day cycle. However, the type (partial, total) and exact location of the eclipse can vary significantly.

Key Takeaways

• Phases of the Moon result from the illumination from the Sun and the relative positions of the Earth, Moon, and Sun.
• Eclipses occur due to the alignment of the Earth and Moon's shadows, with distinct conditions governing whether a lunar or solar eclipse occurs:
  • Lunar Eclipse: Earth's shadow on the Moon during a full moon.
  • Solar Eclipse: Moon's shadow on Earth during a new moon.
  • The tilt of the Moon's orbit restricts the occurrences of these events to specific periods each year.