Lunar Phases and Motions Study Pack

Kibin's free study pack on Lunar Phases and Motions includes a 3-section study guide, 8 quiz questions, 10 flashcards, and 1 open-ended Explain review question. Sign up free to track your progress toward mastery, plus upload your own notes and recordings to create personalized study packs organized by course.

Last updated May 21, 2026

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Lunar Phases and Motions Study Guide

Trace the Moon's changing appearance from new moon to full moon and back through all eight recognized lunar phases, and understand exactly why they occur. Covers the geometry of Earth-Sun-Moon alignment, the difference between the 27.3-day sidereal and 29.5-day synodic periods, synchronous rotation, and the conditions required for solar and lunar eclipses.

Key Takeaways

  • •The Moon's phases result from the changing angle between the Moon, Earth, and Sun as the Moon orbits Earth, which alters how much of the sunlit half of the Moon is visible from Earth.
  • •The Moon completes one orbit around Earth in approximately 27.3 days (the sidereal period), but the lunar cycle of phases takes about 29.5 days (the synodic period) because Earth is simultaneously moving around the Sun.
  • •The eight recognized lunar phases — new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, third quarter, and waning crescent — progress in a fixed, predictable sequence.
  • •The same side of the Moon always faces Earth because the Moon's rotation period equals its orbital period, a gravitational phenomenon called synchronous rotation.
  • •Solar and lunar eclipses occur only when the Moon crosses the ecliptic plane at new or full moon respectively, not every month, because the Moon's orbital plane is tilted about 5° relative to Earth's orbital plane.
  • •The Moon rises roughly 50 minutes later each day, and its visible shape in the sky can be used to determine whether it is waxing (growing) or waning (shrinking) based on which edge is illuminated.

Why the Moon Changes Appearance: Geometry of Illumination

The Moon produces no light of its own — it reflects sunlight, and at any given moment exactly half of its surface is illuminated by the Sun. What changes night to night is the observer's viewing angle onto that illuminated hemisphere as the Moon travels along its orbit.

The Illuminated Hemisphere

  • •At all times, the half of the Moon facing the Sun is fully lit; the other half is in shadow.
  • •As the Moon orbits Earth over roughly 29.5 days, Earth-based observers see progressively more or less of that sunlit half.
  • •The Moon itself does not change shape — only the visible fraction of its lit surface changes.

Why Phases Are Not Caused by Earth's Shadow

  • •A common misconception is that Earth's shadow causes the Moon's phases; in reality, Earth's shadow only falls on the Moon during a lunar eclipse.
  • •Phases occur because of the Sun-Moon-Earth geometry, not because anything is blocking sunlight from reaching the Moon.
  • •The Moon's shadow, cast away from the Sun, is irrelevant to the phase cycle — it only matters during a solar eclipse.

The Eight Lunar Phases in Sequence

The progression of lunar phases follows a consistent cycle tied to the Moon's orbital position relative to the Sun and Earth, cycling through eight recognizable stages every 29.5 days.

New Moon and Crescent Phases

  • •A new moon occurs when the Moon is positioned roughly between Earth and the Sun; the lit side faces away from Earth, making the Moon invisible or barely visible near the horizon.
  • •As the Moon moves eastward from the Sun's position in the sky, a waxing crescent appears — a thin sliver of light on the Moon's right side (as seen from the Northern Hemisphere).
  • •During a waxing crescent, the unlit portion sometimes shows a faint glow called earthshine, which is sunlight reflected off Earth onto the Moon.

Quarter and Gibbous Phases

  • •At first quarter, the Moon has completed one-quarter of its orbit; observers see the right half of the Moon illuminated, and the Moon is roughly 90° east of the Sun, rising near noon and setting near midnight.
  • •The waxing gibbous phase follows first quarter: more than half but less than the full disk is lit, and the illuminated area continues to grow.
  • •At full moon, the Moon is on the opposite side of Earth from the Sun (about 180° away); the entire Earth-facing hemisphere is lit, and the Moon rises at sunset and sets at sunrise.

Waning Phases Back to New Moon

  • •After full moon, the illuminated portion decreases through the waning gibbous phase, then third quarter (left half lit in the Northern Hemisphere), and finally waning crescent before returning to new moon.
  • •Waning phases are distinguished from waxing phases by which edge is illuminated: waxing moons are lit on the right side, waning moons on the left side (Northern Hemisphere perspective).
  • •The term waxing means the lit fraction is increasing; waning means it is decreasing.

About this Study Pack

Created by Kibin to help students review key concepts, prepare for exams, and study more effectively. This Study Pack was checked for accuracy and curriculum alignment using authoritative educational sources. See sources below.

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