Overview
This lecture explains why the Moon has phases, how its appearance changes over the month, and what causes these patterns as seen from Earth.
The Moon’s Appearance and Behavior
- The Moon is a rocky sphere about 3500 km across that reflects sunlight, making it appear bright.
- Half of the Moon is always illuminated by the Sun, just like Earth or any spherical object in space.
- The visible phase of the Moon depends on the Sun’s angle and our viewpoint from Earth.
Causes and Cycle of Lunar Phases
- The Moon's phases are caused by its orbit around Earth and the changing angle of sunlight as seen from Earth.
- The Moon completes a full cycle of phases in about 29.5 days, defining the length of a month.
- The terminator is the line dividing the day and night sides of the Moon.
Lunar Phases Explained
- New Moon: The Moon is between Earth and the Sun; only the dark side faces Earth, making it nearly invisible.
- Waxing Crescent: A thin crescent appears as the Moon moves east; visible after sunset near the horizon.
- First Quarter: The Moon is ÂĽ through its orbit; half of its face is illuminated.
- Waxing Gibbous: More than half is lit as the Moon approaches full phase.
- Full Moon: Earth is between the Moon and Sun; the fully-lit side faces Earth, visible all night.
- Waning Gibbous: After full, the visible lit portion shrinks.
- Third Quarter (Last Quarter): Moon is Âľ through its cycle; again half-lit, but the opposite side as first quarter.
- Waning Crescent: Only a thin crescent is lit before the cycle repeats at new Moon.
Special Observations
- The Moon is visible as often during the day as at night, depending on its phase.
- The timing of moonrise and moonset shifts with each phase.
- From the Moon, Earth goes through opposite phases (e.g., full Earth during new Moon).
Earthshine
- Earthshine is faint illumination of the Moon’s dark side by sunlight reflected off Earth.
Key Terms & Definitions
- Terminator — the dividing line between the lit day side and dark night side of the Moon.
- Waxing — describes phases when the lit portion of the Moon is increasing.
- Waning — describes phases when the lit portion of the Moon is decreasing.
- Gibbous — a phase where more than half of the Moon is illuminated.
- Earthshine — sunlight reflected from Earth that lights up the dark side of the Moon as seen from Earth.
Action Items / Next Steps
- Observe the Moon at different times over a month to identify its phases.
- Review diagrams or animations illustrating the Moon’s phases and orbit.