Lecture on the Phases of the Moon

Introduction

• The Moon is a prominent object in the sky besides the Sun.
• Features of the Moon captured imagination, poetry, science, and even space exploration.
• The Moon's appearance changes daily; it can be seen during the day and night.

Understanding the Moon’s Phases

• The Moon is a 3,500-kilometer-wide rock.
• Surface is dark, with reflectivity similar to a chalkboard.
• Appears bright due to reflection of sunlight.
• Phases are caused by the Moon's spherical shape and orbit around Earth.
• Terminator: the line dividing the lit (day) side from the unlit (night) side.

Phases of the Moon

1. New Moon

   • Sun, Moon, and Earth are aligned.
   • Moon is between Earth and Sun; only the dark half is visible from Earth.
   • Moon is up during the day.

2. Waxing Crescent

   • A few days post-New Moon, a thin crescent is visible.
   • Crescent appears in the western sky after sunset.

3. First Quarter

   • Occurs about 7 days after New Moon.
   • Moon is 90 degrees from the Sun; half-lit.
   • Termed as First Quarter because it's a quarter way through its orbit.

4. Waxing Gibbous

   • More than half of the Moon is visible.
   • Rises in the late afternoon.

5. Full Moon

   • Two weeks after New Moon, the Moon is opposite the Sun.
   • Fully illuminated side is visible; rises at sunset, sets at sunrise.

6. Waning Gibbous

   • Post-Full Moon, the lit side begins to shrink.

7. Third Quarter

   • Occurs three weeks after New Moon; half-lit.
   • The side that was lit is now dark.

8. Waning Crescent

   • Crescent is visible a few days before New Moon.

Additional Points

• The Moon orbits the Earth every 29.5 days.
• The term 'month' is derived from 'Moon'.
• Moonrise and set times vary with phases.
• Earthshine: light from Earth's reflection illuminates the Moon's dark side.

Observational Highlights

• Moon's consistent yet changing phases offer a comforting familiarity.
• Observing with binoculars or telescopes reveals surface features.
• The Moon illustrates the dynamics of celestial mechanics.

Conclusion

• Understanding the Moon's phases provides insight into its orbit and illumination by the Sun.
• The cyclic nature of phases enriches our connection with the celestial world.

Credits

• Written by Phil Plait, edited by Blake DiPastino, consulted by Dr. Michelle Fowler.
• Co-directed by Nicholas Jenkins and Michael Aranda with graphics by Thought Cafe.