Cycles of The Moon

1. Cycles of The Moon Chapter 3

2. The Motion of The Moon • What direction does the moon move in the sky? • East… about 13º each day • Result of moon’s orbit around Earth • Do you see the same side or different sides of the moon? • Same side… the markings on the face don’t change

3. Phases • Sunlight illuminates ½ of the moon… just like Earth • The moon appears to go through phases because we see different amounts of the sunlit side… depending on where it is in its orbit. • Full: our side fully lit, far side dark, opposite Sun in the sky • New: our side dark, far side fully lit, close to Sun in the sky

6. Phases Continued… • Full – disc facing Earth lit (50% of the moon) • Waxing – right side of disc facing earth is lit • Waning – left side of disc facing earth is lit • Gibbous – more than light than shadow • Crescent – more shadow than light

7. More Phases… • 1st Quarter – the right ½ of the disc facing Earth is lit (1/4 of the moon) • 3rd Quarter – the left ½ of the disc facing Earth is lit (1/4 of the moon)

8. Phases In Order • Full • Waxing Gibbous • 1st Quarter • Waxing Crescent • New • Waning Crescent • 3rd Quarter • Waning Gibbous… then back to Full

9. Moon During The Day?? • You can see the moon during the day depending on where it is in its orbit around Earth. • It appears pale because the Sun is so bright.

10. Orbital Period vs Phase Cycle • Sidereal Period – time it takes the moon to circle the sky once, 27.3 days • Synodic Period – one complete cycle of lunar phases, 29.5 days

11. At New Moon, the moon is near the Sun in the sky. • After 27.3 days, the moon has returned to its same position in the sky but the Sun has moved along the ecliptic. We don’t “see” a New Moon… • Wait another 2 days and the moon has caught up with the Sun… New Moon