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Cycles of The Moon

Cycles of The Moon. Chapter 3. 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. Phases.

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Cycles of The Moon

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  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

  4. 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 light than shadow • Crescent – more shadow than light

  5. 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)

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

  7. http://www.almanac.com/moon/calendar/GA/Atlanta/2011-09

  8. 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.

  9. 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

  10. 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

  11. Lunar Eclipse • Occurs at Full Moon • Moon moves through Earth’s shadow

  12. Earth’s Shadow • Umbra: total shadow, about 1.4 million km long, points directly away from sun • Penumbra: partial shadow

  13. Total Lunar Eclipse • The entire moon passes through Earth’s umbra. • The moon appears red in color because of sunlight bent by our atmosphere.

  14. Interesting Tid-Bits • Aristotle concluded that Earth is a sphere because of the shadow projected causing a lunar eclipse. • A total lunar eclipse takes about 6 hours.

  15. Partial Lunar Eclipse • The moon is tilted about 5° to the plane of Earth. • It may not pass through the center of Earth’s umbra and only be partially eclipsed. • A penumbral eclipse occurs when the moon passes through the penumbra but misses the umbra entirely.

  16. Solar Eclipse • Occurs at New Moon • Moon moves between Earth and the Sun

  17. Why? • The moon and the sun have the same angular diameter. • This allows the moon to cover the sun in our sky.

  18. Angular Diameter

  19. The Math… Small Angle Formula • Angular Diameter = Linear Diameter 206,265 Distance ** We use 206,265 because that is the # of seconds of arc in 1 radian… you don’t have to remember that!  ** Make sure linear diameter and distance are in the same unit.

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