*The moon travels in a regular and predictable motion.  After today’s lecture, you should be able to:

-Describe the motions of the moon.

-Explain the reason the moon goes through phases.

-Analyze how the Earth-moon-sun geometry causes lunar and solar eclipses.

*Like the sun, the moon rises in the east and sets in the west, due to the rotation of the Earth.

*Unlike the sun, the moon is in orbit around the Earth.

-Each orbit takes about 27 1/3 days to complete, thus, we see the different phases of the moon about once a month.

*The phases of the moon are the daily changes in the moon’s appearance as viewed from Earth.

Two reasons the moon has phases:

  1. We see the moon only because it reflects sunlight.
  2. The moon is in orbit around Earth.
  • The sun can only light the half of the moon that is facing it.
  • We see changing amounts of the sunlit half, or, in other words, a different portion of the bright half is visible from Earth during each phase.
  • Phases 1-5: The moon changes from all dark (new moon) to all light (full moon). 
    • During this time, the moon is said to be waxing (getting more lit up!).
    • Phases 5-8: The moon changes from all light (full moon) to all dark (new moon).
      • During this time, the moon is said to be waning (getting more dark!)
      • New Moon: bright half faces away from Earth; moon can’t be seen
      • Crescent Moon: an edge of the bright half faces Earth
      • Quarter Moon: half of the bright half of the moon (1/4) faces Earth
      • Gibbous Moon: almost all of the bright half of the moon faces Earth
      • Full Moon: the entire bright half of the moon faces Earth

*Lunar Eclipse: When the Earth passes between the sun and the moon and the Earth’s shadow prevents the sunlight from reaching the moon

  • Can occur only at the full moon phase.
  • Only occurs once a year because the moon’s orbit is not in line w/Earth’s orbit. (The moon is usually above or below the path of the Earth’s orbit, thus, preventing an eclipse from occurring.)

*Solar Eclipse: When the moon passes between the sun and the Earth and the moon’s shadow prevents the sunlight from reaching Earth

  • Can occur only at the new moon phase.
  • Again, because of the moon’s orbit, the moon’s shadow usually falls above or below Earth, thus preventing an eclipse from occurring.  Therefore, it only occurs once a year.

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