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The moon and its motions

Review your understanding of the moon and its motions in this free article aligned to NGSS standards.

Key points:

  • The moon is Earth’s only natural satellite. It rotates once on its axis and revolves once around Earth about every 27 days.
  • A moon phase is the shape of the moon’s sunlit portion as seen from Earth. There are four major moon phases, which repeat every 29.5 days on average.
    • The new moon appears completely dark from Earth. This phase occurs when the moon is on the same side of Earth as the sun.
    • The full moon appears completely lit from Earth. This phase occurs when the moon is on the opposite side of Earth from the sun.
    • The first and third quarter moons both appear half-lit from Earth but on opposite sides. These phases occur when the moon forms a right angle with Earth and the sun.
  • A lunar eclipse occurs when the moon is covered by Earth’s shadow. When the moon is partially covered, the eclipse is partial. When the moon is fully covered, the eclipse is total. Lunar eclipses can occur only during a full moon.
A reddish-orange full moon against a black night sky.
During a total lunar eclipse, the moon often appears reddish-orange. Image credit: KBOutdoors on Pixabay, Pixabay License.

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