Phases of the Moon

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Diagram showing the Moon's phases.
Diagram of the Moon's phases: The Earth is at the center of the diagram and the Moon is shown orbiting (dashed circle). The Sun lights up half of the Moon and Earth from the right-side. The phase of the Moon is shown next to the corresponding position of the Moon in its orbit around Earth
Waning Moon

The phases of the Moon are the different ways the Moon looks from Earth over about a month.

As the Moon orbits around the Earth, the half of the Moon that faces the Sun will be lit up. The different shapes of the lit portion of the Moon that can be seen from Earth are known as phases of the Moon. Each phase repeats itself every 29.5 days, approximately.

The same half of the Moon always faces the Earth, so the phases will always occur over the same half of the Moon's surface.

Phases[change | change source]

Animation of the moon phases

There are 8 phases that the moon goes through.

  • A new moon is when the Moon cannot be seen because we are looking at the unlit half of the Moon. The new moon phase occurs when the Moon is directly between the Earth and Sun.
  • A waxing crescent moon is when the Moon looks like crescent and the crescent increases ("waxes") in size from one day to the next.
  • The first quarter moon (or a half moon) is when half of the lit portion of the Moon is visible after the waxing crescent phase.
  • A waxing gibbous moon occurs when more than half of the lit portion of the Moon can be seen and the shape increases ("waxes") in size from one day to the next. The waxing gibbous phase occurs between the first quarter and full moon phases.
  • A full moon is when we can see the entire lit portion of the Moon. The full moon phase occurs when the Moon is on the opposite side of the Earth from the Sun.
  • A waning gibbous moon occurs when more than half of the lit portion of the Moon can be seen and the shape decreases ("wanes") in size from one day to the next. The waning gibbous phase occurs between the full moon and third quarter phases.
  • The last quarter moon (or a half moon) is when half of the lit portion of the Moon is visible after the waning gibbous phase.
  • A waning crescent moon is when the Moon looks like the crescent and the crescent decreases ("wanes") in size from one day to the next.

"Blue moon"[change | change source]

  • A second full moon in one calendar month is usually called a "blue moon" and this occurs approximately every 3 years. The saying "Once in a blue moon" refers to something that does not happen often (like a blue moon.)[1][2]

References[change | change source]

  1. However, a blue moon can also refer to the third full moon in a season with four full moons.
  2. Sinnott, Roger W., Donald W. Olson, and Richard Tresch Fienberg (May 1999). "What's a Blue Moon?". Sky & Telescope. http://www.skyandtelescope.com/observing/objects/moon/3304131.html?showAll=y&c=y. Retrieved 2008-02-09. "The trendy definition of "blue Moon" as the second full Moon in a month is a mistake."