Lunar Phases

The Moon

The moon is our closest neighbor in the solar system.  The Moon is roughly the same age as the Earth, and made of the same basic materials.  These coincidences are due to the fact that the Moon formed from the Earth after a massive impact in the early days of the solar system.


The moon is also Earth's only natural satellite, meaning it naturally revolves around the Earth.  

  • The moon's period of revolution is 27.3 days.
  • The moon's period of rotation is also 27.3 days

Because the period of rotation and revolution are exactly the same we say the Moon has a synchronous rotation.  One effect of this synchronous rotation is that the same side of the Moon points towards the Earth at all times.  So we only get to see one side of the moon ever.  The side the points away from the Earth is often referred to as the Dark Side, even though it does get sunlight.

Lunar phases

As the moon revolves around the Earth, we see different amounts of the light half of the moon.  This makes the moon appear as different shapes in the night sky that we call lunar phases.

The lunar phases are cyclical, they occur in a regular pattern and repeat about every 29 days.  Each phase occurs at a specific location on the Moon's orbit.  For each of the phases you need to know:

  • Where it goes on the moon's orbit
  • Name of the phase
  • How it looks from Earth

1.  New moon

  • the new moon occurs when the moon is between the Earth and the Sun
  • The new moon looks completely black


2.  Waxing Crescent

  • the second phase is the waxing cresent
  • It is the first one after the new moon moving in a counterclockwise direction
  • It looks like a crescent shaped sliver of light on the RIGHT side of the Moon


3.  First Quarter

  • By the third phase, the moon is 1/4 of the way around the Earth, so this phase is called First Quarter
  • It looks like the RIGHT HALF of the moon is light


4.  Waxing Gibbous

  • A waxing gibbous looks like almost a full moon, but with a sliver of darkness on the left side



5.  Full Moon

  • In the fifth position the Moon is behind the Earth
  • In this position we get to see the entire light half of the moon
  • It looks like a fully lit circle


6.  Waning Gibbous

  • The waning gibbous is the reverse of the waxing gibbous.
  • We still see almost a full moon, but this time the sliver of darkness is on the RIGHT side


7.  Third Quarter

  • By the time the moon is in it's 7th phase it is 3/4 of the way around the Earth.
  • We call this phase the THRID QUARTER
  • It is the revers of the first quarter.  We see only the LEFT HALF of the moon.



8.  Waning Crescent

  • By now, most of the moon is dark again.
  • The waning crescent is the reverse of the waxing crescent.  We see a sliver of light on the LEFT SIDE.