Phases of Moon Explained Using an Orrery

The moon goes through eight distinct phases over the course of about 29.5 days when using the synodic method of measurement. These phases are caused by the relative positions of the moon, Earth, and sun.

The phases of the moon have been used for centuries to track time and to predict the future. They have also been used to explain natural phenomena, such as the tides, and to inspire works of art and literature.

moon phases

The phases of the moon are as follows:

  1. New Moon: The moon is between the Earth and the sun, and its dark side is facing Earth.
  2. Waxing Crescent: The moon is gradually moving away from the sun, and a small crescent of light is visible from Earth.
  3. First Quarter (Waxing Gibbous): The moon is at a right angle to the sun and Earth, and half of its illuminated side is visible from Earth.
  4. Full Moon: The moon is opposite the sun in the sky, and its entire illuminated side is visible from Earth.
  5. Waning Gibbous: The moon is gradually moving closer to the sun, and more than half of its illuminated side is visible from Earth.
  6. Third Quarter (Waning Crescent): The moon is again at a right angle to the sun and Earth, and half of its illuminated side is visible from Earth.
  7. Waning Crescent: The moon is almost between the Earth and the sun, and only a small crescent of light is visible from Earth.
  8. New Moon: The moon returns to its starting position between the Earth and the sun, and its dark side is again facing Earth.

 

The length of time it takes for the moon to go through each phase is roughly 29.5 days when seen from Earth. On average, each phase lasts for about 7 days. The  length of each phase is as follows:

  • New Moon: 2-3 days
  • Waxing Crescent: 7-8 days
  • First Quarter (Waxing Gibbous): 7 days
  • Full Moon: 2-3 days
  • Waning Gibbous: 7-8 days
  • Third Quarter (Waning Crescent): 7 days
  • Waning Crescent: 7-8 days

An easy way to remember if the moon is moving towards first or third quarter in the Northern Hemisphere is to use, “light on right, getting bright.” Each cycle starts at new moon, so if the light is on the right the moon is moving towards 1st quarter or full moon. If the shadow is on the right the moon is waning and moving towards third quarter or new moon. The moon waxes to full moon and wanes to new moon.

moon phases

Sidereal and Synodic Moon Periods

The sidereal moon period is the time it takes the Moon to make one complete orbit around the Earth relative to the stars. This period is approximately 27.322 days.

The synodic moon period is the time it takes for the Moon to return to the same phase as seen from Earth. This period is approximately 29.531 days.

The difference between the sidereal and synodic moon periods is due to the fact that the Earth is also orbiting the Sun. During the course of one synodic month, the Earth moves approximately 27 degrees around its orbit. This means that the Moon must travel an additional 27 degrees in order to return to the same position relative to the Sun.