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How to Determine Moon Phases

The angle between the earth, moon and sun determines the phase of the moon. The side of the moon facing the sun is always illuminated while the side of the moon facing away from the sun is always dark. The amount of light side and dark side showing on the side of the moon facing the Earth changes as the moon orbits Earth. A new moon appears as a dark silhouette of the moon when Earth is behind the moon in relation to the Sun. A completely illuminated, or full moon, occurs when Earth is in front of the moon in relation to the Sun, allowing an observer on Earth to see the sunlit side of the moon. Due to the nearly monthlong orbit of the moon, various intermediate stages of illumination may be seen.

Things You'll Need

Pencil
Paper

Instructions

- 1
  Draw a circle to represent the moon on a piece of paper.
- 2
  Observe the moon. Note the side of the moon that is illuminated, the side that is dark and the terminator. The terminator is the boundary between the light side and dark side of the moon.
- 3
  Draw a line on the circle to indicate the terminator.
- 4
  Shade the circle representing the moon to correspond with the dark side of the moon.
- 5
  Identify the moon phase. It can be new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, third quarter or waning crescent, according to the size and orientation of the shading. A new moon will be completely shaded. Waxing moons occur as the moon phases change from new to full with the left side of the moon darkened. After a full moon, the moon begins to wane with the right side of the moon darkened.
- 6
  Repeat this observation each night for about a month (29.5 days specifically) to create a record of the moon progressing though all of its phases (weather permitting).

Astronomy