Moon Phase: The Moon's Visible Lighted Surface

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Author: Janice VanCleave

As seen from Earth, the apparent changes of the Moon's shape are called phases of the Moon. A complete cycle of the Moon's phases takes about 291/2 days. The phases were observed but not understood by ancient astronomers.

In this project, you will determine what causes the different phases of the Moon. You will investigate the degree of change daily during a synodic month, the time between two successive new moons. You will also investigate the times of moonrise and moonset each day.

Getting Started

Purpose: To determine the cause of the phase of the Moon called the new moon.


  • walnut-size piece of modeling clay
  • pencil
  • flashlight


  1. Shape the clay into a ball and stick it on the point of the pencil. The clay ball is the model Moon.
  2. In a darkened room, hold the flashlight at arm's length and shine the light toward your face.
  3. Hold the model Moon midway between you and the light with the model aligned with the light bulb and slightly above it (see Figure 18.1).
  4. Observe the surface of the clay ball.

Moon Phase: The Moon's Visible Lighted Surface


The light is very bright and the surface of the ball is difficult to see.


The Moon shines because it reflects light from the Sun. The side of the Moon facing the Sun is always sunlit. The side away from the Sun is always dark. As the Moon orbits Earth, an observer on Earth sees different portions of the Moon. The appearance of the illuminated surface of the Moon facing Earth is called a phase of the Moon. The Moon's orbit is at an angle of about 5° to the ecliptic (plane of Earth's orbit). Thus as seen from Earth, it usually appears to pass above or below the Sun's disk. When the Moon is in conjunction with the Sun, the side facing Earth is not illuminated. This phase is referred to as the new moon. The new moon rises with the Sun in the east and sets with the Sun in the west.

Try New Approaches

  1. About 1 week after the new moon, the Moon has moved about one quarter of the way around Earth. In this position, one-half of the side facing Earth is lit. Since this illuminated side is equal to about onefourth of the Moon's total surface, this phase is called the first quarter. It is also called the first quarter because the Moon is one-quarter of the way through its lunar cycle. Demonstrate this phase by holding the model Moon to your left. Shine the flashlight directly at the model.
  2. After about 2 weeks, the Moon is on the opposite side of Earth from the Sun. The side facing Earth is completely illuminated. This phase is called the full moon. Show a full moon by holding the model Moon behind you and shining the light directly on it.
  3. The third quarter occurs around the beginning of the fourth week, which is three-fourths of the way through the lunar cycle. The illuminated quarter is opposite that of the first quarter. Show this by holding the model Moon in your right hand and shining the light from the left. Science Fair Hint: Diagram the positions of Earth, the Sun, and the Moon during the phases (see Figure 18.2). Add a Moon Data table showing the Moon's appearance at each phase.
  4. Moon Phase: The Moon's Visible Lighted Surface

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