The Earth, Sun and Moon 37 37.4 The phases of the moon
The moon does not generate its own light. The moon looks bright because it reflects the light of the sun. The sun is said to illuminate the moon. Half of the moon is always illuminated by the sun while the other half is in darkness. How much of the illuminated side you are able to see depends on where the moon is in relation to Earth. The moon rotates around Earth. As it does so, the portion of illuminated moon you are able to see changes. These changes are called phases of the moon.
Illuminate To light up.
1. New moon
2. First quarter
3. Full moon The main phases of the moon are explained by this model:
4. Third quarter
Lunar cycle A period of
approximately 29.5 days during which the moon completes a full orbit of Earth and has appeared in all of the lunar phases.
Fig. 6 The phases of the moon as it orbits Earth.
Lunar cycle It takes about 29.5 days for the moon to make a complete orbit around Earth. This period of time is called the lunar cycle. For half of the lunar cycle the illuminated area of the moon visible on Earth is increasing. The moon is said to be waxing during this half of the lunar cycle. During the other half of the lunar cycle, the illuminated area gradually decreases. During this half of the lunar cycle the moon is said to be waning.
Waxing moon When the illuminated area of the moon is increasing.
Waning moon When the illuminated area of the moon is decreasing.
