Phases of the Moon

The phases of the moon are caused by the relative position of the Sun, Earth and Moon.

In the sketch below, the Sun is located far off to the left, well off the screen. The small arrows on the left show the direction of incoming sunlight. Both the Earth and Moon always have one half illuminated by sunlight. The sketch is not to scale, and the view is from above Earth's North Pole. As the Moon orbits the Earth, we see all the phases cycle through once per 29 days, labeled with numbers 1-8. Meanwhile, the Earth spins counter-clockwise once per day. Sunrise, noon, sunset and midnight are labeled A, B, C and D.

1. New The dark side of the Moon faces the Earth. Person A sees the new moon rise at sunrise. Person B sees the new moon at its highest point at noon. Person C sees the new moon set at sunset. Person D does not see the new moon at midnight. A solar eclipse (moon blocking the sun) is possible at this phase.

2. Waxing crescent Less than 1/2 of the Moon's illuminated surface can be seen from Earth. Each day the crescent appears a little thicker. Person A does not see the moon at sunrise. Person B sees the moon rising in the East at noon. Person C sees the moon setting in the West at sunset. Person D does not see the moon at midnight.

3. First Quarter Exactly 1/2 of the Moon's illuminated surface can be seen from Earth. Person A cannot see the moon at sunrise. Person B sees the moon rise at noon. Person C sees the moon at its highest point at sunset. Person D sees the moon set at midnight.

4. Waxing gibbous More than 1/2 of the Moon's illuminated surface can be seen from Earth. Each day the gibbous appears a little thicker. Person A does not see the moon at sunrise. Person B does not see the moon at noon. Person C sees the moon rising in the East at sunset. Person D sees the moon setting in the West at midnight.

5. Full The illuminated side of the Moon faces the Earth. Person A sees the full moon set at sunrise. Person B does not see the full moon at noon. Person C sees the full moon rise at sunset. Person D sees the full moon at its highest point at midnight. A lunar eclipse (Earth's shadow on the moon) is possible at this phase.

6. Waning gibbous More than 1/2 of the Moon's illuminated surface can be seen from Earth. Each day the gibbous appears a little thinner. Person A sees the moon setting in the West at sunrise. Person B does not see the moon at noon. Person C does not see the moon at sunset. Person D sees the moon rising in the East at midnight.

7. Third Quarter Exactly 1/2 of the Moon's illuminated surface can be seen from Earth. Person A sees the moon at its highest point at sunrise. Person B sees the moon set at noon. Person C does not see the moon at sunset. Person D sees the moon rise at midnight.

8. Waning crescent Less than 1/2 of the Moon's illuminated surface can be seen from Earth. Each day the crescent appears a little thinner. Person A sees the moon rising in the East at sunrise. Person B sees the moon setting in the West at noon. Person C does not see the moon at sunset. Person D does not see the moon at midnight.

Jason Harlow