1. 1 Phases of the Moon The Phases of the Moon are the variations in the Moon’s appearance as the Moon orbits the Earth. They are due to the changing Sun-Earth-Moon angle through each month. The following is the cycle of lunar phases: new, crescent, first quarter, gibbous, full, gibbous, third quarter, crescent, new

2. 2 Moon Phases The Moon orbits the Earth in about one month (29.5 days) Over one orbit the appearance and rise and set times change with the cycle of Lunar Phases. The figure is not to scale

3. 3 Light & Dark We only see the Moon because sunlight reflects off its surface. At any one time, half the Moon is always lit and half is in darkness. The amount of the illuminated half we see from Earth depends on the position of the Moon in its orbit The time of day the Moon is visible also depends on its phase. How the Earth and Moon would look high above Earth’s North Pole. The figure is not to scale.

4. 4 Example: Observing the Moon Between January 23-25, 2004, at dusk, the Moon was visible in its waxing crescent phase. The first night it appeared close to the Sun. Subsequent nights it appeared higher in the sky (farther from the Sun) and the lit portion appeared to be growing larger. What do you think you would see the next night? How about a week later?

5. 5 Lunar Phase Terms We can only see the half of the Moon above and to the right of the diagonal line drawn on the Moon in the figure above. The cycle of Lunar Phases starts at the New Moon. At New Moon the Moon is not visible to us on Earth. From New Moon to Full Moon the illuminated fraction of the Moon we see from Earth grows. It is said to be waxing. After Full Moon the fraction of the illuminated Moon visible from Earth shrinks. It is said to be waning.

6. 6 The Cycle of Lunar Phases The Lunar cycle is split into quarters –New Moon –1st Quarter –Full Moon –3rd (or Last) Quarter Since the whole cycle lasts about a month each quarter lasts about a week.

7. 7 As you view the moon from different directions (relative to the Sun) its appearance changes.

8. 8 crosses meridian at sundown crosses meridian at about 9:00 pm crosses meridian at midnight crosses meridian at about 3:00 am crosses meridian at dawn crosses meridian at about 9:00 am crosses meridian at noon crosses meridian at about 3:00 pm sunlight

9. 9 The Lunar Phases Web Tool This is an interactive tool that helps illustrate how –the time of day, –the phase of the Moon and, –the position of the Moon in its orbit are all related to one another. The time that is used in this tool is only an average 6 PM represents sunset, 6 AM represents sunrise, etc. “Lunar Phases Web Tool” located at http://www.calvin.edu/~lmolnar/moon/index.html