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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

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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

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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.

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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?

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Lunar Phase Terms

We can only see the half of theMoon above and to the right of thediagonal line drawn on the Moonin 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.

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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.

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

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crosses meridianat sundown

crosses meridianat about 9:00 pm

crossesmeridianat midnight

crossesmeridianat about 3:00 am

crosses meridianat dawn

crosses meridianat about 9:00 am

crossesmeridian at noon

crosses meridianat about 3:00 pm

sunlight

sunlight

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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