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Lunar phases

Why do you see different portions of the Moon lit by the sun

The Moon has no light source of its own. It "shines" because it reflects sunlight. At any given moment, half of the surface is lit while the other half is dark. On the Moon there is night an day, just as on Earth. But what causes the changing appearance of the Moon, what causes the lunar phases?
Lets have a look at the diagram below. The Moon travels around the Earth (blue arrow), and half of the lunar surface always is lit by the Sun (the half that is facing the Sun), while the other half is dark. The lunar phases are caused by the relative positions of the Sun, Earth, and Moon. Because the Moon orbits the Earth, different portions of the Moon seem to be lit by the Sun.

The phases on the image below are numbered from 1 through 8. At the bottom of the image you can see which bright part of the Moon we see.

© Image by

1. New Moon (day 0)
The Moon is between the Sun and the Earth. Its dark side is facing the earth and we cannot see the Moon in the nighttime sky. Its bright side is facing the Sun. The new Moon rises in the east around sunrise, and sets in the west near sunset.

2. Waxing (growing bigger) crescent Moon (day 3)
The Moon looks like a crescent. In the Western evening sky, it can be found to the east of the Sun. Its bright "horns" always point away from the setting Sun. It sets a few hours after sunset.

3. First quarter (day 7)
After 7 days the Moon has travelled 1/4 of its way around the Earth since new Moon. We see 50% of the sunlit half of the Moon and 50% of the lunar disk is dark. It rises around noon and sets at midnight.

4. Waxing gibbous Moon (day 10/11)
After the first quarter we see more than half of the bright side of the Moon. The visible part of the sunlit side of the Moon grows every night until the Earth is between the Sun and the Moon and we reach full Moon)

5. Full Moon (day 14)
The Moon is now halfway in its orbit around Earth, and we can now see the entire bright side of the Moon. The full Moon rises at sunset and sets in the Morning at sunrise. It is visible all night.

Now the Moon starts with its second half of the orbit around earth. It proceeds like the first half but then in reverse.

6. Waning (growing smaller) gibbous Moon (day 18/19)
The visible bright part of the Moon grows smaller night after night.

7. Last quarter or third quarter (day 22)
Again we see 50% of the bright half of the Moon and 50% of the dark half of the Moon. It rises at midnight and sets at noon.

8. Waning crescent (day 26)
Only a small part of the sunlit half of the Moon is visible. The waning crescent rises in the morning sky, before the Sun rises. It's bright "horns" are always pointed towards the rising Sun.

After 29.5 days the Moon is new again (number 1 in the diagram) and the cycle starts all over again. The period of 29.5 days (29 days, 12 hours and 44 minutes) between two successive new Moons is called a synodic month or lunation.
Posted by Math on 02/20 at 05:27 AM
Lunar basics • (3) Comments • (0) TrackbacksPermalink






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