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Observing the Sky Solar System Satellites Navy Research Resources

Solar System: The Moon - Phases of the Moon

The Moon shines by reflecting the Sun's light--just like all the other bodies in our solar system except for the Sun itself. And just like those other bodies, one half of the Moon is always lit by the Sun. The Moon seems to change its shape over the course of a month because our view from Earth lets us see a different amount of that sunlit half every day. You can keep track of the Sun's changing phases by watching it rise about one hour later each day.

Phase

New Moon: when the Moon is between the Earth and Sun, and we cannot see it at all. Occasionally, the Sun, and Moon, Earth, will line up and the Moon will block our view of the Sun, creating a solar eclipse. Usually, the Moon is several degrees off from the Sun, so it passes above or below our line of sight and does not block our view.

Waxing Crescent: as the Moon moves away from the Sun, we begin to see a sliver of its sunlit side. The crescent Moon rises just after the Sun, and follows it across the sky through the day, setting not long after the Sun. The word "to wax" means to increase in size.

First Quarter: when we see half of the Moon's face, we are seeing one quarter of the entire sphere. This phase also marks the end of the Moon's first week in its four-week journey around the Earth (if we count the New Moon phase as a starting point). Therefore this phase marks the first quarter of the Moon's orbit. On a clear day, the half Moon is easy to see in the sky, rising in the East when the Sun is high over head.

Waxing Gibbous: this phase follows the half moon, when the visible Moon's shape is hard to describe.

Full Moon: when the Moon lies opposite the Earth from the Sun, we can see the entire half that is illuminated by the Sun. Sometimes, the Moon will pass through the Earth's shadow and we can witness a lunar eclipse. The full moon rises as the Sun sets, and will set when the Sun rises.

Waning Gibbous: as the Moon moves on its journey back toward the Sun, we see less and less of its sunlit side. "To wane" means to decrease in size.

Last Quarter: now we see the quarter of the Moon that was in shadow during the first quarter phase.

Waning Crescent: as the Moon completes its orbit, and dwindles each night, it nears the Sun again, but this time from the West. You can look for the crescent Moon rising just before the Sun.

The Moon, Sun, and planets seem to travel along a common pathway across our sky, called the ecliptic. Therefore, you will sometimes see the planets Venus and Mercury shining brightly in the evening or morning skies, right near the crescent Moon.

 

Sometimes, you can even see the Moon reflecting light that has already bounced off the Earth. Look for this "earthsine" during the crescent moon phase or a lunar eclipse--when the Moon moves into the Earth's shadow.

 

 

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