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Lunar Phases

Lunar Phases

Introduction: Have you ever heard the phrase, "Harvest Moon?" It is the full moon, when the Moon appears fully lit, closest to the autumnal equinox. But the Moon isn't always full. Why is that?

The Moon revolves, or orbits, around Earth. Just as with Earth, sunlight shines on the side of Moon facing the Sun. To us here on Earth, it may seem that the Moon produces light, but really it is reflecting sunlight. As the Moon orbits Earth, the side of it facing the Sun is always illuminated. However, the amount of the Moon that appears lit as viewed from Earth changes depending on its position compared to Earth and the Sun. The apparent changing shapes of the Moon as it orbits Earth are phases.

Phases of the Moon as seen from Earth

Listed below are descriptions of the eight phases of the Moon. It is important to understand that the lunar cycle is continuous. Although we recognize these eight phases, the Moon's appearance will continue to change throughout its 29.5 day lunar month, the time it takes for the Moon to go through all of its phases once.

Lunar Phases
New MoonThe Moon is positioned between Earth and the Sun during a new moon. Sunlight reflects only off of the far side of the Moon. The side facing Earth is left in darkness.
Waxing CrescentA waxing moon is the period when the amount of the Moon that we see illuminated increases. The Moon waxes between new and full moons. A thin sliver of the Moon is lit at the start of a waxing crescent moon. The amount of the Moon lit increases until it reaches the first quarter.
First QuarterDon't let the name fool you! At the first quarter, half of the Moon appears illuminated. However, it is one-quarter the way through its lunar month. The first quarter moon can be viewed about one week after a new moon.
Waxing GibbousThe amount of the Moon lit by the Sun continues to increase as the Moon begins to move behind Earth. Now, more than half of the Moon appears illuminated. When the Moon appears to be more than half lit, but not a full moon, it is called a gibbous moon.
Full MoonAbout two weeks after the new moon, the Moon appears to be fully lit. It is now positioned so that Earth is between the Moon and the Sun. The Moon orbits Earth in a plane that is slightly tilted, allowing the sunlight to illuminate the full moon. When the Sun, Earth, and Moon are in perfect alignment during a full moon, a lunar eclipse may be viewed from certain locations.
Waning GibbousA waning moon is the period when the amount of the Moon that is illuminated decreases. The Moon wanes between full moon and new moon. As with a waxing gibbous moon, more than half of the Moon is lit during this phase.
Third QuarterThe third quarter moon is also called the last quarter moon. Half of the Moon appears lit. From Earth, this is viewed as the opposite half of the Moon lit than during the first quarter moon. It is now about three weeks after the new moon.
Waning CrescentThe amount of the Moon that is lit continues to decrease as the Moon begins to complete its orbit around Earth and end its lunar month. As it once again approaches a new moon, only a thin slice of the Moon will shine.

Be careful! The Moon does rotate on its axis. Then why does same side of the Moon always face Earth? We do not see the opposite side of the Moon because the Moon spins on its axis at the same rate it orbits Earth.


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