The Moon's appearance in the sky changes over the course of about a month. These changes are known as the lunar phases or cycles of the moon. These phases are the result of the changing angles at which we view the Moon's illuminated surface as the Moon orbits the Earth.
Overview of Lunar Phases
There are eight principal lunar phases, which occur in a continuous cycle.
New Moon
The New Moon phase occurs when the Moon is between the Earth and the Sun, making the side facing Earth unilluminated. The Moon is generally not visible during this phase.
Waxing Crescent
Immediately following the New Moon, the Waxing Crescent phase begins. In this phase, a small sliver of the Moon's right side becomes visible, and the illuminated portion grows larger ("waxes") each night.
|
Phase |
Description |
Key Characteristic |
|---|---|---|
|
New Moon |
Not visible from Earth |
Moon is between the Earth and the Sun |
|
Waxing Crescent |
A small sliver of light appears |
Illuminated portion increases (waxes) |
First Quarter
When exactly half of the Moon's face is illuminated (the right half), it is in the First Quarter phase. It appears a quarter of the way through its cycle from new moon to new moon.
Waxing Gibbous
After the First Quarter, the illuminated portion continues to increase, and the Moon is in the Waxing Gibbous phase. "Gibbous" means more than half.
Full Moon
The Full Moon occurs when the Earth is between the Sun and the Moon. The entire face of the Moon visible from Earth is illuminated.
Waning Gibbous
Immediately after the Full Moon, the illuminated portion begins to decrease ("wanes"). This is the Waning Gibbous phase, where more than half of the Moon is still visible, but the light is shrinking from the right side.
Last Quarter
The Last Quarter (or Third Quarter) phase occurs when the left half of the Moon is illuminated. Similar to the First Quarter, half the Moon is lit, but the illuminated side is now the reverse.
Waning Crescent
The final phase before the New Moon is the Waning Crescent, where only a small, shrinking sliver of the Moon's left side remains visible.
|
Phase |
Description |
Key Characteristic |
|---|---|---|
|
Full Moon |
Entire face visible from Earth is illuminated |
Earth is between the Sun and Moon |
|
Waning Gibbous |
Illuminated portion decreases (wanes) |
More than half of the Moon is lit |
|
Last Quarter |
Left half of the Moon is illuminated |
Half the moon is lit, illuminated portion decreases |
|
Waning Crescent |
Only a small sliver of light remains |
Illuminated portion decreases (wanes) |
The Synodic Month
The time it takes for the Moon to complete one full cycle of phases, from New Moon to New Moon, is called the Synodic Month and averages about (about 29.5 days).