What Factors Determine the Phases of the Moon?

Phases of the Moon

• The phases of the moon begin with the full moon, which means its entirety is visible in the sky. Half-way through the cycle, the moon is completely obscured by its own shadow and is not visible at all. This is called the new moon. Half-way between the full moon turning into the new moon, the left half of the moon is visible. This is called the first quarter. Half-way between the new moon turning into the full moon, the right half of the moon is visible. This is called the third quarter.
  
  

Crescent Moons

• As the moon reveals itself from the new moon to the third quarter, or as it changes from the first quarter to the new moon, you will see a crescent in the sky. This is called the crescent moon. As the moon changes to the new moon, it is said to be waxing, so the crescent between the first quarter and the new moon is called the waxing crescent. As the moon changes to the full moon, it is said to be waning, so the crescent between the new moon and the third quarter is called the waning crescent.
  
  

Gibbous Moons

• As the moon reveals itself from the third quarter to the full moon, or as it changes from the full moon to the first quarter, you will see the moon to appear to bulge toward its shadow. This is called the gibbous moon. The gibbous moon between the full moon and the fist quarter is called the waxing gibbous, while the gibbous moon between the third quarter and the full moon is called the waning gibbous.
  
  

Length of a Moon Phase

• It takes 27.3 days for the moon to orbit the Earth. However, the Earth is also orbiting the sun, which changes the three bodies' perspective of one another. During one orbit of the moon, Earth travels 45 million miles in its own orbit around the sun. This change in position causes the moon to appear through a complete cycle in 29.5 days. This full cycle takes about a month, but because it is slightly less than that, the date of the moon's phases change on the calendar.
  
  

Moon Light

• Even though the moon appears bright in the night sky and people often talk about "moonlight," the moon does not generate its own light. All the light we see from the moon -- and any other object in the solar system -- is actually reflected from the sun. Because of this, only the part of the moon that is facing the sun will be visible.
  
  

Moon's Orbit

• The position of the moon, along with that of the Earth and the sun, determine the phases of the moon. As the Earth rotates even at night, people on the surface can still see the moon, which is illuminated by the sun. During the new moon, the moon is blocking the Earth from seeing any reflected light. During a full moon, the moon's side that is lighted by the sun faces the Earth. As the phases of the moon change, we see more or less of the reflected sunlight off the surface of the moon.