What Are The Phases Of The Moon?
QUICK ANSWER
The Moon has 8 phases that repeat every 29.5 days: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, third quarter, and waning crescent. The phases happen because as the Moon orbits Earth, we see different amounts of its sunlit half lit up by the Sun.
The Moon goes through 8 distinct phases over about 29.5 days, the same cycle that gives us our months. The shape doesn't actually change. What changes is how much of the Moon's sunlit half we can see from Earth as the Moon moves through its orbit. Same Moon, different viewing angle.
What are the 8 phases of the Moon?
In order: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, third quarter (also called last quarter), and waning crescent. According to NASA, the cycle takes about 29.5 days from new moon to new moon, called the synodic month. The phases are caused by the changing angle between the Sun, Moon, and Earth as the Moon orbits. The same half of the Moon always faces the Sun; what changes is how much of that lit half is pointed toward Earth.
Why does the Moon change shape?
It doesn't actually change shape. The Moon is always a sphere, and the Sun always lights up exactly half of it. What changes is our viewing angle. When the Moon is between Earth and the Sun (new moon), we see the dark side. When Earth is between the Sun and the Moon (full moon), we see the fully lit side. At first and third quarter, we see exactly half of the lit side, which looks like a half-circle from Earth. The crescent and gibbous phases are points in between.
What's the difference between waxing and waning?
Waxing means growing; waning means shrinking. After a new moon, the visible lit portion grows night by night (waxing). It passes through waxing crescent, first quarter, and waxing gibbous before reaching full moon. After full moon, the lit portion shrinks (waning), passing through waning gibbous, third quarter, and waning crescent before returning to new moon. A simple trick: if the lit side is on the right, the Moon is waxing (in the Northern Hemisphere). If the lit side is on the left, it's waning.
Why is the cycle 29.5 days?
Because Earth is also moving. The Moon takes 27.3 days to complete one orbit around Earth (called the sidereal month), but the Sun-Moon alignment takes 29.5 days to repeat because Earth has moved partway around the Sun during the same period. The Moon has to catch up an extra two days' worth of orbit to reach the same Sun-Earth-Moon configuration. The 29.5-day cycle is called the synodic month, and it's what governs the visible phases we see.
The Moon's 8 phases cycle every 29.5 days, with the appearance driven entirely by the changing geometry between Sun, Earth, and Moon. The Moon itself stays the same; only our view of it changes. The cycle is the basis for the word month and shapes calendars, holidays, and tides on Earth. It's the most visible astronomical pattern from our planet.
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