The Moon's Phases
The Moon is one of the most familiar objects in Earth’s sky. Some nights it looks like a bright round disk. On other nights, only half of it can be seen. Sometimes it looks like a thin crescent, and sometimes it is not visible at all.
These changing shapes are called the Moon’s phases. The Moon is not really changing shape, of course. It remains a round world orbiting Earth. What changes is how much of its sunlit side can be seen from Earth.
Why does the Moon change shape?
The Moon does not produce its own light. The only object in the Solar System which produces light is the Sun. The Moon shines because it reflects sunlight, and only the side of the Moon facing the Sun is lit up. As the Moon travels around Earth, its position changes. Sometimes the lit side is facing mostly away from Earth. Sometimes part of the lit side can be seen. Sometimes the whole lit side faces Earth. This is what creates the Moon’s phases. So the Moon is not being eaten, sliced, stretched or turned into a banana. It is simply showing Earth different amounts of its sunlit half.
Two useful words for understanding the Moon’s phases are waxing and waning.
When the Moon is waxing, the visible lit part is growing. Each night, the Moon appears to get a little brighter and fuller as it moves from New Moon towards Full Moon.
When the Moon is waning, the visible lit part is shrinking. Each night, less of the Moon’s lit side can be seen as it moves from Full Moon back towards New Moon.
The word crescent describes a Moon that looks like a thin curved slice. It is less than half lit. This is the banana-shaped Moon. Scientists like to call it a "crescent" because it sounds more sensible and grown up.
The word gibbous describes a Moon that is more than half lit, but not completely full. A gibbous Moon looks big and bright, but it still has a curved shadow along one side.
The angle of the crescent or gibbous shape can also look different depending on where you are on Earth and where the Moon is in the sky. The phase is still the same, but the Moon may appear tilted, upright, or even like a smile. This is because people in different places are viewing the Moon from different angles on a round Earth.
How long does the Moon’s phase cycle take?
The Moon takes about 27.3 days to travel once around Earth. However, the full cycle of phases takes about 29.5 days.
That sounds odd, but there is a reason for it. While the Moon is orbiting Earth, Earth is also moving around the Sun. By the time the Moon has completed one orbit around Earth, Earth has moved farther along its own path through space.
This means the Moon has to travel a little bit farther before it lines up with the Sun and Earth in the same way again. That is why the Moon’s phase cycle takes about 29.5 days rather than 27.3 days.
This cycle is sometimes called a lunar month.
Eclipses are connected to Moon phases, but they do not happen every month.
A solar eclipse can only happen during a New Moon, when the Moon is between Earth and the Sun. If the alignment is just right, the Moon blocks the Sun from view.
A lunar eclipse can only happen during a Full Moon, when Earth is between the Sun and Moon. If the alignment is just right, Earth’s shadow falls across the Moon.
Most months, there is no eclipse because the Sun, Earth and Moon do not line up perfectly. The Moon’s orbit is slightly tilted, so it usually passes a little above or below the exact line.
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