(EMS) Phases of the Moon Lesson

Phases of the Moon

The appearance of the lighted portion of the moon changes in a predictable cycle as a result of the relative positions of Earth, the moon, and the sun. In the last lesson we learned Earth turns on an axis that is tilted relative to the plane of Earth's yearly orbit. The tilt causes sunlight to fall more intensely on different parts of the Earth during various parts of the year. The differences in heating of Earth's surface and length of daylight hours produce the seasons.

The moon rotates on its axis at the same pace as it revolves around Earth. As a result, the moon always keeps the same side pointed toward us throughout its orbit. Astronomers call the side we see from Earth the "nearside of the moon," and the side we never see from Earth the "far side of the moon." The moon orbits the earth, so it is called a satellite of Earth. The moon is a natural satellite.

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During the moon's cycle, the actual shape of the moon never changes. It is always a sphere. However, it appears to change shape. We only see the moon because sunlight reflects back to us from its surface; it has no light source of its own. Gravitational forces between the moon and the earth keep the moon in orbit. Remember the moon does not produce light. But it does reflect light from the sun, which is how we are able to see it from Earth. (Only stars produce light and they are called luminous for that reason.) What changes is the portion of the moon that can be seen from Earth. Half of the moon is always illuminated by the sun.The half of the moon facing the sun is always lighted; but the lighted side does NOT always face Earth.

As the moon circles Earth, the amount of its disk facing us that is lighted by the sun changes, altering how much of the lunar surface appears bright and how much is in darkness. The changes are known as phases, and repeat in a specific cycle. These are the primary phases: New Moon, First Quarter, Full Moon, Last Quarter. (It takes 27 days to go from one New Moon to the next.)

It takes the moon 27 days to make a complete orbit of the Earth. As it orbits, we see the moon lit from different angles. This is why we see phases of the moon.

Sometimes the moon looks like a full circle. That is called a 'full moon'. At other times we see a crescent shaped moon, because we can only see the edge of the part that is lit by the sun.

Remember that we can only see the part of the moon that reflects the sun's light. How much of that we can see depends upon the position of the moon in its orbit.

During the time it takes to move from one phase to another, the amount of the moon's surface lighted by the sun changes gradually; it's not an abrupt change from one phase to the next.

There are times during the cycle when the moon can be seen during the day. These times are predictable. The following chart gives the times when each phase rises and sets.

Moon Cycles

Phases	Image	Rises	Highest in Sky	Sets
New Moon		Sunrise	Noon	Sunset
First Quarter		Noon	Sunset	Midnight
Full Moon		Sunset	Midnight	Sunrise
Last Quarter		Midnight	Sunrise	Noon

Phases of the Moon Interactive

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