The Phases Of The Moon

The diagram shows the Moon in different positions along its orbit around the Earth. The Sun is off in the distance, lighting the Earth-Moon system. At any position, half of the Moon is illuminated by the Sun (the light side of the Moon) and half is not (the dark side). Also, half of the Moon is visible to the Earth (the near side of the Moon) and half is not (the far side). As the Moon moves around the Earth, we can see different fractions of the illuminated half of the Moon.

When the Moon is between the Earth and the Sun (1), the near side of the Moon is the dark side. The Moon cannot be seen. We call this New Moon, the beginning of a new cycle of lunar phases. When the Earth is between the Sun and the Moon (5), the near side is the light side. We call this Full Moon, even though we only see half the Moon.

Halfway in between these times (3 & 7), only half of the near side of the moon is illuminated by the Sun. So we can only see one quarter of the Moon. We call these phases First and Third Quarters.



The Names Of The Phases Of The Moon

When the moon is getting bigger (phases New to Full) it is waxing. When it is getting smaller (phases Full to New) it is waning.

For example, if today the Moon were a waxing crescent, then tomorrow the crescent shape would continue to grow larger, approaching first quarter. After first quarter, the Moon would be a waxing gibbous, and continue growing until it reached full. The Moon would then begin to shrink, becoming first a waning gibbous and eventually reaching third quarter. Following third quarter it becomes a waning crescent, and continues to shrink until it becomes invisible at new Moon.

If the edge of the Moon (the real edge of the Moon, not the edge of night on the moon) is curved like a "C", the gibbous Moon is shrinking. Another way to think of it is that the Moon always grows or shrinks from the right to the left.



The Lunar Eclispe

Lunar eclipses are special events that only occur when certain conditions are met. First of all, the Moon must be in full phase. Secondly, the Sun, Earth and Moon must be in a perfectly straight line. If both of these are met, then the Earth's shadow can block the Sun's light from hitting the Moon.

There are three types of lunar eclipses. Which one we will see depends on the alignment of the three celestial objects. But first, you need to know that the Earth's shadow is broken up into two parts. The umbra is the darker part of the shadow, where no part of the Sun can been seen. The penumbra is lighter than the umbra, because part of the Sun can be seen.

So, when part of the Moon passes through the umbra, this is called a partial eclipse. When all of the Moon passes through the umbra, this is called a total eclipse. Finally, when the Moon only passes through the penumbra, this is called a penumbral eclipse.


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