Phases of the Moon

By: Ruilin Chen

Comparing the First Quarter and Third Quarter Moons

Both the first quarter phase and the third quarter phase involve one half of the moon being illuminated. During the first quarter phase, the moon's right side lights up because the moon is waxing. During the third quarter phase, the moon's left side lights up because the moon is waning.


The moon waxes when the amount that is illuminated is increasing, and it wanes when that amount that is illuminated is decreasing. The first quarter moon is in the process of waxing because it is moving from new moon to full moon. The third quarter moon is waning because it was previously a full moon and is moving towards becoming a new moon.

View from Earth vs View from ISS

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The perspective from which we view the moon on Earth is different from the perspective of an astronaut on the ISS because the moon and ISS are both constantly orbiting the Earth. The same half of the moon is illuminated all the time. We are not always able to see the portion of the moon that lights up because different parts of the moon face Earth at different times as it orbits Earth. This is why the percentage of the moon that is illuminated appears to change, and why moon phases are experienced on Earth. Moon phases are experienced from the perspective of the ISS as well, since the ISS orbits the Earth at a lower height than the moon does. The moon appears to rise and set on Earth because of Earth's rotation around its axis. There is about 12 hours between the time the moon rises and sets. From the ISS, the interval between the moon rising and setting is smaller because the ISS orbits the Earth every 92 minutes.