WX 101: The Moon and Moon Phases

Published 08/20 2013 09:34PM

Updated 02/15 2016 06:16PM

The only natural satellite of earth is our moon.  It is located about 239,000 miles from earth and only about 1/4th the size of the earth.  That puts the moon’s circumference at about 6750 miles.  Its surface area is about 14.6 million square miles, which is about the same surface area of Asia. 

The moon orbits around the earth every 29.5 days.  In fact, the reason our calendar has months is because of the moon.  The moon also completes a rotation 27 days, meaning that we pretty much see the same side of the moon from here on earth as it orbits around the earth. 

Because the Moon orbits around the earth, we see different shapes of the moon from here on earth.  In fact, we sometimes even see the moon during the day.  This is a part of what’s called the moon
cycles.  You can see in the picture to the right.  Half of the moon is always lit by the sun, but we can only see part of that light here on earth.

The moon cycle starts with the new moon, which actually means that we won’t see any moon at all.  This is when the moon is on the sunny side of the earth.  As the moon orbits around the earth the illuminated side of the moon can be partially seen on earth.  We call this the First Quarter.  7 Days after the first quarter, the moon’s illuminated side is facing the earth.  This is when we have full moon.  The moon continues to move around the earth and we start to see less of the illuminated moon from earth.  This phase is known as the third quarter.  The moon then moves to its starting position and is once again a new moon.

There are also some intermediate phases of the moon cycle.  These are called the Waxing Crescent, Waxing Gibbous, Waning Gibbous, and Waning Crescent.  The crescent phases are when we see less than half of the illuminated moon (between the quarter phases and the new moon).  The gibbous moons are when we see more than half of the illuminated moon (between the quarter and full moon phases).  The waxing phases are before the full moon, and the waning phases are after the full moon.  The easy way to remember this is “I wax my car and it wans on it.”


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