Crescent Moon

To me one of the prettiest moon phases to see on a clear night is a thin, brightly lit crescent.  It’s like a big jewel hanging in the sky, and always gives me pause.  Since the moon is an ever changing presence in our night sky, I thought it good to visit it a while.


To review your science class astronomy, the lit up part of the moon you see is caused by sunlight shining on it.  The darker part of the moon you can dimly see is shaded from the sun, but is lit up enough to see by “earthshine”, sunlight reflected off the earth to the moon’s surface.  The phases that the moon goes through are caused by the changing locations of the sun, moon, and earth in respect to each other.  A full moon occurs when the earth is between the sun and moon so that from our perspective all of the moon is receiving sunlight.  Gradually things shift  and the sun gradually moves  behind  the moon (again from our perspective), causing the side of the moon we see to become more and more in shadow,  until finally it’s all dark, and is called the “new moon”.


As mentioned this changing light show is identified by various moon phases, so let’s review those.  Starting with the new moon (all dark), the right side will start getting some sunlight and gradually grow larger each night, which is called “waxing”.   The phase where the darker portion is bigger than the lit portion is called a crescent moon.  The half-moon phase is called “first quarter”, referring to the time in the moon’s monthly cycle and not its shape.  The phase from half-moon to full moon is referred to as a “gibbous waxing”.  A gibbous moon means the lit up portion of the moon is larger than the shaded portion.  Gibbous is an old English word that means “bulging”.  After the moon finally reaches its bright “full moon” phase, the shadow will start showing up on the right side as we see it, and the lit portion gets smaller.  This phase from full moon back to half-moon is called “waning gibbous”, and this second half moon in the monthly cycle is called “3rd quarter or last quarter”.  Yeah, it’s a little confusing. So to summarize, the phases of the moon during a 29.5 day cycle are new (all shaded), waxing crescent, 1st quarter (a half moon), gibbous waxing, full, gibbous waning, 3rd quarter (another half-moon), waning crescent, and finally back to new moon.  It helps to remember that crescent means mostly shadow and gibbous means mostly lit, and if the shadow is showing on the left side of the moon it’s waxing bigger, and if the shadow is on the right side it’s waning smaller.  A simpler way to identify phases is to call the new moon  to half-moon the 1st quarter, half-moon  to full moon the 2nd quarter, full moon back to half-moon the 3rd quarter, and half-moon back to new moon as the 4th quarter.   One last piece of geeky stuff and I’ll quit: the line between the lit portion and shadowed portion of the moon is called the terminator (like the movie….”I’ll be back”).


As with many natural occurrences, there is some folklore to go with moon phases.  Planting crops and killing plants has long been timed for what phase the moon is in, and there appears to be some scientific backing for this due to the varying gravitational pull of both the sun and moon on the Earth.  Some phases are good for above ground crops, others for root crops, and so on.  Then there is the wet moon/dry moon thing to predict drought.  The story goes that if you have a crescent moon and the points are turned upward, the moon is holding water and it means dry weather.  If the points are turned in a downward direction, the moon will not hold water but pour it out, predicting rainy weather.  Oh, and it’s good luck to look at a crescent moon over your left shoulder.

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