Expert: Ed - 4/28/2009

Question
I have examined charts of the moon's phases, and the crescent moon is always pictured vertically.  Yet, I occasionally see the crescent "on its back".  I have not found an explanation for it and wonder what causes it? Also, why would the "on its back" position exist, but not "on its head?"

Answer
Jessica,

I am having trouble visualizing this.  However, here is a NASA website which answers your question.
http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question43.html

Here is another discussion of the problem, from an educational website run in Wisconsin.
http://planetariumweb.madison.k12.wi.us/mooncal/crescent-tilt/Crescent

I am still trying to wrap my own brain around this.  I think it's because my allergies are driving me crazy today, I can't concentrate on anything.  Basically, it seems to have to do with the angle of the moon's orbit, as it revolves around Earth, combined with the tilt of the Earth's axis (which is what causes us to have seasons).

Give me a day to think this over, and then ask me again, if you like.

Keep Looking Up,
--Ed