Why is the moon not full when the sun and moon are on opposite sides of the sky with no object between them to cast a shadow?
A viewer asks: the other day I saw the sun in the western sky and a half moon in the eastern sky and wondered why the moon was not full, since I can see no object between the sun and the moon to cause a shadow.
Answer
Yes, sometimes it will look like that, and there are several reasons for it. One is that in order to get a full moon the sun has to be at 180 degrees; it cannot be at 160 or 130, it has to be exactly 180. If you do not have 180 degrees of separation between the sun and the moon, you will see something less than a full moon. The other thing is that the moon revolves around the earth every 27 or 28 days, so it has an independent movement from the sun and the stars. And its orbit sits on a five degree plane of variation. Picture a straight plane, then tilt it five degrees. The circle of the moon's orbit can vary by five degrees either way, so that is a ten degree difference, and it changes from month to month. So though it may look like the sun is on the other side of the world, it may not be exactly there, and that makes all the difference. That is also why we do not have eclipses all the time. We are going to have a total eclipse on April 8th that runs from Texas up to Maine. The most total eclipses you can have in a year is five, and most of them are partial, not total.