During the day on December 30th I noticed a half moon sitting vertical during the day. That same night and into the AM hours of Dec 31st, the moon was sitting horizontal with the bottom half iluminated. What caused this?
The illuminated half of the Moon is always toward the Sun. A first quarter Moon rises at noon and transits the meridian at sunset. At sunset the right half will be illuminated. When the moon sets at midnight the Sun is on the bottom of the Earth, and the illuminated portion of the Moon is on the bottom. <div class="Discussion_UserSignature"> <p> </p><p> </p> </div>
There is an interesting effect that led astronomers thousands of years ago to conclude that the sun must be much, much farther than the moon. If you look at the half illuminated moon (commonly referred to as first quarter) at sunset when it directly in the south, the line dividing the illuminated and dark sides is vertical. However the sun, which is illuminating the moon, is not in the direction perpendicular to the line as it would be if the sun an moon were at the same distance, but low on the horizon. The only way the sun can create a vertical division between light and dark is if the sun were essentially at infinity compared to the moon.