Why does the moon appear different on different days?

Answer 1

This sounds strange, but this is what happens. Over the course of a lunar cycle, the time from one new moon to another, we can look up and see the progress of a lunar day. The lunar day is one lunar cycle long, approximately one month of earth time. We see the same face of the moon all the time. When we look up at the full moon, we are seeing lunar 'noon' at the center of the moon. As the lunar day progresses, the same thing happens that happens on earth. The wash of light falling on the moon from the sun changes position. Every part of the moon has a sunrise, a 'noontime', a sunset and a night time. It takes a little time and thought to get the idea, but we are simply watching a lunar day unfold. This is DIFFERENT from what happens during what we call a lunar eclipse. That is a moderately rare event where the sun's light is blocked from the moon for several minutes because the earth is in the way. The dark part of the moon during the ordinary phases of the moon has nothing to do with the earth casting a shadow on the moon. Lunar eclipses happen every few years, and most people have many opportunities to see one over the course of a lifetime. The only disadvantage is that they happen late at night. Anyone who has not seen one should take the opportunity at least once, if interested at all.

As a footnote: On those spectacular occasions when we can safely witness an eclipse of the sun, we are looking up and witnessing lunar midnight at the center of the moon's face. This is just to ease the doubts of those who may think that the far side of the moon is always 'dark'. It certainly is not.