Lunar Phases

The moon appears to change its shape as it goes through its monthly cycle of phases. It of course retains its spherical shape as it orbits the earth, but we see the different apparent shapes as we see different percentages of the illuminated portion of the moon.

It is a common misconception that the lunar phases are caused by the earth's shadow falling on the moon. A simple observation can show how this idea must be incorrect. Notice the crescent moon. It is either low in the west just after sunset (waxing crescent) or low in the east just before sunrise (waning crescent). In either of these cases, both the sun and moon are on the same side of the earth, so the earth's shadow can not fall on the moon. Yet we see only a sliver of illuminated moon. Hence the earth's shadow falling on the moon can not cause the cycle of phases.

The inner circle shows the moon illuminated in space.
The outer circle shows how moon appears in the sky.

The lunar phases are caused by the relative orientation of the earth, sun, and moon as the moon orbits the earth. The cycle takes about 29.5 days and is the origin of the time period we call a month. At any given time, half of the moon is illuminated by the sun and half is dark. As the moon orbits the sun we see different amounts of the illuminated portion and the dark portion. That’s why we see phases.

During the new moon phase, the moon is between the sun and the earth, allowing us to only see the dark half of the moon. Therefore we do not see the moon at all. As the moon continues on its monthly orbit, we see a small portion of the illuminated half of the moon. We have the waxing (growing) crescent phase. When the moon's orbit has advanced to the point where we see half of the illuminated portion of the moon (half of a circle), and the moon is 90° from the sun in the sky, we have the first quarter moon. This phase is often incorrectly called the half moon. As the orbit continues, we see more than half a circle illuminated and have the waxing gibbous phase. When the moon is directly opposite the sun in the sky, we see the entire illuminated portion of the moon (a full circle) and have the full moon phase. The orbit then continues through the waning (shrinking) phases: waning gibbous, third quarter, and waning crescent. The cycle then repeats itself.