What we see as mysterious moonlight is actually sunlight. It falls on the moon and is reflected to earth. Though we see it as having phases, the moon, in one sense, is always full. The half of it that faces the sun is always completely sunlit, just as the half of our earth that is facing the sun is always completely sunlit, making it daytime there.
What changes with the phases of the moon is how much of the sunlit half of the moon we can see from earth. When the entire sunlit half is visible from earth, we call it a full moon, and when the earth and moon are positioned so that only the dark half is visible to us, we call it a new moon. Through the synodic month, the sunlight area of the moon becomes more and more visible on earth and then less and less visible, as the angle we are observing it from changes.
A synodic month is the period of time it will take the moon to get back to the same position it holds now with respect to the earth and sun. That period is, on average, 29 days, 12 hours, 44 minutes, and 2.8 seconds. The actual length can be quite different, but for general use, the synodic month is considered to be about 29.5 days. Another way to define the synodic month is as the time it takes for the moon to go from the new moon to the new moon again.
The synodic month determines the phases of the moon. The relative motions of the sun, moon, and earth determine the phases of the synodic month, and the forces generated by the motions of these massive bodies create the tides. The shadows these bodies cast, and the way they intersect the earth, also cause eclipses.
Strictly speaking, the moon does not revolve around the earth. Both the moon and the earth revolve around a common center that is located on a line between them, about three-fourths of the way between the center of the earth and its crust. The earth’s mass affects the moon, of course, but the moon’s mass also affects the orbit of the earth. We actually travel around the sun in a very slightly wobbling, hobbled path, leashed by gravitation to the tugging moon. However, to visualize the phases of the moon, it is a useful simplification to picture the moon going around the earth, while the earth goes around the sun.
The new moon begins the synodic month. The moon is approximately between the earth and the sun. We see the unlit half of the moon. Actually, the moon is seldom directly between the earth and the sun. If it were, the moon’s shadow would fall on earth every month, and, within the circle of that shadow, there would be a solar eclipse each time. However, the orbit of the moon is tilted about 5.09 degrees, so it is seldom directly between the earth and the sun, and eclipses are rare. Therefore, during the new moon, the moon’s shadow usually falls through space, missing the earth. We see the moon’s shaded side, and we call it the dark of the moon.
There is another reason we don’t see the new moon on the first day it appears. It rises at the same time as the sun, so we do not see it in the sunlight, just as we do not see the stars by day. The sun’s glare overwhelms it. If we could see the new moon in daylight though, we would see the sun and moon rise in the east together, and move westward together across the sky.
However, the moon would appear to lag, and would finally hang in the sky after the sun has set. The sun of course, is not really moving. The earth is turning, moving observers away from their view of the sun. Finally, the curve of the earth comes between the observer and the sun, and it appears to set.
Sometimes the new moon, or the faint beginning of a crescent, is visible after sunset. The dark part of the moon may be faintly illuminated by earthshine, the reflected light of the earth. The eerie glow is fainter than moonlight. The moon is much more reflective than our watery earth, and earthshine has been reflected twice.
After the new moon, the moon appears as a growing crescent. It is a waxing moon. Its horns are pointed away from the sun. Now the moon is farther separated from the sun in the sky, so both may be visible in daylight. The moon still lags though, so the sun sets first, while the waxing crescent moon hangs low in the twilit western sky.
About 7.5 days after the new moon, the moon reaches its first quarter. It appears as a half disk of light. (It appears as half of a disk, but is called a quarter moon.) By now, the moon’s path has lagged the sun to the point that the moon rises about noon. It will be at its highest about sunset, and set about midnight.
After that, it becomes a waxing gibbous moon. Gibbous means bulging, and the moon now appears to be bulging toward the full. About 11 days into the synodic month about three quarters of the moon is illuminated.
The full moon comes at nearly 15 days into the cycle. The earth is now between the sun and the moon. (Not exactly between them or there would be a lunar eclipse.)The sun’s light now shines past us, and we see the completely sunlit side of the moon reflecting light. The moon will be its highest at about midnight. If an equinox is near, the moon will rise at sunset, and set with the dawn.
The cycle continues, with the moonlight seen from earth diminishing. There is now a waning gibbous moon. At 22.5 days or so, the moon is in the third quarter. A third quarter moon is at its highest around sunrise, and sets around noon. At 26 days the crescent moon shines before dawn, points its horns towards the sun, and is visible much of the day.
The moon goes through phases because we see smaller and larger portions of its sunlit side. The phases are regular, though their timing varies somewhat throughout the year. The same conjunctions that show us the phases of the moon also produce the tides, and occasional eclipses.