Phases of the Moon
The moon's phases depend on what fraction of its sunlit hemisphere can be seen from the earth. As the moon orbits earth, it "grows in size" from new moon, to waxing crescent, first quarter (one-fourth of a completed revolution around the earth), waxing gibbous, to full moon. It then decreases in reverse order to waning gibbous, third (or last) quarter (three-quarters of a completed revolution around the earth), waning crescent, and back to the new moon (completing the full revolution around the earth).
The same side of the moon always faces the earth because the moon rotates on its axis in the same period that it revolves around the earth. The moon takes 27.3 days to complete one revolution around the earth with respect to the stars (sidereal month, 27 days, 7 hours, 43 minutes, 11.5 seconds), but the synodical month takes 29.5 days (29 days, 12 hours, 44 minutes, 3 seconds) with respect to the sun because of the earth's simultaneous motion around the sun. Sidereal rotation is anything that is measured with respect to the stars rather than with respect to the Sun or the primary of a satellite.
The "new moon", or first light after the "dark of the moon" is how people have measured time for thousands of years. The responsibility for keeping time was originally the responsibility of priests of the state religion of the time. The meeting of priests and other religious officials is known as a synod and it was such a group that determined when the new moon had arrived. The astronomical period from "new moon" to "new moon" is even today called the synodical month. The term synodical is defined as an astronomical term meaning "relating to the alignment of celestial bodies".
During Roman times, the Pontifex Maximus (high priest) would be responsible for proclaiming the beginning of a new month when he saw the new waxing crescent of the moon. The Latin word for "proclaim" or "announce" is kalare and that's why the Roman calendar always started with the Calends (Kalends) and that's where the English word for "calendar" comes from.
Examples of the various phases of the moon
The "new moon", or as some say, the "dark of the moon".
The waxing-crescent moon indicates that it is "growing", or even "increasing", in the size of its lighted portion, or it is becoming gradually fuller. "Waxing-crescent" is a redundancy because both waxing
mean "to grow" or "to increase". The waxing moon
is the right-hand moon
which is determined by holding up your right hand and looking at the moon between the right-hand index finger and the thumb and noting how the sun's reflected light from the moon follows the curve made by the your finger and thumb.
Festivals are observed among many people and thankful prayers are said for the reappearance of the missing light in the sky when it starts to wax again; that is, when the first thin slice of light appears along the moon's eastern edge is seen from the earth.
The first-quarter moon is actually considered by most people to be the "first half" of the moon because it looks like a "half" from our perspective, but astronomers call it the first quarter because it has completed one-fourth of a revolution around the earth.
The waxing-gibbous moon indicates that the moon is still growing but that now it is over half full ("gibbous" or "hump"), but not yet full.
The full moon, of course, indicates that the moon is fully reflecting the sun's light. The full moon is looked upon as a climactic period of the month; the Jewish Passover is celebrated at the full moon, and the Christian Easter, from which the dates of all other movable religious feasts in the calendar are determined, occurs on the first Sunday following the full moon after the vernal equinox.
The waning-gibbous moon is an indication that it is gradually decreasing, a reference to the visible face of the moon during the period after it has become full. As with the waxing-gibbous moon, the waning gibbous refers to the moon being over half full, but not completely dark. The waning moon is determined by holding up your left hand and looking at the moon between the left hand index finger and thumb and noting how the crescent moon follows the curve of your finger and thumb; so, this phase of the moon could be considered the "left-hand" moon phase.
The last quarter moon (or waning half) indicates that the moon is in its last quarter or three-quarters of a completed revolution around the earth.
The waning-crescent moon could be considered to be an oxymoron because waning
means "decreasing" while crescent
meant to "increase". It's always considered the "left-hand" moon as indicated at the waning-gibbous moon above.
So it is that we come to another new
(or fully "dark") moon that starts a new cycle. Note that most people consider the "new moon" to be the first waxing crescent, not the dark of the moon now indicated by astronomers as the "new moon".
Like the earth, half of the moon is always lighted by the sun's rays except during an eclipse. Usually the far side of the moon (from the earth) is in full sunlight even though it is out of earth's view.