Phases of the Moon,
Part 1 of 5

As the moon orbits the 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 revolutioon 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. The term synodical is defined as an astronomical term meaning "relating to the alignment of celestial bodies".

More about the Waxing and Waning of the Moon

When the moon waxes, it is growing, or increasing in the size of its lighted portion, or it is becoming gradually fuller. When the moon is waning, it is gradually decreasing, said of the visible face of the moon during the period after it has become full.

Festivals of the new moon 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.

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 feasts in the calendar are determined, occurs on the first Sunday following the full moon after the vernal equinox.

The waxing moon is the "right-hand moon", because the curve of the right hand index finger and thumb follows the curve of the crescent while the waning moon is the "left-hand moon".

Phases of the moon

The moon's phases depend on what fraction of its sunlit hemisphere can be seen from earth.

• As the moon orbits the earth, it grows in size from new moon, to crescent, first quarter, waxing gibbous, and to the full moon.
• After the full moon, it decreases in reverse order to waning gibbous, third quarter, crescent, and back to the new moon.
• 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, but 29.5 days with respect to the sun because of the earth's simultaneous motion around the sun.
• During a synodic month [the time between repeated lunar phases, such as new moon to new moon], we can see the moon "change" from a slim crescent to a full circle and back again.
• These changes in the moon's shape and size are actually different conditions of lighting called phases.
• The phases are caused by changes in the amount of sunlight reflected by the moon toward the earth.
• The moon seems to change shape because we see different parts of its sunlit surface as it orbits the earth.
• 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.
• A day after a new moon, a thin slice of light appears along the moon's eastern edge.
• The line between the sunllit part of the moon's "face" and its dark part is called the terminator.
• Each day, more and more of the moon's sunlit side is seen as the terminator moves from east to west.
• In about seven days, after the first quarter, the moon has moved to a point where the earth is between the moon and the sun.
• It is at this point that we can see the entire sunlit side of the moon or the full moon.
• A full moon seems to be bright on a clear night, but a whole sky of full moons would be only about one fifth as bright as the sun.
• In about seven days after the full moon, we will see half of a full moon.
• This phase is called the last quarter, or the third quarter.
• After another week, the moon returns to a point between the earth and the sun for a new phase of the moon.
• As the moon changes from new moon to full moon, it is said to be waxing [from Old English, weaxan, "to increase"; and related to Greek auxanein and Latin augere, "to increase"].
• During the period from full moon back to new moon, the moon is said to be waning [from Old English, wanian, "wanting, deficient; and Latin, vanus, empty].
• When the moon appears smaller than half of a full moon, it is called the crescent [from Latin, crescere, "to grow, increase"].
• When the moon looks larger than half of a full moon, but is not a full moon, it is called gibbous [from Latin, gibbus, "hump"].
• The moon rises and sets at different times.
• In the new moon phase, it rises above the horizon with the sun in the east and travels close to the sun across the sky.
• Of all of the visible heavenly bodies, the moon is the closest to earth.
• It is about 240,000 miles away on average.
• Venus is the next closest visible heavenly body, but it is never closer than 25,000,000 miles from the earth.
• The moon appears to be as large as it is because of its closeness, but it is comparatively small since it is only a quarter of the diameter of the earth.
• The sun is a hundred times the diameter of the earth, but it is also about 93,000,000 miles away.
• It just happens that size and distance cancel each other and both the sun and the moon appear to be the same size.
• The moon is close enough to the earth to be caught in earth's gravitational field.
• The moon is the only heavenly, non-man made, body that really moves (revolves) around the earth.
• Although the moon is moving from west to east, the earth's spin overtakes it and makes it seem to move from east to west. It appears to "rise" in the east, moves westward, and "sets" in the west.

The sidereal month

• The time the moon takes to travel from a position in the heavens (using the stars as markers) back to the same position is 27 days, 7 hours, 43 minutes, 11.5 seconds is called a sidereal month.
• The sidereal month is not the time from new moon to new moon and it is not what ancient men (nor modern humans) would call a month.
• It is the relationship of the moon to the sun that marks the month which is used by mankind not the relationship of the moon to the stars.
• As the moon travels around the earth, once every revolution it gets between the earth and the sun.
• Sometimes the moon comes exactly between the sun and the earth and partly, or entirely, "hides" the sun.
• When the moon "covers" the sun, we have an eclipse of the sun, or a solar eclipse.
• Most of the time, the moon is slightly above or slightly below the sun so it is not covered up entirely.
• Naturally, when the moon is between the sun and the earth, we can not see the moon because its dark side faces the earth and this is called the "new moon."
• As the moon moves eastward from the sun around the earth, a sliver of light on one edge can be seen.
• As the moon continues to "drift" eastward from the sun, more and more of the sunlit half of the moon can be seen each night.
• When the moon has completed one-fourth of its revolution, we can see exactly half the sunlit portion.
• As the moon continues to move eastwasrd, it grows gibbous as more of the sunlit portion is visible.
• When it is on the side of the earth almost directly opposite to the sun, it is usually a little above or just below the earth's shadow; however, when it is exactly opposite and is in the earth's shadow, we have an eclipse of the moon, or a lunar eclipse.
• When the moon is on the side of the earth opposite to the sun, the face of the moon which is toward the earth is also toward the sun and, since is entirely lit up, we have a "full moon."
• The full moon is at the eastern horizon when the sun is at the western horizon; therefore, the full moon "rises" at sunset and "sets" at sunrise.
• There are two "half moons", one coming to the full moon and one on the way back to the new moon.
• Calendars show the first half moon as the first quarter and the second half moon as the last (or third) quarter of the moon.
• The half moons are called "quarters" because half the visible face of the moon is one quarter the entire surface of the moon, if the unseen face on the other side is counted.
• The time from new moon to new moon is measured as 29 days, 12 hours, 44 minutes, 3 seconds.
• New moon to new moon is how people have measured time for thousands of years.

Synodical month

• The responsibility for keeping time was originally the responsibility of priests.
• 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.
• During Roman times, the Pontifex Maximus (high priest) would be responsible for proclaiming the beginning of a new month when he saw the crescent of the waxing moon.
• The Latin word for "proclaim" or "announce" is kalare and that's why the Roman calendar always started with the Calends and that's where the English word for "calendar" comes from.
• The Romans called the day which came at or near the middle of each month the Ides (usually the 13th of each month, except for March, May, July, and October which had their Ides on the 15th).
• The 9th day before the Ides (including the Ides and the Nones) was the Nones, which in Latin means nine.
• The Nones fell on the 7th day of some months (March, May, July, and October) and on the 5th day of the others.
• The Babylonians broke the month into parts which is still used even to our present time: the new moon, the first quarter, the full moon, and the last quarter.
• The so-called parts of the moon are known as phases from Greek meaning "appearance."
• The time between any of the four phases of the moon is one-quarter of synodical month (seven days, nine hours, and eleven minutes).
• The word "week" comes to us from Teutonic, or Old English wicu to Middle English weke, and means "change" or "alteration" [perhaps as a reference to the moon].
• The Greeks and Romans did not have the seven-day week in pre-Christian times.

Moon Phases: Part 2 of 5.

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