Calendar, Moon Facts, Part 1 of 6

(waxing, waning and phases of the moon)


The Views of the Moon's Phases as seen from the Earth

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


Phases of the Moon Chart.
—Based on information presented by
"Lunar Phases" in The Facts on File Dictionary of Astronomy;
Edited by Valerie Illingworth; Facts On File, Inc.;
New York; 1979; page 241.

Phases of the Moon animation.

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.
Today is the tomorrow you worried about yesterday.
—Anonymous

Moon Facts, Part 2, is available.
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A cross reference of word units that are related, directly or indirectly, to the: "moon": Chemical Element: selenium; Gods and Goddesses; luna, luni-; Luna, the earth moon; menisc-; meno-; Planets in Motion; plano-; seleno-.