THE MOON

REVOLUTION AND ROTATION• Relative to stars, Moon moves eastward by about 13° per

day.

• Eastward motion is faster than that of Sun; meaning Moon moves with respect to Sun as well.

• Times of moonrise and moonset grow later throughout the month; an hour later each day.

• Moon returns to same place among stars in ≈ 27.3 days; this is known as the sidereal month (one orbit around Earth)

• It takes the Moon more than a sidereal month to lap Sun & return to same position relative to the Sun; the length of time required for this to occur is called the synodic month (≈ 29.5 days).

REVOLUTION AND ROTATION

SYNCHRONOUS ROTATION

• As moon revolves about Earth, it rotates on its axis w/a period that is = its orbital period.

• Moon keeps same side (face) turned toward Earth; people think it doesn’t rotate.

• If moon didn’t rotate, we would see entire surface of it each month.

• Hence, a planet, satellite, or star that revolves & rotates in same length of time is said to be in synchronous rotation.

SYNCHRONOUS ROTATION

THE PHASES OF THE MOON

Full Phase – phase of Moon @ which the bright side of it is the face turned toward the Earth.

Gibbous Phase – phase of Moon @ which the near side of it is more than ½ illuminated by sunlight. Occurs just before and after full moon.

PHASES OF THE MOON

• First Quarter – phase when Moon has moved a ¼ of the way around the Earth; only half of the bright hemisphere of the Moon can be seen.

• Waxing Crescent – The horns of the crescent Moon point away from Sun; Moon is only 15° east of the Sun and will set about an hour after sunset. sometimes called a young moon – is always seen in the west after sunset.

• New Moon - first phase of the Moon, when it lies closest to the Sun in the sky as seen from the Earth; it is the instant when the Moon and the Sun have the same ecliptical longitude. The Moon is not normally visible at this time except when it is seen in silhouette during a solar eclipse.

Waxing Gibbous Phase First Quarter Phase

PHASES OF THE MOON

• Waning Crescent -sometimes called an old moon, occurs when the Moon is nearly, but not precisely, on a line with the Earth and Sun. Part but less than ½ of the Moon is illuminated by direct sunlight, creating a crescent moon. Because the fraction of the Moon’s disk that is illuminated is decreasing, we consider the Moon to be “waning.”

• Third Quarter - often called a "half moon", happens when the moon is at a 90 degree angle with respect to the earth and sun. So we are seeing exactly half of the moon illuminated and half in shadow.

Waning Crescent

PHASES OF THE MOON

• An easy way to remember and understand those "between" lunar phase names is by breaking out and defining 4 words: crescent, gibbous, waxing, and waning.

• The word crescent refers to the phases where the moon is less than half illuminated. The word gibbous refers to phases where the moon is more than half illuminated.

• Waxing essentially means "growing" or expanding in illumination, and waning means "shrinking" or decreasing in illumination.

PHASES OF THE MOON

PHASES OF THE MOON

• Animation of the Moon as it cycles through its phases, as seen from the Northern Hemisphere. The apparent “wobbling” of the Moon is known as libration. The apparent change in size is due to the eccentricity of the lunar orbit.