Download The Moon

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
 Waxing Gibbous Phase
First Quarter Phase 
• 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.
PHASES OF THE MOON
 Waning Crescent
• 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.
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.