Description
A lunar phase or
phase of the moon is the appearance of the illuminated (lighted) portion of the
Moon as seen by an observer, usually on Earth. The lunar phases change
cyclically as the Moon orbits the Earth, according to the changing relative
positions of the Earth, Moon, and Sun. One half of the lunar surface is always
illuminated by the Sun (except during lunar eclipses), and hence is bright, but
the portion of the illuminated hemisphere that is visible to an observer can
vary from about 100% (full moon) to 0% (new moon). The lunar terminator is the
boundary between the illuminated and unilluminated hemispheres.
The lunar phase
depends on the Moon's position in orbit around the Earth and the Earth's
position in orbit around the sun. This diagram looks down on Earth from north.
Both Earth and the Moon's orbit are rotating counter-clockwise. Sunlight is
coming in from the right. One can see, for example, that the full moon will
always rise at sunset and that the waning crescent moon is high overhead around
9:00 am local time.
Lunar phases are
the result of looking at the illuminated half of the Moon from different
viewing geometries; they are not caused by the shadow of the Earth or umbra
falling on the Moon's surface (this occurs only during a lunar eclipse).
The actual time
between two syzygies or two phases is quite variable because the orbit of the
Moon is elliptic and subject to various periodic perturbations, which change
the velocity of the Moon. When the moon is closer to the earth, it moves
faster; when it is farther, it moves slower. The orbit of the Earth around the
Sun is also elliptic, so the speed of the Earth also varies, which also affects
the phases of the Moon.
It might be
expected that once every month when the Moon passes between Earth and the Sun
during a new moon, its shadow would fall on Earth causing a solar eclipse.
Likewise, during every full moon one might expect the Earth's shadow to fall on
the Moon, causing a lunar eclipse. Solar and lunar eclipses are not observed
every month because the plane of the Moon's orbit around the Earth is tilted by
about five degrees with respect to the plane of Earth's orbit around the Sun.
Thus, when new and full moons occur, the Moon usually lies to the north or
south of a direct line through the Earth and Sun. Although an eclipse can only
occur when the Moon is either new or full, it must also be positioned very near
the intersection of Earth's orbit plane about the Sun and the Moon's orbit
plane about the Earth. This happens about twice per year, and so there are
between four and seven eclipses in a calendar year. Most of these are quite
insignificant; major eclipses of the Moon or Sun are rare.
Names of Lunar Phase:
|
Phase
|
Northern Hemisphere
|
Southern Hemisphere
|
Visibility
|
Standard
time of culmination (mid-phase)
|
|
New
moon
|
Not visible, traditionally Moon's first visible
crescent
|
after sunset
|
12 pm
|
|
|
Waxing crescent moon
|
Right 1–49% visible
|
Left 1–49% visible
|
afternoon and post-dusk
|
3 pm
|
|
First quarter moon
|
Right 50% visible
|
left 50% visible
|
afternoon and early night
|
6 pm
|
|
Waxing gibbous moon
|
Right 51–99% visible
|
Left 51–99% visible
|
late afternoon and most of night
|
9 pm
|
|
Full moon
|
Fully visible
|
Fully visible
|
sunset to sunrise (all night)
|
12 am
|
|
Waning gibbous moon
|
Left 51–99% visible
|
Right 51–99% visible
|
most of night and early morning
|
3 am
|
|
Last quarter moon
|
Left 50% visible
|
Right 50% visible
|
late night and morning
|
6 am
|
|
Waning crescent moon
|
Left 1–49% visible
|
Right 1–49% visible
|
pre-dawn and morning
|
9 am
|
|
Dark moon
|
Not visible, traditionally Moon's last visible
crescent
|
before sunrise
|
12 pm
|
|
When the Sun and
Moon are aligned on the same side of the Earth the Moon is "new", and
the side of the Moon visible from Earth is not illuminated by the Sun. The
lunar phases progress from new moon, crescent moon, first-quarter moon, gibbous
moon and full moon phases, before returning through the gibbous moon,
third-quarter (or last quarter) moon, crescent moon and new moon phases. The
terms old moon and new moon are interchangeable, although new moon is more
common. Half moon is often used to mean the first- and third-quarter moons,
while the term 'quarter' refers to the extent of the moon's cycle around the
Earth, not its shape. When a sphere is illuminated on one hemisphere and viewed
from a different angle, the portion of the illuminated area that is visible
will have a two-dimensional shape defined by the intersection of an ellipse and
circle (where the major axis of the ellipse coincides with a diameter of the
circle). If the half-ellipse is convex with respect to the half-circle, then
the shape will be gibbous, whereas if the half-ellipse is concave with respect
to the half-circle, then the shape will be a crescent. When a crescent Moon
occurs, the phenomenon of Earthshine may be apparent, where the night side of
the Moon faintly reflects light from the Earth.
Conclution:
The time between
two full moons (a Lunar month) is about 29.53 days(29 days, 12 hours, 44
minutes) on average. This synodic month is longer than the time it takes the Moon
to make one orbit around the Earth with respect to the fixed stars (the
sidereal month), which is about 27.32 days. This difference is caused by the
fact that the Earth-Moon system is orbiting around the Sun at the same time the
Moon is orbiting around the Earth.
0 komentar:
Posting Komentar