Mechanisms of The
Lunar Eclipse
A lunar
eclipse can only happen at Full Moon, it is because the Earth comes between
the Sun and the Moon as if the Moon slips into the shadow of the Earth. Usually,
people on the Earth’s night hemisphere can see the same lunar eclipse for
the whole night.
This
animation illustrates a total lunar eclipse as the Moon passes through the
umbra of the Earth's shadow. Distances are not to scale, and we have idealized
the illustration by fixing the Earth in space relative to the Sun. In reality,
the Earth will be in motion around its orbit as the Moon moves around the
Earth.
*A Lunar Eclipse
~~ Full Moon
*Why doesn’t A
Lunar Eclipse be seen every month ??
As people may think that they can view a lunar eclipse once a
month during the Full Moon but it is not true. The moon’s orbit about the Earth
is inclined about 5 degree with respect to the Earth’s orbit of the sun. As a
result, the moon only crosses the Earth’s orbital plane (the ecliptic) twice
every orbit, at points called nodes. The moon is usually above or below the sun
in our sky at New Moon and misses the Earth’s shadow at Full Moon. In order for
lunar eclipses to occur, the Moon must be on the line of nodes during Full Moon, that means to be in line with the ecliptic plane.
* An Eclipse
year.
First
penumbral contact occurs when the Moon first enters Earth’s penumbra, while
first umbral contact marks when the Moon is first
shadowed by Earth’s darker umbra.
Refer to Figure 1.13, first penumbral contact
(P1) is difficult to discern visually, while first umbral
contact (U1) is clear with or without optical aid (however trying to tell exactly when first umbral
contact occurs is difficult).
Second umbral
contact (U2) takes place at the beginning of totality, when the Moon is fully
eclipsed by the Earth’s umbra, and third umbral contact
(U3) occurs at totality’s end.
Last umbral
contact (U4) marks when the Moon completely leaves Earth’s umbra, and second
penumbral contact (P2) occurs when the Moon leaves the penumbra.
Partial
lunar eclipses do not have U2 and U3 values, since those are reserved for when
the Moon fully enters the Earth’s umbra. Rather, contact times for partial
lunar eclipse are designated only P1, U1, U4, P2 respectively.
Sometimes,
the umbra appears a bright red-orange, dark coppery red or brownish gray. As coloration of the Earth’s shadow, the result of sunlight
refracting through our atmosphere, can change remarkably from one eclipse
to another.
* Points of
different Contact Times of The Lunar Eclipse