The Earth casts a shadow that the
Moon can pass through. When this happens we say that a
LUNAR ECLIPSE occurs.
A Lunar Eclipse
Lunar eclipses (like solar eclipse) can be partial or total,
depending on whether the light of the sun is partially or completely blocked
from reaching the moon. The figure shows a total lunar eclipse
with the moon lying in the UMBRA of the Earth's shadow.
The UMBRA is the region of total shadow, while the PENUMBRA is the surrounding region of partial shadow. In the penumbra, sunlight is only dimmed and not blocked.
The umbra of the Earth's shadow is about 1.4 x 10^6 km long
and points directly away from the sun.
If we place a screen at the distance of the moon,
then the diameter of the dark umbral projection is 16,000 km,
and the moon's diameter is only 3476 km.
A TOTAL LUNAR ECLIPSE occurs when no part of the moon is
outside the umbra. We can see (fig 3-10) the stages as the eclipse
proceeds from partial to total. It can only occur when
the moon is in the FULL phase.
The moon travels its diameter in 1 hour, so it takes about 1 hour
to become totally eclipsed when favorably aligned to enter the umbra.
A total eclipse, including the penumbral stage takes about 6 hours.
It is important to note that
during a total lunar eclipse, the moon takes on a dark red color because it is
being lighted slightly by sunlight passing through the Earth's atmosphere
whose ray paths are bent because the light scatters off of the gas
particles in the Earth's atmosphere.
It turns out that the physics of light scattering tells that, for
the conditions of the Earths atmosphere, blue light (high frequency)
is scattered more
than red light (lower frequency).
Blue light is scattered too much to
be seen at the small angles of scattering needed to illuminate
the moon in a total eclipse. Only the red light ray paths are scattering
This particular scattering is called "Rayleigh Scattering"
and is the SAME REASON THAT THE SKY IS BLUE IN THE DAY,
but the sun looks red at the horizon! The blue light is scattered more and so our more direct view
of the sun means that we see the light rays which are not bent.
If the the Earth's atmosphere is particularly cloudy or dusty (e.g.
from Volcanos) then
the moon can be darker as some of the light is further absorbed by
By similar reasoning, if we travel to
the moon during a total lunar eclipse and looked at Earth,
we would not see the sun, but we would see a reddish glow around the edge
of the Earth.
Aristotle (384-322 BC) noticed that the shadow cast by the
Earth on the moon was circular, and thus concluded that the Earth was
Because moon's orbital plane is inclined a little more than 5 degrees
with respect to the Earth's orbital plane, the moon might not
pass through the center of the umbra.
PARTIAL LUNAR ECLIPSE occurs when part of the moon remains outside
the umbra. Any fainter red glow, as in a total eclipse, is drowned
out by the glare of the illuminated part of the moon
for all but nearly total eclipses.
If the moon passes far enough north or south of the
umbra, and the moon only passes through the penumbra, then the
lunar eclipse is called a PENUMBRA ECLIPSE and are uneventful.
The last total eclipse of the moon ended at Jan 9 at 3:52pm and
the duration was 1 hour.