(bright music)
- [Narrator] A lunar eclipse
happens when the earth blocks
some or all of the sun's direct light
from reaching the moon.
This cosmic event only takes
place during a full moon,
which happens once every
29 and a half days,
or the length of one full orbit
of the moon around the earth.
So why don't we have
an eclipse every month?
The moon's orbit is tilted a few degrees
in relation to the earth,
so the earth, moon, and
sun don't always align.
When the earth does eclipse the sun,
it casts two types of shadows on the moon,
a larger shadow, known as the penumbra,
and a smaller, darker
shadow, known as the umbra.
There are three types of lunar eclipses.
The first is a total lunar eclipse,
when the sun, moon, and earth
are in perfect alignment
and the moon falls within
the earth's umbral shadow.
Total lunar eclipses are the most striking
of the three types because they
turn the moon a sunset red.
While shorter, blue wavelengths of light
are scattered outward by
the earth's atmosphere,
longer, red wavelengths
are refracted, or bent,
inward toward the moon,
making it appear red.
The brightness of the moon's red glow
depends upon how much dust and clouds
are in the earth's atmosphere.
Following volcanic activity,
ash can block out enough light
to render the moon a darker
red, or even near black.
A partial lunar eclipse, the second type,
occurs when the earth, moon, and sun
don't perfectly align, so only part
of the moon passes into earth's umbra.
Earth's shadow appears very dark
on the side of the moon facing earth.
Last, a penumbral lunar eclipse,
occurs when the moon passes
through earth's penumbral shadow.
The event is so subtle that
most people don't even notice.
The moon will appear just
slightly darker than normal.
Lunar eclipses occur up
to three times a year
and can be observed from the entire
nighttime half of the earth.
Unlike during a solar eclipse,
it's safe to look at the moon
with the naked eye during a lunar eclipse.
It is only because of the
distances of the sun and moon
from the earth that we are able to witness
total lunar eclipses.
As the moon inches away
from the earth each year,
one day, billions of years from now,
the moon will be too far
away to fall completely
within earth's umbral shadow.
Until then, we can
occasionally enjoy seeing
our own planet's shadow cast upon
the largest object in the night sky.
