What
is…
…an
Eclipse?
projected upon another object. Eclipses are most
Throughout the history of this earth there commonly referred to as being in relation to the
Sun, Earth, and the Moon, but can certainly
have been recorded instances where the sun’s
rays have been hidden for a brief time when they happen elsewhere as well.
Because of the 23 ½ degree tilt of the
should otherwise not be. Ancient civilizations
earth, the moon does not often cross directly
did not yet fully
between the earth and the sun or fall directly
understand the
behind the earth from the sun. The moon
circumstances that
follows the tilt of
brought about this
earth’s axis like the
rare display. Often
hammer, or
these events were
weighted ball at the
attributed to an act
end of the cable that
of intervention from
is thrown Olympians. All of the planets orbit on
the gods and
an ecliptic, or tilted plane to some degree, and
foretold some form
any moons that those planets might have will do
of imminent doom.
much the same thing.
Today we know that eclipses are one of
Through the course of one year there are
the most awe inspiring natural events that can be
at
most
only seven instances out of the twenty
witnessed by the inhabitants of this earth. They
are neither scary, nor ominous. They can occur five or so positions where the moons orbital path
crosses directly between or behind the earth and
in any part of the world, and therefore are
the sun. This limited number of actual eclipse
generally available for everyone to see
personally at some point in their lifetime. They causing instances happens because most of the
are more common than most comet appearances, times when the path presents an opportunity, the
moon’s position is actually far away from being
like the famous Halley’s Comet which comes
in the right place to make it happen. Everything
around once every seventy-five years or so.
needs to be just right for even a partial eclipse to
Eclipses, while amazing to watch, are not as
happen.
frequently occurring as sunsets, or shooting
The Eclipsed Past
stars, two other natural wonders.
Pieces Parts
Anatomy of an Eclipse
There are parts of the shadow that is cast
by the planetary bodies that must be discussed in
order to more fully understand what causes an
eclipse. The diagram below will be used to
show these features.
An eclipse happens when a light source
is either obscured for the viewer by another
object, or when the shadow of the viewer is
When a shadow is cast by the moon, and
that shadow reaches the earth at any point, an
eclipse occurs. The darkest cone shape, directly
behind the moon is called the umbra. No direct
sunlight exists in this area of the shadow. The
Penumbra encircles the umbra, being itself part
of the overall shadow of the moon, but also
containing at least a part of the visible and direct
light from the sun within its zone. Beginning
directly behind the focal point of the umbra
starts a zone where a part of the direct rays of the
sun are always visible. This zone is called the
antumbra. Which type of eclipse occurs will
depend on these four zones, which are directly
affected by how close the moon is to the earth
when it passes between the earth and the sun.
earth that receives no direct sunlight. All who
view the eclipse within the area of the umbra
will view a total solar eclipse. These usually last
a few minutes long. The shadow moves across
the face of the earth at around1000 miles per
hour, so it goes away all too quickly.
The last type of eclipse that will be
mentioned is the annular eclipse. This eclipse is
experienced when the viewer is within the region
of the antumbra. The sun’s outside diameter is
then visible, because the moon is farther away
from the earth, making it appear smaller than the
apparent diameter of the sun. This phenomenon
is commonly referred to as the “Ring of Fire.”
Phases of an Eclipse
There are four phases of an eclipse. The
first, called “first contact” happens when the
moon’s outside diameter (OD) touches the sun’s
OD. Second Contact is when the moon’s OD
first completely enters the sun’s OD. Third
Contact is when the moon’s OD once again
touches the exiting edge of the sun’s OD.
Lastly, fourth contact is when the moon leaves
the sun’s OD altogether. It should be noted that
Three Major Types
the second and third contacts do not always
A lunar eclipse happens when the moon happen during every eclipse event. During a
crosses either partly or completely through the
partial eclipse, only the first and last contacts
shadow of the earth. It may happen around the
might be the only visible events.
time when the moon is in its fully lit up phase,
Finally, at no time should anyone look
called the full moon phase. The moon does not
directly at the sun during one of these events.
have to be completely full for a lunar eclipse to
The annular eclipse, even though as little as six
happen, however. Small crossings of the earth’s
percent of the sun is visible, is still fully able to
shadow can still happen when the moon is in a
cause permanent retinal damage. Alternate
gibbous, or nearly full phase. While lunar
viewing methods like special solar viewing
eclipses take place mostly during the night time,
glasses, or
they can be viewed during twilight hours as
homemade pinhole
well.
projection boxes
A solar eclipse happens when the shadow are readily
of the moon falls on the earth such that a portion available and
of the umbra cone reaches the earth. This
highly encouraged.
creates a small circular area on the face of the