Educator's Guide to Eclipses
Courtesy of the Jet Propulsion Laboratory
Astronomical Alignments
Eclipses have long been a source of mystery and spectacle. These
events were viewed with fear and dread in the past and, even
today, still thrill.
There is a lot of special vocabulary involved in eclipses but
there is a way to keep from being confused. The eclipse is named
for the object that is being eclipsed, or obscured. In a
solar eclipse
you observe the Sun (using only safe methods, of course).
You will see the Sun with a piece apparently cut out of it. In a
lunar eclipse
you observe the Moon. A portion of its surface
will be obscured.
Another way to avoid confusion is to consider the time at which
you will be viewing the eclipse. Because of the geometry
described below, you can only view a solar eclipse when the Sun
is up, and the Moon is nowhere to be seen. You view lunar
eclipses when the Moon is up.
Eclipses occur when the Sun, Earth and Moon line up. They are
rare because the Moon usually passes above or below the imaginary
line connecting Earth and the Sun. In a solar eclipse the Moon
passes directly in front of the Sun. This can only happen when
the phase of the Moon is "new." That occurs because, for
Earth-based observers, the far side of the Moon is illuminated
while the side facing Earth is in darkness. The Moon, like any
sphere, casts a shadow. A solar eclipse occurs when that shadow
sweeps across Earth. The black cone is called the
umbra,
as in umbrella. An observer anywhere in that region is completely in
shade. None of the Sun is visible from there.
Surrounding the umbra is the
penumbra.
An observer there will
see some, but not all, of the Sun. Outside of these regions, all
of the Sun is visible. Note that the tip of the umbra barely
touches Earth. At the current time the position of the Moon
relative to the Sun is such that the Moon, which is 400 times
smaller that the Sun, is 400 times closer! This means that the
two objects appear to be the same size in the sky. Only
observers at the tip of the umbral cone will see a total solar
eclipse. A large number of observers across the globe will see a
partial solar eclipse if they are in the penumbra.
An annular eclipse is a special partial solar eclipse. Because
the Moon's orbit around Earth is an ellipse, not a circle, the
Moon's distance from Earth varies. When the Moon is far from
Earth it appears slightly smaller in the sky. (Earth's orbit
around the Sun is also an ellipse, and during January, Earth is
at its closest point to the Sun. The Sun's size is slightly
larger than during the rest of the year.) With a "small" Moon
and a "large" Sun the Moon will not completely block out the Sun.
The umbra does not touch Earth. An observer would have to be
above the surface of Earth to see a total eclipse. For
individuals in just the right location, the Sun appears as a ring
(annulus)
around the silhouetted Moon.
In a lunar eclipse the Moon moves into Earth's shadow. They can
only occur when the moon is "full." Observers on the night side
of Earth see the Moon take on a reddish hue as it moves into
Earth's umbra. If the entire disk of the Moon falls into the
umbra it is total lunar eclipse. If only a portion does, then it
is a partial lunar eclipse. Penumbral lunar eclipses are very
difficult to detect because the Moon dims only slightly while
moving through that region. Lunar eclipses are more common than
solar eclipses. Total eclipses of the Sun and Moon are partial
before and after totality.
Popular astronomy magazines, available on many news stands,
always give timely eclipse details.
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