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being that
some have a
total phase.
The partial
phases of
total eclipses
differ in
length at
each eclipse.
Places
outside
the path of
totality only
see a partial
eclipse.
Sometimes
the center of
the Moon’s
shadow
completely
misses Earth
COURTESY OF Steve Edberg
and only a
This January 4, 1992, annular eclipse was technically below the horizon by the time it reached the
partial eclipse California coast, Black Mountain, where this image was taken. Refraction (light bending) by Earth’s
is visible over atmosphere raised the Sun above the physical horizon and made the ring visible. Notice the many
a limited area layers apparent in Earth’s atmosphere, presented by the variation in brightness of the ring.
of our planet.
But there are also solar eclipses wherein
the center of the Moon’s shadow does
cross our planet but the eclipse isn’t
total. In this case, the Moon is too far
from Earth and it does not completely
cover the Sun even when it is centered on
the Sun. This is a central partial eclipse,
better known as an annular eclipse.
During the central period (sometimes
called annularity), the Moon’s silhouette
is surrounded by the bright sun, making
a ring (annulus) that changes shape
all through the central phase as the
COURTESY OF Steve Edberg
silhouette moves across the Sun’s disk.
The tiny silhouette of Mercury is the dot, smaller than the sunspot on
Annular eclipses can last as long
the left, visible on the right half of the solar disk below the equator.
as about 12 minutes or be almost
instantaneous.
See Sidebar One on the next page for more
Another phenomenon, similar to an eclipse,
information on these infrequent events.
is called a transit. On Earth we can observe
Eclipses Seen on Other Solar System Planets
transits of Mercury and Venus occasionally.
With no natural satellites, Mercury and Venus
The silhouettes of these two planets cross the
don’t get to enjoy eclipses, though someone
disk of the Sun, taking a few hours to do so.
Sky ’ s
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sidebar one
Phenomena
related to eclipses
About 13 times a century for Mercury and in pairs
eight years apart separated by more than a century
for Venus, we see these planets transit the Sun’s
disk. The silhouette of the planet passes across the
disk of the Sun, taking a few hours to make the full
crossing. Mercury’s silhouette is tiny, invisible without
a telescope. The silhouette of Venus is large enough
that with a safe solar filter you can see it without a
telescope; with a telescope Venus is a small but distinct
disk.
Asteroids and comets can also transit. Halley’s
Comet was predicted to transit the Sun in 1910 but
the passage was not visible in telescopes. This told
astronomers that the solid nucleus of the comet was
small. Earth satellites also transit the Sun (and Moon)
on occasion. The pass takes about 1 second. The
distinctive shape of the International Space Station can
be seen and imaged.
Sometimes spacecraft see the Moon crossing the Sun’s
disk. It’s sometimes not obvious whether to call the
event an annular eclipse or a transit.
We call an event when a larger body passes in front
of a smaller body in the sky an occultation. If we
were consistent using definitions of astronomical
phenomena, a total solar eclipse would be called a total
occultation of the Sun. Only events during which we
see the shadow of one object fall projected on another
object would be called an eclipse (like an eclipse, total
or partial, of the Moon when it passes through Earth’s
shadow).
But in common usage, an occultation refers to
(usually) night time events such as the Moon passing
in front of (occulting) a star or asteroid or planet. There
are also occasions when an asteroid or planet occults
a star. See the discussion of Jupiter in Sidebar Two for
more information.
COURTESY OF Steve Edberg
Venus is much larger than the sunspots visible on the Sun’s disk
to its right.
COURTESY OF NASA/Bill Ingalls
Above, this composite image made from five frames shows the International
Space Station, with a crew of nine onboard, in silhouette as it transits the Sun
at roughly five miles per second on Sept. 6, 2015. Left, in this view, the Moon
is partially occulting Saturn. At most locations for viewing this event, Saturn
eventually disappeared completely behind the Moon and reappeared an hour or
less later. From this site, the Moon never completely covered Saturn and spent
about five minutes sliding by, partially covering the planet. Notice that Saturn is
covered by a narrow piece of the night side of the Moon, not by the mountains
illuminated by Sun in the foreground.
COURTESY OF Steve Edberg
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