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Earth’s eclipses are special |
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August 3, 2017 |
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By Larry Sessions
EarthSky.org
You might have heard that the Earth is the only
planet in our solar system that experiences
eclipses. That is not true. Granted, our
eclipses are special, but hypothetical observers
on other planets also could experience partial
and total solar eclipses.
Read on to learn more about eclipses on other
planets, and to learn why Earth’s eclipses are
unique.
Venus and Mercury
Generally speaking, an eclipse occurs
when one astronomical body (such as a moon)
passes in front of another (such as our sun).
Mercury and Venus, having no moons at all, never
have eclipses of any kind.
Observers on the searing hot surface of Venus,
would never directly experience an eclipse even
if the planet had a moon, simply because it is
constantly enveloped by thick clouds.
Mars
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| Martian moon
Phobos passes in front of the sun as
viewed by the Mars rover Curiosity on
August 20, 2013. |
Martians, if they existed, would occasionally
experience transits by the planet’s moons Deimos
and Phobos.
Transits occur when the nearer body appears too
small to cover the farther body. During these
events, the nearer body (for example, a moon of
Mars) would appear through appropriately
filtered, large-enough telescopes as black dots
moving in front of the farther body (in this
case, the sun).
A transit of a moon of Mars is essentially a
partial eclipse. It can be geometrically
equivalent to an annular solar eclipses seen on
Earth. During an annular eclipse, the moon is in
a far part of its orbit and appears too far from
Earth to cover the sun completely, even when the
moon passes directly in front of the sun.
The moons of Mars – Phobos and Deimos – are
always too small and too far from the planet for
anything but transits, or partial eclipses.
Jupiter, Saturn, Uranus, Neptune, Pluto
Elsewhere in the solar system,
observers floating at cloud-top level on
Jupiter, Saturn, Uranus or Neptune could
experience eclipses by the various moons of
those worlds. Even far-off Plutonians would, on
quite rare occasions, see a moon pass in front
of the sun from their distant dwarf world.
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| Simulation
showing the approximate relative sizes
of the sun with Pluto’s large moon
Charon, as seen by an observer on the
surface of Pluto. To a real observer,
however, the sun would be dazzlingly
bright, and Charon would show up
silhouetted as a black disk. |
From Pluto, the sun is little more than an
intensely bright star. In fact, it is only about
the same size as the planet Jupiter as viewed
from Earth. But Pluto’s large moon Charon
appears from Pluto nearly four times as large as
Earth’s moon from Earth. A solar eclipse from
Pluto would be more like a lunar occultation of
a star from Earth, an event where the moon
covers the star.
So if solar eclipses are not exclusive to Earth,
why are they so special?
Not to stress the obvious, but there are human
observers on Earth to view eclipses, something
no other planet in our solar system can boast.
Aside from that, the main reason is that the sun
and moon appear roughly the same size in the
sky, allowing particularly impressive total
solar eclipses. During totality of a solar
eclipse, the silhouette of the moon leaves a
gaping “black hole” in the sky, surrounded by
the ghostly glow of the sun’s outer atmosphere,
the corona.
This fortuitous circumstance happens because
although the moon’s diameter is about 400 times
smaller than the sun’s, it also is about 400
times closer. That makes the sun and moon to
appear roughly the same size in the sky, about a
half-degree. If the moon were 10% closer, as it
was about a billion years ago*, it would always
appear appreciatively larger than the sun, and
some of the magic of today’s total solar
eclipses would be lost.
If the moon were 10% farther away, as it will be
roughly a billion years in the future, it would
appear too small to completely cover the sun’s
disk, and we would never experience a total
solar eclipse.
So while our eclipse experience on Earth today
is virtually unrivaled anywhere in the solar
system, it is simply a temporary coincidence. |
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