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Observing Mercury
Mercury's Caloris Basin is one
of the largest impact features in the Solar System.
Observation of Mercury is complicated by its proximity
to the Sun, as it is lost in the Sun's glare for
much of the time, and at most other times can
be observed for only a brief period during either
morning or evening twilight.
Like Venus, Mercury exhibits
moon-like phases as seen from Earth, being
"new" at inferior conjunction
and "full" at superior conjunction,
rendered invisible on both of these occasions
by virtue of its rising and setting in concert
with the Sun in each case. The half-moon
phase occurs at greatest elongation, when
Mercury rises earliest before the Sun when
at greatest elongation west, and setting
latest after the Sun when at greatest elongation
east (its separation from the Sun ranging
from 18.5° if it is at perihelion at
the time of the greatest elongation to 28.3°
if at aphelion). |
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Unlike Venus, which is brightest when it is between
new and half full, Mercury is brightest as seen
from Earth when it is at a "gibbous"
phase, between half full and full. This is because
Venus is much closer to the Earth when in its
crescent phase than it is in its gibbous phase,
while Mercury's smaller orbit means it is not
much further away and the fuller phase more than
outweighs its greater distance from Earth.
Mercury attains inferior conjunction every 116
days on average, but this interval can range from
111 days to 121 days due to the planet's eccentric
orbit. Its period of retrograde motion as seen
from Earth can vary from 8 to 15 days on either
side of inferior conjunction, this large range
also arising from the planet's high degree of
orbital eccentricity.
Mercury is more often easily visible from the
Earth's Southern Hemisphere than from its Northern
Hemisphere; this is due to the fact that its maximum
possible elongations west of the Sun always occur
when it is early autumn in the Southern Hemisphere,
while its maximum possible eastern elongations
happen when the Southern Hemisphere is having
its late winter season. In both of these cases,
the angle Mercury strikes with the ecliptic is
maximized, allowing it to rise several hours before
the Sun in the former instance and not set until
several hours after sundown in the latter in countries
located at South Temperate Zone latitudes, such
as Argentina and New Zealand. At northern temperate
latitudes, by contrast, Mercury is never above
the horizon of a more-or-less fully dark night
sky.
Mercury can, like several other planets and the
brightest stars, be seen during a total solar
eclipse.
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