Sky & Telescope - January 2023 - 52

JANUARY 2023 OBSERVING
Exploring the Solar System by Thomas A. Dobbins
Observing on the Edge
January offers a rare opportunity to glimpse one of the Moon's most challenging features.
Grimaldi
T
he Moon spins once on its axis and
completes one revolution around
Earth in the same interval of time, so
our satellite always presents the same
familiar face. But it's never exactly the
same face.
The shape of the Moon's orbit is
Lacus Autumni
Crüger
Lacus Veris
slightly elliptical, causing the distance
between the Moon and Earth to vary
between about 28 and 32 Earth diameters
every month. In accordance with
Kepler's second law of motion, our satellite's
orbital speed is fastest when it's
at perigee (closest to Earth) and slowest
when it's at apogee (farthest away). As
a result, the Moon's constant speed of
rotation alternately leads and lags its
orbital position, making it appear as if
the Moon is slowly oscillating from east
to west with an amplitude of almost 8
degrees. Known as libration in latitude,
this apparent wobble allows terrestrial
observers to peer around the Moon's
eastern and western limbs.
Favorable librations that occur when
Montes Rook
the southwestern limb is sunlit provide
opportunities to observe one of
the most imposing formations on the
Moon, Mare Orientale, the Eastern
Sea. Centered at 95° west longitude,
just beyond the Moon's western limb, it
marks the site of the impact of a large
asteroid around 3.8 billion years ago.
Despite its age, Orientale's almost pristine
state of preservation makes it the
most striking example of a multi-ring
impact basin on the entire Moon.
The name Mare Orientale is an
This month, a favorable libration brings the
southwestern limb of the Moon into view.
52 JANUARY 2023 * SKY & TELESCOPE
anachronism. In 1961 the International
Astronomical Union dispensed with the
astronomical convention of lunar coordinates
based on telescopic views and
adopted the astronautical convention
that defines east as the direction of sunrise
for someone standing on the Moon.
Suddenly the Eastern Sea was awkwardly
located on the Moon's western
limb. A proposal to rename the feature
Mare Pacificus was briefly entertained,
but the old name was deemed too well
established to be changed.
Images taken by the Lunar Orbiter
4 spacecraft in 1967 clearly revealed
Orientale's bulls-eye structure for the
first time. Flooded by dark basalt lavas,
the central mare measures almost 330
kilometers (205 miles) in diameter and
is completely encircled by a pair of concentric
mountain ranges. Resembling
ripples made by throwing a stone into
a pond, these circular scarps are the
result of shock waves in the lunar crust,
which was liquefied by the colossal,
basin-forming impact. The inner rim,
measuring 620 km across, is marked by
Montes Rook (the Rook Mountains).
The more rugged Montes Cordillera
defines the basin's 930-km-wide outer
rim. Isolated peaks in this range tower
to heights of 6 km above the central
floor of the basin.
Even the best telescopic views of
Mare Orientale are impaired by foreshortening.
At mean libration, the surrounding
mountain ranges often appear
silhouetted against the background
sky as irregularities on the lunar limb.
German astronomer Johann Hieronymus
Schröter charted them during
the 1780s. However, only slivers of the
central basin's dark floor can be seen.
The most prominent are two dusky ribbons
nestled in the lowlands between
the concentric mountain rings known
as Lacus Autumni (Lake of Autumn)
and Lacus Veris (Lake of Spring). They
Mare Orientale
SEAN WALKER / S&T
Sky & Telescope - January 2023

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