Sky and Telescope - June 2017 - 52

JUNE 2017 OBSERVING
Exploring the Moon by Charles Wood

Peaks of "Eternal" Light
If we ever try to live on the Moon, the best locations will be polar mountains
bathed in nearly continuous sunlight.

the rims of some deep polar craters, and
so their ﬂoors receive very little direct
sunlight, if any at all. Thermal maps
created by the Diviner instrument on
NASA's Lunar Reconnaissance Orbiter
(LRO) show that the always-dark ﬂoors
of these craters are some of the coldest
locations in the solar system.
Offsetting this absence of light are
some nearby polar mountains that
nearly always bask in sunlight. The
pioneering German lunar mapper
Johann Heinrich von Mädler was the
ﬁrst to realize this, but it was French
astronomer and author Camille Flammarion who, about 40 years later in
1879, romanticized the idea by calling
these always-sunlit places pics de lumière
éternelle - peaks of eternal light.
Actually, no lunar real estate is 100%

Earth

Scott

Malapert
Massif

Demonax

Cabeus
Haworth
H
Shoemaker
Amundsen

SOUTH POLE

Shackleton

100 km

-8

Elevation (km)

p Spacecraft-borne laser altimeters have mapped the topography near the Moon's south pole.
High points on the rims of Malapert and Shackleton receive sunlight about 90% of the time.

J U N E 2 0 1 7 * SK Y & TELESCOPE

illuminated, though a few peaks are
lit by the Sun more than 90% of the
time. In any case, these locations are
always difﬁcult to observe and identify.
Even when seen with favorable lunar
libration, the extreme foreshortening
created by looking sideways at the lunar
poles compresses circular crater rims
into apparently straight ridges.
However, using images from the
lunar-orbiting Clementine, Kaguya, and
LRO spacecraft, which have been able
to look straight down on the poles, scientists have disentangled these distortions and mapped lunar locations that
are nearly permanently shadowed and,
conversely, rarely so.

Take the Polar Challenge
For a telescopic observer, identifying the
"pics" near the lunar south pole is especially difﬁcult because the area is heavily
cratered. The most continuously sunlit
peaks typically occur as small pieces of
rim crests and ridges around Shackleton, a 21-km-wide crater located right at
the pole. But I have yet to come across
an Earth-based image of Shackleton.
Instead, peaks on the rims of the
craters Cabeus and Malapert are the
most southerly named features that can
usually be identiﬁed telescopically when
librations tip the south pole toward
Earth. They're both in view in the image
at the top of the facing page.
Most conspicuous are informally
named Malapert Massif (a 5-km-high
crest along that crater's southwestern
rim) and other isolated high points that
are parts of the rim of the enormous
farside basin called South Pole-Aitken.

CR
N
ASA
EDIT
/ GODDA RD SPACE FLIG H T CEN TER

his is the time of year when we
become keenly aware that Earth's
equator is inclined 23½° to the plane
deﬁned by its orbit around the Sun.
The solstice falls on June 21st at 4:24
Universal Time (12:24 a.m. Eastern
Daylight Time), and that's when the
northern pole of our planet's spin axis
is maximally tilted toward the Sun.
In addition to causing seasons, this
axial tilt (known as obliquity) allows
sunlight to fully illuminate the poles -
periodically even at midnight - so that
over a year, remote-sensing satellites can
image the entire surface of our planet.
The Moon, however, is tilted only
1½° with respect to the ecliptic, and
hence the Sun is always near or just
below the horizon as seen from its
poles. In fact, the Sun never rises above

Sky and Telescope - June 2017

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