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OBSERVING BASICS
Mario Motta
really hates
the Moon. In
fact, he would nuke it
into oblivion if he had
the means. You see,
Mario is an accom-
plished astroimager
whose work appears in
the second edition of
Stephen O’Meara’s
Deep-Sky Companions:
The Messier Catalog.
And as anyone who
images clusters, nebu-
lae, and galaxies knows,
a bright Moon obscures
these cosmic delights.
But even the most
vocal Moon-hater has to acknowledge the jaw-dropping
visual impact of the Moon when viewed through a
telescope. Nothing in the evening sky packs a “Wow!”
punch like the sight of its rugged, cratered surface. No
cluster. No nebula. No galaxy. Saturn, with
its fabled rings, is a distant second.
The Moon is resplendent with lofty moun-
tain ranges, sinuous ridges and rilles, and
hundreds of craters. And, to sample a variety
of lunar features, you can do no better than to
explore the expanse that runs along the north-
ern border of Mare Imbrium (Sea of Rains)
from the Montes Alpes (Lunar Alps) west-
ward to the Montes Jura (Jura Mountains).
The Lunar Alps come into view around
First Quarter. Located at the northeast edge
of the Sea of Rains, this range is about
155 miles (250 kilometers) long. Most of its
peaks reach elevations between 6,000 and
8,000 feet (1,800 to 2,400 meters). They arose from the
impact that created the Sea of Rains some 4 billion years
ago. Bisecting the Lunar Alps is the Vallis Alpes (Alpine
Valley), a gash a little over 100 miles (160 km) long and
5 to 6 miles (8 to 9.6 km) wide. Once thought to be the
re su lt of a n obl ique st r i ke by a n objec t t h row n out by t he
Sea of Rains impact, it’s now thought to be a graben — a
segment of the lunar crust thrust downward by the event.
Near the southern end of the Montes Alps and juttingout of the grayish Sea of Rains is the bright peak Mons
Piton. When near the terminator, Piton casts a long,
pointed shadow, giving the impression that this mountain
is a tall, needle-shaped structure. In reality, Piton rises
just 1.4 miles (2.2 km) above its 16-mile-wide (25 km)
base. The lofty spire is actually a gently rolling hill!
By the evening following First Quarter, the lunar
terminator (the boundary between day and night on the
Moon) has drifted westward to reveal a dark oval patch
encircled by a bright rim. This is the 63-mile-wide
(101 km) crater Plato, which appears dark because its
f loor is covered by low-ref lectivity lava rocks. A popular
observing challenge for lunar observers is to search for
the tiny craterlets that dot the crater’s floor. If seeing
conditions are favorable, a 6-inch scope and a magni-
f ic at ion of 175x or more shou ld re ve a l a few. How ma ny
can you spot?
Directly south of Plato and rising out of the Sea of
Rains is the 8,000-foot-high (2,400 m) Mons Pico. Like
Piton, Pico casts a sharp and lengthy shadow when near
the terminator, belying its relatively f lattened shape.
Another evening will reveal more mountains that
emerge from the Sea of Rains. Northwest of Pico is a
70-mile-long (113 km) jumble of peaks known as the
Montes Teneriffe (Teneriffe Mountains), whose tallest
members reach a height of 8,000 feet (2,400 m). Further
west is a mountain chain that’s shorter (56 miles
[90 km]) and less lofty (average height 6,000 feet
[1,800 m]) but far more eye-catching. Appearing more
like a massive, artificially created wall than a natural
lunar feature, this line of mountains is appropriately
named the Montes Recti (Straight Range).
Three nights after First Quarter, the ter-
minator has moved westward enough to
reveal the entire north shore of the Sea of
Rains and our pièce de résistance, the mag-
nificent Sinus Iridum (Bay of Rainbows).
The “bay” is actually the remnant of a crater
160 miles (258 km) across. Its northern rim
forms the craggy Jura Mountains. The
remaining rim was erased by the lava f low
that created the Sea of Rains. The Bay of
Rainbows is f lanked by the two mountain-
ous capes of Promontorium Laplace and
Promontorium Heraclides.
So, my deep-sky-imaging friends, stop
cursing the Moon. Set up a small telescope in the back-
yard and spend some time exploring this amazing
world.
Questions, comments, or suggestions? Email me at
[email protected]. Next time: Here comes the Sun!
Clear skies!Our natural satellite has many treasures to observe.
Appreciating
the Moon
The lunar disk is
shown in crisp detail
in this mosaic of an
11-day-old-gibbous
Moon imaged on
March 17, 2019. ALAN DYERNothing in
the evening
sky packs a
“Wow!”
punch like
the sight of
its rugged,
cratered
surface.
BY GLENN CHAPLE
Glenn has been an
avid observer since
a friend showed
him Saturn through
a small backyard
scope in 1963.BROWSE THE “OBSERVING BASICS” ARCHIVE
AT http://www.Astronomy.com/Chaple