Sky & Telescope - July 2024 - 38

Lunar Features
Nectaris (the Sea of Nectar). Its northern wall was wiped out
by the advancing lava, leaving a horseshoe-shape remnant.
First Quarter and Beyond
At first quarter, when the Moon is half-lit and half-dark, we
get a bonanza of great features. The most obvious is probably
another crater trio: Ptolemaeus, Alphonsus, and Arzachel.
They're different sizes and have vastly different histories.
Ptolemaeus, the largest one, is fully flooded with lava,
leaving no central peak at all. Alphonsus, in the middle, is
partially flooded but still has a central peak, while Arzachel,
the southernmost of the three, is a classic large impact crater
with slumped walls and a ragged central peak.
There's a smaller feature visible at first quarter that is quite
possibly my favorite feature on the Moon. It's the lunarism I
call Elvis, the lunar guitarist. It's a trick of the shadow that
pools into a V-shaped pair of ridges next to the crater Ukert.
When the light is just right - and it's only right for a few
hours every other lunar cycle - the shadow looks like the
rock 'n' roll guitarist with his feet wide apart and guitar held
straight out. You can even see the crook of his arm around
the body of the guitar.
While you're in the neighborhood, check out Rima Hyginus,
a volcanic caldera with collapsed lava tubes radiating
outward to the southeast and northwest.
At the same time when Elvis is visible, there's another
lunarism called the Purbach Cross (or the Lunar X) that
appears down in the southern highlands, a large swath of
ancient, rugged terrain blanketed with craters. The X is formed
by the overlapping walls of Purbach and three or four other
craters and is only visible for a few hours. When sunlight first
touches the crater rims, you see a luminous X surrounded by
darkness on all sides, but a few hours later encroaching light
reveals the lower parts of the craters and the X fades.
A day after first quarter, look for Plato, a distinct, flatbottomed
crater at the northern edge of Mare Imbrium.
Plato has been completely flooded with lava, and its floor
is as smooth as a baby's bottom . . . until you crank up the
power and look for craterlets. Plato has several distinct
craters on its flat floor, including four that range from 2.0
to 2.4 km across (1.2 to 1.5 miles), which makes them just
barely visible with a medium-size telescope on a night of
good seeing. When the light angle is low and Plato's floor
is lit but the craters are filled with shadow, it's fun to see if
you can spot them.
At about the same time, in the southern highlands, look
for Clavius, a crater that holds an interesting mystery.
Within Clavius lies a perfect arc of smaller craters, diminishing
in size as you work your way counterclockwise around
the arc. There are six craters in the arc (five of them obvious),
and they're so perfectly spaced that there's no doubt
in my mind that they're related. It seems clear to me that
they must have all happened at the same time as an asteroid
approached the Moon and was torn apart by tidal forces, the
asteroid's rotation flinging the smaller pieces outward farther
than the larger pieces. That's the only explanation that
fits, except craterlet counts within the individual craters
indicate that they aren't all the same age. They're apparently
randomly timed and not-so-randomly placed. No, I can't
believe that either.
Near the time that Plato and Clavius are looking good, so
is the Straight Wall to the north in Mare Nubium (the Sea
of Clouds). It's actually a long fault scarp, but at low light
angles it draws a straight line of shadow that stands out easily
against all those circular craters elsewhere.
A day or two later, look for the " Golden Handle " of Sinus
Iridum (the Bay of Rainbows) that sticks out northwest of
Mare Imbrium (the Sea of Rains). When the light is just
right, Iridum's rim is sunlit but the floor is still in darkness,
making this huge feature stand out like the handle on an
enormous celestial coffee cup.
ICE CREAM SPLAT Copernicus looks rugged when near the
terminator, but when fully sunlit it looks like nothing so much as
a scoop of ice cream that landed with a splat on hot pavement.
38 JULY 2024 * SKY & TELESCOPE
Fattening to Full
Later on in the waxing gibbous phase, look for Copernicus,
a beautiful crater in the Oceanus Procellarum (the Ocean
Sky & Telescope - July 2024

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