Sky & Telescope - July 2019 - 24

The Moon: Just Look Up

The Moon increases its distance from the Sun about 13°
per day, delaying its rising time an average of about 50 minutes a night. After full phase, the Moon continues to rise later
and later after sunset until it reaches last-quarter phase, when
it rises around midnight and stands due south at sunrise.
Morning crescent phases follow until the terminator shaves
away our satellite to nothing, and a new Moon returns to
begin the cycle afresh. As the Moon waxes to full, the terminator deﬁnes the line of advancing sunrise; during the Moon's
waning phases it becomes the line of advancing sunset.
Along the way, the Moon passes near the bright planets in
eye-catching pairings called conjunctions. Conjunctions are a
natural consequence of the Moon and planets following similar paths through the zodiacal constellations. Novice astronomers can be grateful for the Moon's help in pointing out both
the planets and brighter stars as it makes its monthly rounds.

Naked-eye Sights
You might be surprised by how much you can see on the
Moon without optical aid. In addition to phases and conjunctions, treat yourself to an eyeful of earthshine, the soft, faint
glow that illuminates the shadowed portion of the Moon
when it's a crescent. Sunlight reﬂected from Earth to the
Moon and back is the source of this ghostly light.
WILD IMAGINATION The contrast of the Moon's dark maria
(seas) with light-colored highlands offers a chance for the creative mind to run wild. How many ﬁgures can you compose out
of the variegated lunar surface?

Many people can ﬁnd the Man in the Moon, a crude face
formed by the contrast between light-toned lunar highlands
and the darker seas or maria (MAH-ree-uh). But have you
ever seen the rabbit? How about the woman wearing a jeweled
pendant? Or the moose? Our natural tendency to see distinct
shapes in random patterns is known as pareidolia. These
shapes are fun to ﬁnd, but what they truly represent on the
Moon is far more fascinating.
The highlands comprise the original lunar crust that
cooled from a global magma ocean around 4.5 billion years
ago. Lighter materials rich in calcium and aluminum ﬂoated
to the top and solidiﬁed to create a white, rocky landscape.
Intense cratering from meteorite and asteroid bombardment
between 4 and 4.5 billion years ago pulverized the outer crust
into the lunar soil, or regolith, that now bears the boot prints
of twelve Apollo astronauts. (For more on this, see page 52.)
The seas, so called because pre-telescopic observers mistook them for bodies of water, are actually gigantic impact
basins blasted out by asteroid collisions roughly 4 billion
years ago. Between 3.2 and 3.9 billion years ago, the ﬁssures
that riddled their fractured ﬂoors bled magma to ﬁll these
gigantic holes and create vast, dark plains. Our ancestors had
the "sea" part correct, just not the liquid that ﬁlled them.
Pale minerals dominate in the crust, but the basaltic lavas
were rich in darker minerals containing iron, magnesium,
and titanium, which is why the plains appear dark in contrast
to less disturbed regions of the Moon's surface.
The lunar landscape is truly ancient. Although subsequent
impacts have churned the regolith and blotched the maria,
the Moon would appear much the same to the naked eye if

R A BBIT M OON: G REGORY H. RE V ER A / CC BY-SA 3.0;
A RISTA RCHUS PL ATE AU: N ASA / JSC / A RIZON A STATE UNIV ERSIT Y

q SOURCE OF THE TROUBLE This stunning image of the Aristarchus
plateau was captured with a 76-mm Fairchild mapping camera using
5-inch film by the Apollo 15 astronauts. The "Cobra head" volcanic vent
released a river of lava that flowed down the plateau and onto the lava
plains of Oceanus Procellarum. Aristarchus Crater is east (left) of the
vent, the shallower crater Herodotus west of the vent.

J U LY 2 0 1 9 * S K Y & T E L E S C O P E

Sky & Telescope - July 2019

Table of Contents for the Digital Edition of Sky & Telescope - July 2019