Astronomy - USA (2020-06)

14 ASTRONOMY • JUNE 2020

STRANGE UNIVERSE

The overall theme of this page is the odd or

unusual. So, the sky’s No. 1 most-observed

target shouldn’t qualify. But let’s give it a shot

anyway and focus on the Moon, just this once.

For non-astronomers, the lunar phase that

first comes to mind is “Full Moon,” with

“crescent” serving as runner-up. Cartoonists

portraying a night sky always include one of

those two. Ask friends to name a lunar phase

and no one will say “First Quarter” or “Last

Quarter.” Many don’t even know those are

the names of the half Moons — or appreciate

the oddity that, in our little corner of science,

“half ” and “quarter” mean the same thing.

So, to explore the public’s least-familiar

lunar phase, we’ll choose a half Moon. But

which one is more neglected — First Quarter

or Last Quarter? It’s actually an easy choice,

since we only ever observe one of them.

That’s because the First Quarter Moon, illuminated

on its right side for North Americans and Europeans,

stands highest at nightfall. It’s so conveniently placed

we observe it monthly. We can’t not see it. But now

consider the Last or Third Quarter Moon, the one lit up

on its left side. This is our strange, orphaned, unob-

served Moon. The Third Quarter doesn’t even rise until

midnight or later, and can’t lift itself out of blurry hori-

zon air until the wee hours. It’s highest at dawn, but who

drags their telescope out at 5 A.M.?

I won’t pretend to be special. I only started observing

this phase last year when I finally had an observatory

near my home. But then — wow!

Observers know that only around a half Moon phase

can they see optimally illuminated craters near the

middle of the Moon rather than those closer to the edge,

which are foreshortened and have a faraway appear-

ance. So, people who enjoy up-close lunar features —

and who doesn’t? — drool on their eyepieces when

treated to the rugged front-and-center mountain chain,

the magnificent lunar Apennines, whose western end

abruptly stops at the gorgeous crater Eratosthenes.

Adjacent to it, a few days after First Quarter, the termi-

nator shadow uncovers Copernicus, possibly the

Moon’s most handsome feature.

But at Third Quarter, sunlight comes from the oppo-

site direction, making the familiar lunar features

weirdly alien. The edge of the Apennines, now hit with

direct sunlight, looks oddly white and harsh.

Copernicus, invisible at the First Quarter phase, is now

intriguingly illuminated. The same is true in the rugged

southern highlands, where we optimally see the famous

crater Clavius, featured in the film 2001: A Space

Odyssey. Clavius’ f loor has six evenly spaced craterlets,

each slightly smaller than the preceding one. Together

they form a remarkable French curve, an arc so perfect

it seems unnatural, almost a lunar Mount Rushmore.

But the Last Quarter’s visceral eeriness mostly stems

from its reversed lighting, added to its unique position

in the sky. It is the only lunar phase that f loats exactly

in the direction toward which Earth is

traveling. When you look up at the Last

Quarter Moon, you see the place that we

ourselves will occupy three and a half hours

later. If this reconnaissance scout appears

tranquil and unruff led, we know it’s safe to

keep going.

Even to the naked eye, the Last Quarter

vividly pops out as it hovers against the dark-

est, most polarized part of the blue sky,

smack on the meridian at dawn.

Accompanied by birdsong, it’s a scene woven

of earthly and celestial threads, a tapestry of

timeless design that f loats balanced and

motionless — until life snaps out of its crepuscular

reverie to resume its customary frenzy.

In this calm 5 A.M. air you are almost certainly the

only one in your community looking up at the sky. This

is the Moon that is yours alone.

There’s a stunning lunar phase you’re likely missing.

Strange Moon rising

The Third Quarter

Moon, which is half

illuminated, throws

the craters and

mountains along the

terminator into stark

relief. NASA’S SCIENTIFIC

VISUALIZATION STUDIO

Consider

the Third

Quarter

Moon. This

is our

strange,

orphaned,

unobserved

Moon.

BY BOB BERMAN

Join me and Pulse

of the Planet’s

Jim Metzner

in my podcast,

Astounding Universe,

at http://www.astounding

universe.com

BROWSE THE “STRANGE UNIVERSE” ARCHIVE

AT http://www.Astronomy.com/Berman