Sky & Telescope - July 2024 - 34

Loony
LUNAR FEATURES by Jerry Oltion; Illustrations by Cindy Krach
Go
A
mateur astronomers love to disparage the Moon. It's
nature's own glare-bomb, casting its spotlight glow
across the sky to drown out galaxies, nebulae, and any
hope of dark-adapting our eyes. We often put our scopes away
as the Moon reaches first quarter and leave them untouched
until last quarter, only observing during the dark phase.
That's a sad waste of photons, because the Moon offers
more bang for the buck than any other object in the sky, bar
none. In fact, if the Moon were the only thing we could see
with a telescope, amateur astronomy would still be the coolest
hobby in the world.
I'm something of a Moon nut, and by the end of this article
I hope you will be, too. There are so many neat things to
see! I can't cover a tenth of what's up there, but I'll describe
some of my favorite sights, illustrated with Cindy Krach's
amazing sketches.
Before we start, let's take a moment to talk about equipment.
For lunar observing, you don't need much. The
for Luna!
Look into the light for a change.
Moon is a big, bright rock hovering out there in space close
enough that you can see features with your naked eye. Even
a modest telescope, the kind we often call a " hobby killer "
(S&T: Dec. 2019, p. 36), will show you tons of detail on the
Moon. Binoculars are great, too. But the more scope you can
throw at it, the more details you can see.
If you have much aperture, you'll need one piece of vital
equipment: a filter to reduce the brightness. The Moon can
be uncomfortably bright to look at directly. It won't hurt you,
but it will be unpleasant, especially in a modestly large scope.
I recommend a variable-polarizing filter, which lets you dial
in the amount of dimming you want, because the Moon
changes brightness as it waxes and wanes.
Okay, now that we're equipped, what can we look for?
Chiaroscuro World
The Moon is tidally locked in its orbit around Earth, which
means the same side always faces us. There's a little wobbling
about - called libration - due to its inclined and somewhat
eccentric orbit, but even with the effects of libration
we only get to see 59% of the Moon's surface.
That's plenty! There are an estimated bazillion craters
visible on the 22.3 million square kilometers (8.6 million
square miles) of lunar terrain that we can see. Plus there are
volcanic features, stress fractures, fault lines, and lunarisms
(features that look like something familiar to the human
eye) galore. You won't run out of stuff to look at.
So where to start? It depends on the phase, but at any
time other than full or new Moon, look for the terminator,
the band of high-contrast features where light gives way to
darkness. That's where sunlight comes in at a shallow angle
and casts long shadows, highlighting differences in terrain.
Often you'll find craters along the terminator that are completely
filled with shadow, yet the rims and a central peak
will be brightly sunlit. Watch for a few hours during the
Moon's waxing phase and you can see sunlight work its way
down the central peak, eventually reaching the crater floor.
You might even see individual peaks along crater rims even
farther out into the dark side, standing like lone sentinels at
the edge of dawn.
The location of the terminator changes dramatically
RILLEY NEAT! Petavius sports a radial rille that's best viewed
when the terminator is near the crater.
34 JULY 2024 * SKY & TELESCOPE
from night to night. It takes the Moon 29.5 days to cycle
through all its phases (the synodic month), so the sunrise
terminator takes roughly two weeks to sweep across from
limb to limb, followed by the sunset terminator as the
Sky & Telescope - July 2024

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