Astronomy

62 ASTRONOMY • DECEMBER 2018

he Moon, the

brightest object in

the night sky, has a

naked-eye secret: its

limb. The illumi-

nated edge of the Moon’s vis-

ible surface has a remarkable

nature that’s easy to overlook.

What you experience, however,

depends on how carefully you

examine it.

Note: When I write, “illumi-

nated edge,” I’m not talking

about the bright side of the

terminator (the line dividing

day and night, where shadows

highlight the view). I’m refer-

ring to the rounded bright edge

of the Moon that’s not the ter-

minator. At Full Moon, the

illuminated edge goes all the

way around.

Padraig Houlahan of

Flagstaff, Arizona, recently

noticed one peculiarity:

“Regularly when I look at the

Full Moon, I see an effect that

I have not seen documented

anywhere, and I was wonder-

ing if you knew of it.”

Namely, Houlahan noticed

that the edge of the Full Moon

appears “disproportionately

washed out,” meaning it looks

“featureless and brighter” than

the rest of the disk. Houlahan

doubted the phenomenon is

due to contrast effects alone

— and he’s right.

It’s a wonder

Houlahan’s observation of

lunar limb brightening is

worthy of praise because little

naked-eye attention is given

to it during each lunar month,

and some books on the Moon

overlook it altogether. On top

of all that, what one sees along

the limb is not consistent.

If the Moon were a uniform

(smooth) sphere, its edge would

ref lect no sunlight toward

Earth, making the limb appear

dark to us. But the Moon has

great surface irregularities —

rocks, mountains, and other

topographical features — that

effectively scatter sunlight

toward our eyes at optical

wavelengths. (The lunar limb is

dark at radio wavelengths.)

Speaking generally, then, we

say that during the waxing and

waning phases, the Moon’s

circular edge has a bright limb

facing the Sun and a dark limb

shadowed from it.

A casual glance at the illu-

minated edge will show it

brighter than areas closer to

the Moon’s terminator. But if

you take time to study the

limb — running your gaze

along the edge and making

SECRETSKY

Lunar limb magic Explore often-overlooked features of the Moon’s edge.

mental notes — you will see

that the limb is irregularly

bright with darker segments.

What you see at any given

time depends on many factors,

including phase angle effects,

variations in the scattering of

sunlight, and lunar librations

— slight axial nods (in latitude)

and swivels (in longitude) that

swing features gently in and

out of view along the Moon’s

limb as our neighbor in space

orbits Earth.

It’s best to look for the limb

effects in twilight, when the

contrast between the Moon’s

edge and the sky is lowest. I

suggest starting with a young

waxing crescent because there

often are great variations along

that slender arc of illumination.

First compare the general

appearance of the limb to fea-

tures closer to the terminator.

Then go in for the detailed

study using direct vision.

Use the extra magnification

of binoculars if you need to

confirm any uncertain naked-

eye views. Sketch and record

what you see each night, and

compare these views during

another lunation, when the

librations are different.

Lunar highlands will create

the brightest effects, but even

these regions vary in surface

irregularities and extent. Look

also for contrast effects within

the highlands, especially

between craters of high reflec-

tivity (caused by blankets of

ejecta surrounding them) near

the limb.

And, as always, let me know

what you see or don’t see at

[email protected].

BY STEPHEN JAMES O’MEARA

T

BROWSE THE “SECRET SKY” ARCHIVE AT http://www.Astronomy.com/OMeara.

The lunar limb is bright, but with a watchful eye, a skilled

observer can make out subtle (and not so subtle) differences

in its intensity. BOTH IMAGES: STEPHEN JAMES O’MEARA

Darkening the image to the left using software like Photoshop

reveals the lunar limb’s intensity variations more clearly.

Stephen James O’Meara

is a globe-trotting observer

who is always looking for the

next great celestial event.

It’s best to look for the limb effects in twilight,

when the contrast between the Moon’s edge

and the sky is lowest.