Astronomy

16 ASTRONOMY • APRIL 2018

W

hat is the

thinnest cres-

cent Moon

visible to the

unaided eye

during daylight? A good ques-

tion, to which I do not have an

answer, though I do have an

observation.

Seeking the thinnest crescent

Moon during evening or morn-

ing twilight without optical aid

has long been a fun and chal-

lenging pursuit among amateur

astronomers. As early as 1921,

Popular Astronomy asked ama-

teur astronomers to “take up

the matter, in order to deter-

mine how frequently it may be

detected when less than 24

hours.” But I have not encoun-

tered any studies on how thin

of a crescent the eye can detect

with the Sun in the sky.

Studies have shown that

whenever the Moon lies 7.5° or

less from the Sun, the lunar

crescent is invisible, either with

the naked eye, through binocu-

lars, or through telescopes. So

at least we have a starting point.

Out on a whim

The afternoon of November 16,

2017, was extremely clear.

Knowing that New Moon was

near, I grabbed 8x40 binocu-

lars and set out to search for

the thin lunar crescent with

the Sun 40° high in the sky.

I first sighted it through the

binoculars at 3:19 p.m. local

Botswana time (13h19m UT),

then with my unaided eyes

after lining the crescent with

the edge of a tree.

At first, the naked-eye view

was that of a hyperfine, f lat

SECRETSKY

See the daytime

lunar crescent

Even when it’s thin, you can spot our nearest

celestial neighbor while the Sun’s still out.

ellipse. But with attention and

time, I could make out the

whisker-thin crescent; I could

not have picked it out casually

with the eye alone.

I made my last sighting

(before the Moon slipped

behind trees) 13 minutes later

when the crescent was 22.5°

from the Sun, 18° above the

horizon, and 46.1 hours before

New Moon. Fortunately, I was

able to snap an image of it.

Eager to best that sighting, I

tried again the following morn-

ing, when the crescent was only

14° from the Sun. I could not

detect it with my eyes or

through binoculars, even

though I had Venus tack-sharp

in the binocular field only 4°

away from it. I did not try to

spot it through a telescope.

Day vs. dawn

and dusk

Like I said, this was not a

planned observation but a

whimsical one that turned into

something fascinating. The sky

was exceedingly clear (free of

haze), which helped with the

sighting, as did the altitude of

Maun, Botswana, which is at

3,041 feet (927 meters) above

sea level. But can there be bet-

ter sighting conditions for the

lunar crescent?

For twilight observations,

your chances of seeing a thin

lunar crescent are best when

the Moon is at perigee (closest

to Earth) and near its greatest

ecliptic latitude, which is about

plus or minus 5.5°. For my

November 16 sighting, the

Moon stood (serendipitously)

4.8° north of the ecliptic. But it

wasn’t at perigee; it was five

days from apogee (farthest

from Earth). Now I wonder, for

daytime observations, is look-

ing near apogee more benefi-

cial than at perigee because the

Moon appears smaller and its

light more condensed? It’s

worth investigating.

Something else that needs

investigating is how the Moon’s

altitude affects the observation.

Looking for a thin daytime

crescent may be better when

the Sun is hugging the horizon,

but such a time may also hin-

der observations because we

are looking through a denser

atmosphere with more haze

and airborne pollutants, which

would diminish the view. My

observation occurred when the

Moon was relatively low, only

about one-fifth the way from

the horizon to the zenith.

Would it be easier to see when

highest in the sky?

As with all daytime observa-

tions, it’s important to have

your eyes focused at infinity.

I’d love to hear about your

own observations. Send reports

to [email protected].

BY STEPHEN JAMES O’MEARA

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

The author captured this image of the waning crescent Moon on November 16, 2017,

at 13h32m UT, when our satellite was 46.1 hours from New Moon. STEPHEN JAMES O’MEARA

Longtime contributing imager Chris Schur imaged the Moon on May 26, 2017, with

the Sun already halfway up in the east. The Moon stood below the Sun 15° to the east,

and its age was only 19.2 hours. Schur reported that he saw the crescent through a

telescope, though it was difficult to spot. He could see only a portion of the crescent

at one time using a low-power eyepiece. CHRIS SCHUR

Stephen James O’Meara

is a globe-trotting observer

who is always looking for the

next great celestial event.

Studies have shown that whenever

the Moon lies 7.5° or less from the Sun,

the lunar crescent is invisible.