Sky and Telescope - March 2016 - 53

Tom Fleming is a veteran observer who has coordinated an
all-volunteer group of sunspot counters for 12 years.

Tom Fleming

Broadening the observing window in this way yields
several more opportunities during 2016 to hone your
crescent-hunting skills. I'm not talking about setting
a new record for sighting the thinnest possible Moon
(S&T: Dec. 1996, p. 104), but rather pushing yourself in a
way that will be immediately rewarding.
For example, the Moon is new at 1:54 UT on March
9th (coinciding with a total solar eclipse; see page 50).
This timing ﬁts the window for three of the four listed
time zones. From my location in Texas, the Sun slips
below the horizon at 6:31 p.m. on March 9th, and 20
minutes later it's at an altitude of -5°. The Sun's azimuth
at sunset is 265°. At 6:51 p.m. the Moon's azimuth is the
same and its altitude is 7° - making it an easy target.
The Moon's age in this example is 22 hours 57 minutes.
Because these events are speciﬁc to your location,
and particularly to where you are within your time zone,
you'll want to preview the circumstances using Stellarium
or some other sky-simulating program. This will give you
the position of the Sun and Moon along the horizon.
Use the program to note the Sun's azimuth at sunset,
and then advance the time until the solar disk is about
4½° to 5° below the horizon. This provides a practical
starting point to begin the search under normal atmospheric conditions. Next, note the position of the Moon
relative to the sunset point as well as its altitude above
the horizon. Then, once outside, use binoculars to place
that horizon azimuth at the bottom of the ﬁeld of view.

For most circumstances, you should be able to spot the
crescent Moon within the ﬁeld. After spotting it in binoculars, you can then try using just your eyes.
Several variables can aﬀect the outcome. Not surprisingly, atmospheric conditions play a huge part, especially the amount of water vapor contained in the air.
I recall seeing an 18-hour-old Moon on January 9, 1978.
A strong cold front had come through, leaving so little
water vapor in its wake that I could pick up Boeing 727s
cruising 150 miles away while scanning within 2° of the
horizon with my 8-inch Newtonian.
Gaining altitude on a mountainside or hill can give
you a "negative" (geometrically depressed) horizon
and more time to ﬁnd the Moon. Thinner air at higher
altitudes is also an advantage. Cirrus cloud layers as far
as 200 miles away can surprise you when they reveal
themselves as the Sun sets.
Also, the ecliptic tips farther from vertical as your
latitude increases, which narrows the rise or set interval
between the Sun and Moon. If you try out a new location
with a favorable horizon, try previewing the site 24 hours
ahead of time to gauge its suitability.
There is a real thrill in seeing an extremely thin lunar
crescent. Once acquired, it seems to jump out at you, and
you wonder how you'd missed it a few minutes prior to
discovery. And there's the added beneﬁt of developing
observing skills that you can apply to other observing
endeavors. It's an experience you don't want to miss. ✦

The Moon * March 2016
Ecliptic
(seasonal
path of Sun)

Phases

Distances
Perigee
March 10, 7h UT
223,389 miles diam. 33′ 14″

March 1, 23:11 UT

Vertical

Moon

Vertical

LAST QUARTER
NEW MOON

Apogee
March 25, 14h UT
252,354 miles diam. 29′ 25″

March 9, 1:54 UT
FIRST QUARTER

March 15, 17:03 UT
FULL MOON

March 23, 12:01 UT

S&T ILLUSTRATION

LAST QUARTER

South

Sun
30 minutes
after sunset
North

Looking West in March

March 31, 15:17 UT

Mare Marginis
18

North

Mar 3
29

March 3

March 18

Hausen (crater) March 29
For key dates, yellow dots
indicate which part of the
Moon's limb is tipped the most
toward Earth by libration under
favorable illumination.

Looking West in September

Due to the tilt of the ecliptic at the horizon, very thin crescent Moons are easiest
to see after sunset in March and most diﬃcult in September. The reverse is true
before dawn. Note: the Moon can be situated up to 5° above or below the ecliptic.

Pingré (crater)

Cusanus (crater) March 12

12
South

Favorable Librations

NASA / LRO

Sk yandTelescope.com March 2016

http://www.SkyandTelescope.com

Sky and Telescope - March 2016

Table of Contents for the Digital Edition of Sky and Telescope - March 2016