Sky and Telescope - March 2016 - 52

OBSERVING

Exploring the Solar System

Spotting Super-Thin Moons
Learn the where and when to look for dramatically thin lunar crescents.
ary through April at dusk, and August through October
before dawn. (Of course, you can attempt a sighting at
other times, but doing so increases the difficulty.)
Here's an example of a successful hunt: On February 10, 2013, I spotted a very young Moon at 6:40 p.m.
Central Standard Time. At the time, the Sun's altitude
was -7° (sunset was 6:08 p.m.) and the Moon's was just
under 3°. The lunar disk was also riding 5° north of the
ecliptic, which gave a slightly better separation from the
Sun. Since new Moon had occurred at 7:20 UT (1:20 a.m.
CST), I saw and photographed the crescent when it was
just 17 hours 20 minutes past new.

Guidelines for Optimum Viewing

Armed with knowledge of where to look, the author captured a hairline-thin crescent Moon, just 17.3 hours past
new, on February 10, 2013.

Along with all the positive benefits we find in astronomy, observing challenges can add a certain spice to our
experiences. Messier Marathons and splitting difficult
double stars come to mind.
One of my particular favorites is locating extremely
thin crescent Moons. Maybe you've tried this yourself
but came up empty-handed. Or maybe you just didn't
figure out when and where to look. Wouldn't it be nice to
know just which criteria are useful to determine when
the effort might be worthwhile?
First, let's tackle the situational geometry. The Moon
moves eastward in the sky along its orbit by a distance
roughly equal to its own diameter (½°°) every hour. So
24 hours before or after new Moon, the lunar disk won't
appear more than about 15° from the Sun. Therefore,
you'll need the horizon to block the solar disk, and you
need the darkest sky possible to help with contrast.
Ideally, the lunar crescent should be positioned directly
above the Sun to provide the best possible elevation in
a twilit sky. Since the Moon is never more than 5° away
from the ecliptic, you'll want to pick times when this
line intersects the horizon most nearly vertically. With
this geometry in mind, the Northern Hemisphere offers
two "seasons" to hunt for thin lunar crescents: Febru52

March 2016 sky & telescope

If I can do it, you can too! For the purposes of this challenge, I've set the goal of spotting the lunar crescent 16
to 18 hours before or after new Moon. This is admittedly
ambitious, even for practiced observers and especially
if it's your first effort, but the guidelines presented here
should improve your chances greatly.
First, you'll need the dates and Universal Times of
new Moon. Here are this year's, as calculated by the
United States Naval Observatory: January 10, 1:30; February 8, 14:39; March 9, 1:54; April 7, 11:24; May 6, 19:29;
June 5, 3:00; July 4, 11:01; August 2, 20:44; September
1, 9:03; October 1, 0:11; October 30, 17:38; November 29,
12:18; and December 29, 6:53.
Next, compare the list above against the Universal
Times below that correspond to the optimum crescentviewing "window" for the four main U.S. time zones:
Eastern

Central

Mountain

Pacific

Before New (dawn)

2h -4h

3h -5h

4h -6h

5h -7h

After New (dusk)

4h -6h

5h -7h

6h -8h

7h -9h

As you can see, only June (predawn, Eastern and Central), September (dusk, Pacific), and December (dusk,
Central and Mountain) meet the criteria for viewing
a 16- to 18-hour Moon anywhere from the continental
U.S. But let's ease the requirements; the table below lists
optimum times for a 21- to 24-hour Moon:
Eastern

Central

Mountain

Pacific

Before New (dawn)

19h -22h

20 h -23h

21h -0 h

22h -1h

After New (dusk)

22h -1h

23h -2h

0 h -3h

1h -4h



Sky and Telescope - March 2016

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

Contents
Sky and Telescope - March 2016 - Cover1
Sky and Telescope - March 2016 - Cover2
Sky and Telescope - March 2016 - 1
Sky and Telescope - March 2016 - Contents
Sky and Telescope - March 2016 - 3
Sky and Telescope - March 2016 - A
Sky and Telescope - March 2016 - B
Sky and Telescope - March 2016 - 4
Sky and Telescope - March 2016 - 5
Sky and Telescope - March 2016 - 6
Sky and Telescope - March 2016 - 7
Sky and Telescope - March 2016 - 8
Sky and Telescope - March 2016 - 9
Sky and Telescope - March 2016 - 10
Sky and Telescope - March 2016 - 11
Sky and Telescope - March 2016 - 12
Sky and Telescope - March 2016 - 13
Sky and Telescope - March 2016 - 14
Sky and Telescope - March 2016 - 15
Sky and Telescope - March 2016 - 16
Sky and Telescope - March 2016 - 17
Sky and Telescope - March 2016 - 18
Sky and Telescope - March 2016 - 19
Sky and Telescope - March 2016 - 20
Sky and Telescope - March 2016 - 21
Sky and Telescope - March 2016 - 22
Sky and Telescope - March 2016 - 23
Sky and Telescope - March 2016 - 24
Sky and Telescope - March 2016 - 25
Sky and Telescope - March 2016 - 26
Sky and Telescope - March 2016 - 27
Sky and Telescope - March 2016 - 28
Sky and Telescope - March 2016 - 29
Sky and Telescope - March 2016 - 30
Sky and Telescope - March 2016 - 31
Sky and Telescope - March 2016 - 32
Sky and Telescope - March 2016 - 33
Sky and Telescope - March 2016 - 34
Sky and Telescope - March 2016 - 35
Sky and Telescope - March 2016 - 36
Sky and Telescope - March 2016 - 37
Sky and Telescope - March 2016 - 38
Sky and Telescope - March 2016 - 39
Sky and Telescope - March 2016 - 40
Sky and Telescope - March 2016 - 41
Sky and Telescope - March 2016 - 42
Sky and Telescope - March 2016 - 43
Sky and Telescope - March 2016 - 44
Sky and Telescope - March 2016 - 45
Sky and Telescope - March 2016 - 46
Sky and Telescope - March 2016 - 47
Sky and Telescope - March 2016 - 48
Sky and Telescope - March 2016 - 49
Sky and Telescope - March 2016 - 50
Sky and Telescope - March 2016 - 51
Sky and Telescope - March 2016 - 52
Sky and Telescope - March 2016 - 53
Sky and Telescope - March 2016 - 54
Sky and Telescope - March 2016 - 55
Sky and Telescope - March 2016 - 56
Sky and Telescope - March 2016 - 57
Sky and Telescope - March 2016 - 58
Sky and Telescope - March 2016 - 59
Sky and Telescope - March 2016 - 60
Sky and Telescope - March 2016 - 61
Sky and Telescope - March 2016 - 62
Sky and Telescope - March 2016 - 63
Sky and Telescope - March 2016 - 64
Sky and Telescope - March 2016 - 65
Sky and Telescope - March 2016 - 66
Sky and Telescope - March 2016 - 67
Sky and Telescope - March 2016 - 68
Sky and Telescope - March 2016 - 69
Sky and Telescope - March 2016 - 70
Sky and Telescope - March 2016 - 71
Sky and Telescope - March 2016 - 72
Sky and Telescope - March 2016 - 73
Sky and Telescope - March 2016 - 74
Sky and Telescope - March 2016 - 75
Sky and Telescope - March 2016 - 76
Sky and Telescope - March 2016 - 77
Sky and Telescope - March 2016 - 78
Sky and Telescope - March 2016 - 79
Sky and Telescope - March 2016 - 80
Sky and Telescope - March 2016 - 81
Sky and Telescope - March 2016 - 82
Sky and Telescope - March 2016 - 83
Sky and Telescope - March 2016 - 84
Sky and Telescope - March 2016 - Cover3
Sky and Telescope - March 2016 - Cover4
https://www.nxtbook.com/nxtbooks/aas/st_202012
https://www.nxtbook.com/nxtbooks/aas/st_202011
https://www.nxtbook.com/nxtbooks/aas/st_202010
https://www.nxtbook.com/nxtbooks/aas/st_202009
https://www.nxtbook.com/nxtbooks/aas/st_202008
https://www.nxtbook.com/nxtbooks/aas/st_202007
https://www.nxtbook.com/nxtbooks/aas/st_202006
https://www.nxtbook.com/nxtbooks/aas/st_202005
https://www.nxtbook.com/nxtbooks/aas/st_202004
https://www.nxtbook.com/nxtbooks/aas/st_202003
https://www.nxtbook.com/nxtbooks/aas/st_202002
https://www.nxtbook.com/nxtbooks/aas/st_202001
https://www.nxtbook.com/nxtbooks/aas/st_201912
https://www.nxtbook.com/nxtbooks/aas/st_201911
https://www.nxtbook.com/nxtbooks/aas/st_201910
https://www.nxtbook.com/nxtbooks/aas/st_201909
https://www.nxtbook.com/nxtbooks/aas/st_201908
https://www.nxtbook.com/nxtbooks/aas/st_201907
https://www.nxtbook.com/nxtbooks/aas/st_201906
https://www.nxtbook.com/nxtbooks/aas/st_201905
https://www.nxtbook.com/nxtbooks/aas/st_201904
https://www.nxtbook.com/nxtbooks/aas/st_201903
https://www.nxtbook.com/nxtbooks/aas/st_201902
https://www.nxtbook.com/nxtbooks/aas/st_201901
https://www.nxtbook.com/nxtbooks/aas/st_201812
https://www.nxtbook.com/nxtbooks/aas/st_201811
https://www.nxtbook.com/nxtbooks/aas/st_201810
https://www.nxtbook.com/nxtbooks/aas/st_201809
https://www.nxtbook.com/nxtbooks/aas/st_201808
https://www.nxtbook.com/nxtbooks/aas/st_201807
https://www.nxtbook.com/nxtbooks/aas/st_201806
https://www.nxtbook.com/nxtbooks/aas/st_201805
https://www.nxtbook.com/nxtbooks/aas/st_201804
https://www.nxtbook.com/nxtbooks/aas/st_201803
https://www.nxtbook.com/nxtbooks/aas/st_201802
https://www.nxtbook.com/nxtbooks/aas/st_201801
https://www.nxtbook.com/nxtbooks/aas/st_201712
https://www.nxtbook.com/nxtbooks/aas/st_201711
https://www.nxtbook.com/nxtbooks/aas/st_201710
https://www.nxtbook.com/nxtbooks/aas/st_201709
https://www.nxtbook.com/nxtbooks/aas/st_201708
https://www.nxtbook.com/nxtbooks/aas/st_201707
https://www.nxtbook.com/nxtbooks/aas/st_201706
https://www.nxtbook.com/nxtbooks/aas/st_201705
https://www.nxtbook.com/nxtbooks/aas/st_201704
https://www.nxtbook.com/nxtbooks/aas/st_201703
https://www.nxtbook.com/nxtbooks/aas/st_201702
https://www.nxtbook.com/nxtbooks/aas/st_201701
https://www.nxtbook.com/nxtbooks/aas/st_201612
https://www.nxtbook.com/nxtbooks/aas/st_201611
https://www.nxtbook.com/nxtbooks/newtrack/st_201612
https://www.nxtbook.com/nxtbooks/aas/st_201610
https://www.nxtbook.com/nxtbooks/aas/st_201609
https://www.nxtbook.com/nxtbooks/aas/st_201608
https://www.nxtbook.com/nxtbooks/aas/st_201607
https://www.nxtbook.com/nxtbooks/aas/st_201606
https://www.nxtbook.com/nxtbooks/aas/st_201605
https://www.nxtbook.com/nxtbooks/aas/st_201604
https://www.nxtbook.com/nxtbooks/aas/st_201603
https://www.nxtbook.com/nxtbooks/aas/st_201602
https://www.nxtbook.com/nxtbooks/aas/st_201601
https://www.nxtbook.com/nxtbooks/aas/st_201512
https://www.nxtbook.com/nxtbooks/aas/st_201511
https://www.nxtbook.com/nxtbooks/aas/st_201510
https://www.nxtbook.com/nxtbooks/aas/st_201509
https://www.nxtbook.com/nxtbooks/aas/st_201508
https://www.nxtbook.com/nxtbooks/aas/st_201507
https://www.nxtbook.com/nxtbooks/aas/st_201506
https://www.nxtbook.com/nxtbooks/aas/st_201505
https://www.nxtbook.com/nxtbooks/aas/st_201504
https://www.nxtbook.com/nxtbooks/aas/st_201503
https://www.nxtbook.com/nxtbooks/aas/st_201502
https://www.nxtbook.com/nxtbooks/aas/st_201501
https://www.nxtbook.com/nxtbooks/aas/st_201412
https://www.nxtbook.com/nxtbooks/aas/st_201411
https://www.nxtbook.com/nxtbooks/aas/st_201410
https://www.nxtbook.com/nxtbooks/aas/st_201409
https://www.nxtbook.com/nxtbooks/aas/st_201408
https://www.nxtbook.com/nxtbooks/aas/st_201407
https://www.nxtbook.com/nxtbooks/aas/st_201406
https://www.nxtbook.com/nxtbooks/aas/st_mars
https://www.nxtbook.com/nxtbooks/aas/st_201405
https://www.nxtbook.com/nxtbooks/aas/st_201404
https://www.nxtbook.com/nxtbooks/aas/st_201403
https://www.nxtbook.com/nxtbooks/aas/st_201402
https://www.nxtbook.com/nxtbooks/aas/st_201401
https://www.nxtbook.com/nxtbooks/aas/st_201312
https://www.nxtbook.com/nxtbooks/aas/st_201311
https://www.nxtbook.com/nxtbooks/aas/st_201310
https://www.nxtbook.com/nxtbooks/aas/st_201309
https://www.nxtbook.com/nxtbooks/aas/st_201308
https://www.nxtbook.com/nxtbooks/aas/st_201307
https://www.nxtbook.com/nxtbooks/aas/st_201306
https://www.nxtbook.com/nxtbooks/aas/st_201305
https://www.nxtbook.com/nxtbooks/aas/st_201304
https://www.nxtbook.com/nxtbooks/aas/st_201303
https://www.nxtbook.com/nxtbooks/aas/st_201302
https://www.nxtbook.com/nxtbooks/aas/st_201301
https://www.nxtbook.com/nxtbooks/aas/st_201212
https://www.nxtbook.com/nxtbooks/aas/st_201211
https://www.nxtbook.com/nxtbooks/aas/st_201210
https://www.nxtbook.com/nxtbooks/aas/st_201209
https://www.nxtbook.com/nxtbooks/aas/st_201208
https://www.nxtbook.com/nxtbooks/aas/st_201207
https://www.nxtbook.com/nxtbooks/aas/st_201206
https://www.nxtbook.com/nxtbooks/aas/st_201205
https://www.nxtbook.com/nxtbooks/aas/st_201204
https://www.nxtbook.com/nxtbooks/aas/st_201203
https://www.nxtbook.com/nxtbooks/aas/st_201202
https://www.nxtbook.com/nxtbooks/aas/st_201201
https://www.nxtbook.com/nxtbooks/aas/st_201112
https://www.nxtbook.com/nxtbooks/aas/st_201111
https://www.nxtbook.com/nxtbooks/aas/st_201110
https://www.nxtbook.com/nxtbooks/aas/st_201109
https://www.nxtbook.com/nxtbooks/aas/st_201108
https://www.nxtbook.com/nxtbooks/aas/st_201107
https://www.nxtbook.com/nxtbooks/aas/st_201106
https://www.nxtbook.com/nxtbooks/aas/st_201105
https://www.nxtbook.com/nxtbooks/aas/st_201104
https://www.nxtbook.com/nxtbooks/aas/st_201103
https://www.nxtbook.com/nxtbooks/aas/st_201102
https://www.nxtbook.com/nxtbooks/aas/st_201101
https://www.nxtbook.com/nxtbooks/aas/st_201012
https://www.nxtbook.com/nxtbooks/aas/st_201011
https://www.nxtbook.com/nxtbooks/aas/st_201010
https://www.nxtbook.com/nxtbooks/aas/st_201009
https://www.nxtbook.com/nxtbooks/aas/st_201008
https://www.nxtbook.com/nxtbooks/aas/st_201007
https://www.nxtbook.com/nxtbooks/aas/st_201006
https://www.nxtbook.com/nxtbooks/aas/st_201005
https://www.nxtbook.com/nxtbooks/aas/st_201004
https://www.nxtbook.com/nxtbooks/aas/st_201003
https://www.nxtbook.com/nxtbooks/aas/st_201002
https://www.nxtbook.com/nxtbooks/aas/st_201001
https://www.nxtbookmedia.com