Sky & Telescope - March 2021 - 52

MARCH 2021 OBSERVING
Exploring the Solar System by Thomas Dobbins

Seeing for Planetary
Observers
Knowing how Earth's atmosphere functions can improve
your chances of experiencing good planetary views.

W

e live at the bottom of a turbulent ocean of air that is both the
controlling and largely uncontrollable
factor in determining the clarity of telescopic images. Unevenly heated from
place to place, the atmosphere is filled
with currents and eddies.
Warm, rarefied air refracts light less
than cool, dense air. Astronomical " seeing " is caused by packets of thermally
non-homogeneous air passing between
the telescope and the object under
scrutiny. These atmospheric " cells "
typically range in size from 10 to 20
centimeters (4 to 8 inches) in diameter
at most locations.
Each moving air cell acts like a weak
lens, bending incoming rays of light.
This has two distinct effects on a telescopic image, although they invariably
occur in combination to some degree.
Oscillation or " image excursion " displaces the image laterally, causing it to
move randomly around a mean position

52

MARCH 2 021 * SK Y & TELESCOPE

in the field of view. Poor seeing also
creates a defocusing effect, moving the
image in and out of the telescope's focal
plane, giving it an undulating appearance that's intermittently sharp.
The larger a telescope's aperture, the
lower the probability that the air mass
over it will be optically homogeneous.
When the aperture is sufficiently large
to receive light that has passed through a
multitude of air cells, a blurred, washedout image usually results. It's not
uncommon for instruments of modest
aperture to perform to their theoretical
limits, but at most locations large telescopes rarely do so (S&T: Nov. 2018, p.
52). On most nights, only a patient vigil
at the eyepiece allows the observer to
enjoy those fleeting moments of supreme
clarity that Percival Lowell called " revelation peeps. "
Many novices imagine that crystalclear winter nights with stars twinkling
like flickering candles are best for tele-

scopic observing. Such nights invariably
disappoint with fuzzy, swollen images
that seldom sharpen even momentarily,
caused by turbulence in the jet stream
at altitudes of roughly 9,000 to 18,000
meters. In fact, good seeing is often
accompanied by the presence of haze,
a thin deck of high cirrus clouds, or a
temperature inversion. These all retard
ground cooling and the turbulent convection it produces. Humid nights when
the telescope drips with dew are usually
excellent. Mist and fog are harbingers of
superb views of the planets if the transparency doesn't suffer too much.
Virtually every location has its share
of tranquil nights, but some areas have
them more than others. On a global
scale, the tropics and subtropics favor
good seeing due to the more uniform
atmospheric pressure at low latitudes.
The high apparent elevation above
the horizon of the Moon and planets
from low-latitude vantage points is also
a considerable advantage. The thickest
part of the Earth's atmosphere extends
to a height of about 16 kilometers. The
best seeing is found near the zenith,
where the overlying blanket of air is
thinnest and convective disturbances
arising from the ground are minimized.
The light from a celestial object 45°
above the horizon passes through half
again as much air as an object overhead,
while the light from an object near the
horizon must traverse a thickness of
160 kilometers or greater.
At temperate latitudes, the best seeing conditions generally occur from late
spring through early autumn. Winter
nights are frequently plagued by " fast "
seeing when the jet stream is overhead.
Isaac Newton suggested that mountainous regions would enjoy " serene and
quiet air " because there would be less
overlying atmosphere to degrade images.
High elevation alone is no panacea,
however, because wind blowing over

SE A N WA LK ER / S&T

W Atmospheric turbulence can vary greatly from
night to night, or even hour by hour. This pair of
of Mars images was recorded using the same
telescope and camera on November 7th and
8th, 2020, as a high-pressure system settled
over the northeastern United States, significantly
improving telescopic views on the second night.



Sky & Telescope - March 2021

Table of Contents for the Digital Edition of Sky & Telescope - March 2021

Contents
Sky & Telescope - March 2021 - Cover1
Sky & Telescope - March 2021 - Cover2
Sky & Telescope - March 2021 - 1
Sky & Telescope - March 2021 - Contents
Sky & Telescope - March 2021 - 3
Sky & Telescope - March 2021 - 4
Sky & Telescope - March 2021 - 5
Sky & Telescope - March 2021 - 6
Sky & Telescope - March 2021 - 7
Sky & Telescope - March 2021 - 8
Sky & Telescope - March 2021 - 9
Sky & Telescope - March 2021 - 10
Sky & Telescope - March 2021 - 11
Sky & Telescope - March 2021 - 12
Sky & Telescope - March 2021 - 13
Sky & Telescope - March 2021 - 14
Sky & Telescope - March 2021 - 15
Sky & Telescope - March 2021 - 16
Sky & Telescope - March 2021 - 17
Sky & Telescope - March 2021 - 18
Sky & Telescope - March 2021 - 19
Sky & Telescope - March 2021 - 20
Sky & Telescope - March 2021 - 21
Sky & Telescope - March 2021 - 22
Sky & Telescope - March 2021 - 23
Sky & Telescope - March 2021 - 24
Sky & Telescope - March 2021 - 25
Sky & Telescope - March 2021 - 26
Sky & Telescope - March 2021 - 27
Sky & Telescope - March 2021 - 28
Sky & Telescope - March 2021 - 29
Sky & Telescope - March 2021 - 30
Sky & Telescope - March 2021 - 31
Sky & Telescope - March 2021 - 32
Sky & Telescope - March 2021 - 33
Sky & Telescope - March 2021 - 34
Sky & Telescope - March 2021 - 35
Sky & Telescope - March 2021 - 36
Sky & Telescope - March 2021 - 37
Sky & Telescope - March 2021 - 38
Sky & Telescope - March 2021 - 39
Sky & Telescope - March 2021 - 40
Sky & Telescope - March 2021 - 41
Sky & Telescope - March 2021 - 42
Sky & Telescope - March 2021 - 43
Sky & Telescope - March 2021 - 44
Sky & Telescope - March 2021 - 45
Sky & Telescope - March 2021 - 46
Sky & Telescope - March 2021 - 47
Sky & Telescope - March 2021 - 48
Sky & Telescope - March 2021 - 49
Sky & Telescope - March 2021 - 50
Sky & Telescope - March 2021 - 51
Sky & Telescope - March 2021 - 52
Sky & Telescope - March 2021 - 53
Sky & Telescope - March 2021 - 54
Sky & Telescope - March 2021 - 55
Sky & Telescope - March 2021 - 56
Sky & Telescope - March 2021 - 57
Sky & Telescope - March 2021 - 58
Sky & Telescope - March 2021 - 59
Sky & Telescope - March 2021 - 60
Sky & Telescope - March 2021 - 61
Sky & Telescope - March 2021 - 62
Sky & Telescope - March 2021 - 63
Sky & Telescope - March 2021 - 64
Sky & Telescope - March 2021 - 65
Sky & Telescope - March 2021 - 66
Sky & Telescope - March 2021 - 67
Sky & Telescope - March 2021 - 68
Sky & Telescope - March 2021 - 69
Sky & Telescope - March 2021 - 70
Sky & Telescope - March 2021 - 71
Sky & Telescope - March 2021 - 72
Sky & Telescope - March 2021 - 73
Sky & Telescope - March 2021 - 74
Sky & Telescope - March 2021 - 75
Sky & Telescope - March 2021 - 76
Sky & Telescope - March 2021 - 77
Sky & Telescope - March 2021 - 78
Sky & Telescope - March 2021 - 79
Sky & Telescope - March 2021 - 80
Sky & Telescope - March 2021 - 81
Sky & Telescope - March 2021 - 82
Sky & Telescope - March 2021 - 83
Sky & Telescope - March 2021 - 84
Sky & Telescope - March 2021 - Cover3
Sky & Telescope - March 2021 - Cover4
https://www.nxtbook.com/nxtbooks/aas/st_202405
https://www.nxtbook.com/nxtbooks/aas/st_202404_qr
https://www.nxtbook.com/nxtbooks/aas/st_202404
https://www.nxtbook.com/nxtbooks/aas/st_202403
https://www.nxtbook.com/nxtbooks/aas/st_202402
https://www.nxtbook.com/nxtbooks/aas/st_202401_aus
https://www.nxtbook.com/nxtbooks/aas/st_202401
https://www.nxtbook.com/nxtbooks/aas/st_202312_aus
https://www.nxtbook.com/nxtbooks/aas/st_202312
https://www.nxtbook.com/nxtbooks/aas/st_202311
https://www.nxtbook.com/nxtbooks/aas/st_202310
https://www.nxtbook.com/nxtbooks/aas/st_202309
https://www.nxtbook.com/nxtbooks/aas/st_202308
https://www.nxtbook.com/nxtbooks/aas/st_202307
https://www.nxtbook.com/nxtbooks/aas/st_202306
https://www.nxtbook.com/nxtbooks/aas/st_202305
https://www.nxtbook.com/nxtbooks/aas/st_202304
https://www.nxtbook.com/nxtbooks/aas/st_202304_qr
https://www.nxtbook.com/nxtbooks/aas/st_202303
https://www.nxtbook.com/nxtbooks/aas/st_202302
https://www.nxtbook.com/nxtbooks/aas/st_202301
https://www.nxtbook.com/nxtbooks/aas/st_202212
https://www.nxtbook.com/nxtbooks/aas/st_202211
https://www.nxtbook.com/nxtbooks/aas/st_202210
https://www.nxtbook.com/nxtbooks/aas/st_202209
https://www.nxtbook.com/nxtbooks/aas/st_202208
https://www.nxtbook.com/nxtbooks/aas/st_202207
https://www.nxtbook.com/nxtbooks/aas/st_202206
https://www.nxtbook.com/nxtbooks/aas/st_202205
https://www.nxtbook.com/nxtbooks/aas/st_202204
https://www.nxtbook.com/nxtbooks/aas/st_202203
https://www.nxtbook.com/nxtbooks/aas/st_202202
https://www.nxtbook.com/nxtbooks/aas/st_202201
https://www.nxtbook.com/nxtbooks/aas/st_202112
https://www.nxtbook.com/nxtbooks/aas/st_202111
https://www.nxtbook.com/nxtbooks/aas/st_202110
https://www.nxtbook.com/nxtbooks/aas/st_202109
https://www.nxtbook.com/nxtbooks/aas/st_202108
https://www.nxtbook.com/nxtbooks/aas/st_202107
https://www.nxtbook.com/nxtbooks/aas/st_202106
https://www.nxtbook.com/nxtbooks/aas/st_202105
https://www.nxtbook.com/nxtbooks/aas/st_202104
https://www.nxtbook.com/nxtbooks/aas/st_202103
https://www.nxtbook.com/nxtbooks/aas/st_202102
https://www.nxtbook.com/nxtbooks/aas/st_202101
https://www.nxtbook.com/nxtbooks/aas/skywatch_2021
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/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