Sky & Telescope - January 2019 - 7

stant using a very distant source (since the
universe emitted the CMB when it was only
370,000 years old). There are still some discrepancies between data sets: The Hubble
constant may be 68 kilometers per second
per megaparsec or 73 km/s/Mpc, depending whether you measure it from nearby
sources or from the CMB. But there's simply
no way that current measurements would
allow a value of 10 km/s/Mpc, as Harrison
proposed, and ultimately a scientific theory
must be able to predict our observed reality.

Gravity Doesn't Discriminate
In a sidebar (S&T: Sept. 2018, p. 25),
Camille Carlisle mentions black holes
undergoing the EKL mechanism. Is it
also possible for the Hills mechanism to
occur, flinging a black hole away from
the center of our (or any other) galaxy
to travel through intergalactic space? I
don't recall seeing this possibility mentioned as a source of dark matter.
Ed Evans
Seneca, South Carolina

"

Camille Carlisle replies: Any kind
of binary can undergo the Hills
mechanism, since in these situations gravity
doesn't discriminate among stars, black
holes, and other stellar remnants. Unfortunately, a hypervelocity black hole would be
immensely difficult to detect. A star emits
light, so we can see it and calculate both its
proper motion across the sky and its radial
velocity along our line of sight, based on the
redshift of its spectrum. We couldn't do that
with a black hole.
I'm afraid hypervelocity black holes
wouldn't work as a potential explanation for
dark matter. For one thing, a hypervelocity black hole would not be bound to the
galaxy, so it wouldn't create a big, deep well
of gravitationally bound matter for the galaxy
to sit in. For another, the Milky Way has
roughly 10 times more mass in dark matter
than it does in all its stars combined. There
just haven't been enough massive stars to

1969

1994

Here Goes Nothing
"The Void Next Door" (S&T: Oct. 2018,
p. 12) helps a lot in visualizing our cosmic neighborhood. Over and above the
numerous successful large simulations,
is there an intuitive picture of why
voids, sheets, and filaments are what
gravity produces out of Gaussian quantum fluctuations in the early universe?
Ken Wachter
The Sea Ranch, California

"

Ken Croswell replies: Voids have a

lower density, so they expand faster
than the overall universe. I like to think of
voids as pushing against the rest of the
universe, evicting their galaxies and herding
them into the filaments and sheets that
crisscross the cosmos.

SUBMISSIONS: Write to Sky & Telescope, 90 Sherman St., Cambridge, MA 02140-3264, U.S.A. or email: letters@
skyandtelescope.com. Please limit your comments to 250 words; letters may be edited for brevity and clarity.

75, 50 & 25 YEARS AGO by Roger W. Sinnott
1944

make enough black holes to explain all that
dark-matter mass.

º January 1944
Director's Rebuke "[Long ago, on
occasion, George Airy would tell an
assistant at the Greenwich Observatory,] 'The Royal Observatory
was founded for observation of the
moon. We get about 300 observations of the moon during the year in
all; and the Observatory costs the
nation 6,000 pounds a year. Hence
each observation of the moon is
worth 20 pounds; and by losing
one last night you have cost the
nation 20 pounds! . . .'
"[That sum] seems pretty high
for one observation of the moon's
position, whereas Dr. Harlow
Shapley figured a few years ago
that it cost only a few cents . . . to
discover a galaxy!"
This gem is from Roy K. Marshall's ever-popular Astronomical
Anecdotes column.

rapidly pulsating radio sources and
supernova explosions. Two cases
of such association have recently
been announced. . . .
"The first of these pulsars [lies]
near the midpoint of the extended
radio source Vela X [,] believed to
be the result of a prehistoric supernova explosion.
[Secondly,] R. B. E. Lovelace
and his associates observed [Taurus pulsar] NP 0532 with the 1,000foot dish at Arecibo, Puerto Rico.
Several scans on November 15th
placed the pulsar within 10 minutes
of arc of the Crab nebula's center.
. . . The distance of NP 0532 [from
us] agrees fairly well with that of
the Crab nebula, [so perhaps] this
pulsar is, like the Crab nebula, a
product of the supernova observed
in A.D. 1054."
The pulsar-supernova link, now
amply confirmed, was made just a
year after pulsars were discovered.

º January 1969
Growing Evidence "[T]here may
be a close connection between

º January 1994
Distant Blinks "Among the exciting astronomical news of last

year was David Jewitt and Jane
Luu's discovery of objects orbiting the Sun beyond Pluto. . . . They
may lie within the Kuiper Belt, the
hypothetical reservoir of comets
postulated [to exist] just outside the
region occupied by planets. . . .
"Is it possible to see [such
remote comets] where they live
before making their suicidal dives
toward the Sun? [Using] reflected
sunlight to detect them is hopeless.
But there is another way to find
comets; it involves looking for dark
objects by observing occultations
of more distant, bright objects. . . .
If we adopt Kuiper's optimistic
estimate of the Belt population,
1013 bodies, we can calculate that a
star is occulted . . . once every two
days or so."
Princeton physicist Freeman
Dyson, reviving an earlier proposal
by Mark E. Bailey, lamented it had
been largely ignored. In recent
years at least five major observatories have launched occultation
surveys of this type, but so far the
results are inconclusive.

sk yandtele scope.com * JA N UA RY 2 019

7


http://www.skyandtelescope.com

Sky & Telescope - January 2019

Table of Contents for the Digital Edition of Sky & Telescope - January 2019

Contents
Sky & Telescope - January 2019 - Cover1
Sky & Telescope - January 2019 - Cover2
Sky & Telescope - January 2019 - 1
Sky & Telescope - January 2019 - Contents
Sky & Telescope - January 2019 - 3
Sky & Telescope - January 2019 - 4
Sky & Telescope - January 2019 - 5
Sky & Telescope - January 2019 - 6
Sky & Telescope - January 2019 - 7
Sky & Telescope - January 2019 - 8
Sky & Telescope - January 2019 - 9
Sky & Telescope - January 2019 - 10
Sky & Telescope - January 2019 - 11
Sky & Telescope - January 2019 - 12
Sky & Telescope - January 2019 - 13
Sky & Telescope - January 2019 - 14
Sky & Telescope - January 2019 - 15
Sky & Telescope - January 2019 - 16
Sky & Telescope - January 2019 - 17
Sky & Telescope - January 2019 - 18
Sky & Telescope - January 2019 - 19
Sky & Telescope - January 2019 - 20
Sky & Telescope - January 2019 - 21
Sky & Telescope - January 2019 - 22
Sky & Telescope - January 2019 - 23
Sky & Telescope - January 2019 - 24
Sky & Telescope - January 2019 - 25
Sky & Telescope - January 2019 - 26
Sky & Telescope - January 2019 - 27
Sky & Telescope - January 2019 - 28
Sky & Telescope - January 2019 - 29
Sky & Telescope - January 2019 - 30
Sky & Telescope - January 2019 - 31
Sky & Telescope - January 2019 - 32
Sky & Telescope - January 2019 - 33
Sky & Telescope - January 2019 - 34
Sky & Telescope - January 2019 - 35
Sky & Telescope - January 2019 - 36
Sky & Telescope - January 2019 - 37
Sky & Telescope - January 2019 - 38
Sky & Telescope - January 2019 - 39
Sky & Telescope - January 2019 - 40
Sky & Telescope - January 2019 - 41
Sky & Telescope - January 2019 - 42
Sky & Telescope - January 2019 - 43
Sky & Telescope - January 2019 - 44
Sky & Telescope - January 2019 - 45
Sky & Telescope - January 2019 - 46
Sky & Telescope - January 2019 - 47
Sky & Telescope - January 2019 - 48
Sky & Telescope - January 2019 - 49
Sky & Telescope - January 2019 - 50
Sky & Telescope - January 2019 - 51
Sky & Telescope - January 2019 - 52
Sky & Telescope - January 2019 - 53
Sky & Telescope - January 2019 - 54
Sky & Telescope - January 2019 - 55
Sky & Telescope - January 2019 - 56
Sky & Telescope - January 2019 - 57
Sky & Telescope - January 2019 - 58
Sky & Telescope - January 2019 - 59
Sky & Telescope - January 2019 - 60
Sky & Telescope - January 2019 - 61
Sky & Telescope - January 2019 - 62
Sky & Telescope - January 2019 - 63
Sky & Telescope - January 2019 - 64
Sky & Telescope - January 2019 - 65
Sky & Telescope - January 2019 - 66
Sky & Telescope - January 2019 - 67
Sky & Telescope - January 2019 - 68
Sky & Telescope - January 2019 - 69
Sky & Telescope - January 2019 - 70
Sky & Telescope - January 2019 - 71
Sky & Telescope - January 2019 - 72
Sky & Telescope - January 2019 - 73
Sky & Telescope - January 2019 - 74
Sky & Telescope - January 2019 - 75
Sky & Telescope - January 2019 - 76
Sky & Telescope - January 2019 - 77
Sky & Telescope - January 2019 - 78
Sky & Telescope - January 2019 - 79
Sky & Telescope - January 2019 - 80
Sky & Telescope - January 2019 - 81
Sky & Telescope - January 2019 - 82
Sky & Telescope - January 2019 - 83
Sky & Telescope - January 2019 - 84
Sky & Telescope - January 2019 - Cover3
Sky & Telescope - January 2019 - 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