The Great Christ Comet (76 page)

54
 Brandt and Chapman,
Introduction to Comets
, 137.

55
 Mobberley,
Hunting and Imaging Comets
, 9.

56
 Ibid., 168.

57
 However, as the comet gets closer to the Sun, the Sun actually constricts the coma, causing it to shrink.

58
 F. Richard Stephenson, Kevin Yau, and Hermann Hunger, “Records of Halley's Comet on Babylonian Tablets,”
Nature
314 (April 18, 1985): 587–592; Kevin Yau, Donald Yeomans, and Paul Weissman, “The Past and Future Motion of Comet P/Swift-Tuttle,”
Monthly Notices of the Royal Astronomical Society
266 (1994): 314. In a rural location with clear, dark skies, the faintest ordinary star visible to the naked eye is generally sixth magnitude (up to +6.5).

59
 Mark Littmann and Donald K. Yeomans,
Comet Halley: Once in a Lifetime
(Washington, DC: American Chemical Society, 1985), 111.

60
 It should be appreciated that the absolute magnitude of a comet is not constant, and may change during a single apparition, particularly around the time of perihelion.

61
 Gary W. Kronk, personal email message to the author, September 12, 2012 (“something like -8 to -10!”); Mobberley,
Hunting and Imaging Comets
, 74–75 (-5 to -6). Andreas Kammerer reckons that if Hale-Bopp's perihelion distance had been 0.1 AU and its perigee distance 0.1 AU, its apparent magnitude would have been -12 and -8 respectively (personal email message to the author, October 30, 2012).

62
 Mobberley,
Hunting and Imaging Comets
, 34–35; Vsekhsvyatskii,
Physical Characteristics
, 106; Kronk,
Cometography
, 1:320.

63
 Seargent,
Greatest Comets
, 107.

64
 -3 is the value of absolute magnitude favored by Vsekhsvyatskii,
Physical Characteristics
, 51, 124; Donald K. Yeomans,
Comets: A Chronological History of Observation, Science, Myth, and Folklore
(New York: John Wiley, 1991), 160–161; and Kronk,
Cometography
, 1:396. F. G. Watson,
Between the Planets
, rev. ed. (Cambridge, MA: Harvard University Press, 1956), 62, opts for something closer to -6. If we assume the orbit preferred by Kronk (
Cometography
, 1:396) and an (average) brightness slope (n) of 4, then an absolute magnitude of -6 would mean that the comet was first spotted at apparent magnitude +2.6 on August 1, 1729, and last observed at apparent magnitude +4.1 “in slight twilight” (Kronk,
Cometography
, 1:396) on January 21, 1730. (For more on “brightness slope,” see chapter 9.) With an absolute magnitude of -3, the comet would have been spotted first at apparent magnitude +5.6 and last seen at +7.1. All in all, it seems most likely that Sarabat's Comet was first spotted at apparent magnitude +3 to +4 and had an absolute magnitude of -4.6 to -5.6 (if first seen at apparent magnitude +3.4, its absolute magnitude would have been -5.2).

65
 Sagan and Druyan,
Comet
, 131–132.

66
 Zdenek Sekanina, as cited by Schaaf,
Comet of the Century
, 73.

67
 So Wikipedia, s.v. “Great Comet of 1882,”
http://
en
.wikipedia
.org
/wiki/Great
_Comet
_of
_1882
(last modified February 26, 2013).

68
 Gary W. Kronk,
Comets: A Descriptive Catalog
(Hillside, NJ: Enslow, 1984), 69.

69
 John E. Bortle, “The Bright-Comet Chronicles,”
International Comet Quarterly
(1998),
http://
www
.icq
.eps
.harvard
.edu
/bortle.html
(accessed March 26, 2014).

70
 Andreas Kammerer, “Analysis of Past Comet Apparitions: C/1995 O1 (Hale-Bopp),”
http://
kometen
.fg
-vds
.de
/koj
_1997
/c1995o1
/95o1eaus.htm
(last modified June 26, 2007).

71
 See “Brightest Comets Seen since 1935,”
International Comet Quarterly
,
http://
www
.icq
.eps
.harvard
.edu
/brightest.html
(accessed March 26, 2014); Joe Rao, “The Greatest Comets of All Time,” SPACE.com
(January 19, 2007),
http://
www
.space
.com
/3366
-greatest
-comets
-time.html
(accessed March 26, 2014); and Seargent,
Greatest Comets
, 150, 208–224.

72
 “Brightest Comets Seen since 1935.”

73
 Seargent,
Comets: Vagabonds of Space
, 34.

74
 Karl Battams, “The Great Birthday Comet of 2011,”
http://
sungrazer
.nrl
.navy
.mil
/index
.php
?p
=
news/birthday
_comet
(last modified December 28, 2011).

75
 Kronk,
Cometography
, 2:130–133. Other instances of especially bright comets being spotted relatively close to the Sun can be found in Seargent,
Greatest Comets
, 113, 144, and 212; and Kronk,
Cometography
, 2:506–507.

76
 Joseph N. Marcus, “Forward-Scattering Enhancement of Comet Brightness. II. The Light Curve of C/2006 P1 (McNaught),”
International Comet Quarterly
29 (2007): 119, 128.

77
 Gary W. Kronk, “C/1975 V1 (West),”
http://
cometography
.com
/lcomets
/1975v1.html
(last modified October 3, 2006).

78
 “Comet Hale-Bopp: The Great Comet of 1997,” an article on NASA's Stardust website,
http://
stardust
.jpl
.nasa
.gov
/science
/hb.html
(last modified November 26, 2003).

79
 Crovisier and Encrenaz,
Comet Science
, 31.

80
 Cf. Brandt and Chapman,
Introduction to Comets
, 258; Littmann and Yeomans,
Comet Halley
, 74.

81
 “Comet in Major Outburst Now Visible in Evening Sky,” an article on the Armagh Observatory's website in 2010,
http://
star
.arm
.ac
.uk
/press
/2007/cometholmes
(last modified October 10, 2012).

82
 We shall consider comet fragmentation events later in this chapter.

83
 On these and other possibilities, see Sergei I. Ipatov, “Cavities as a Source of Outbursts from Comets,” in
Comets: Characteristics, Composition, and Orbits
, ed. Peter G. Melark (Hauppauge, NY: Nova Science, 2011), 101–112.

84
 Schaaf,
Comet of the Century
, 284.

85
 Ibid.

86
 Joseph N. Marcus, “Forward-Scattering Enhancement of Comet Brightness. I. Background and Model,”
International Comet Quarterly
29 (2007): 61.

87
 Ibid., 58.

88
 Ibid.

89
 Ibid., 56.

90
 Marcus, “C/2006 P1 (McNaught),” 119.

91
 Marcus, “Background and Model,” 40, 62.

92
 Ibid., 59.

93
 For this reason, astronomers refer to “surface brightness,” the magnitude of 1 square arcsecond (1/3600 degree) of any given astronomical object. The surface brightness of a comet that is 1 square arcminute (an arcminute is 60 arcseconds or 1/60 degree) will be 10,000 times, and hence 10 magnitudes, brighter than if it were 100 square arcminutes. In ideal observing conditions, such as would generally have prevailed in ancient Babylon, the surface brightness of the night sky is +22 magnitudes per square arcsecond and that of the clear daytime sky approximately +4 (an overcast daytime sky's surface brightness is about +6). To be seen, an object needs to have a surface brightness greater than the background sky. The Milky Way Galaxy, with its surface brightness of +21, can be seen only in very clear night skies. The full Moon has a surface brightness of approximately +3.6 and so is brighter than a clear daytime sky and hence may be clearly visible even when the Sun is present. Venus, with its surface brightness of +1.9, is also detectable during the day in clear skies to those with excellent eyesight who know exactly where to focus their vision. However, due to its small size, it is more difficult to spot than the Moon. The Sun's surface brightness is -10.7, Jupiter's +5.7, Saturn's +5.9, and Mars's +3.9 (see Roger Nelson Clark,
Visual Astronomy of the Deep Sky
[Cambridge: Cambridge University Press, 1990], 11 table 2.3; Mike Luciuk, “Astronomical Magnitudes: Why Can We See the Moon and Planets in Daylight?,” 7 [
http://
www
.asterism
.org
/tutorials
/tut35
%20Magnitudes.pdf
(last modified April 25, 2013)]; Paul Schlyter, “Radio and Photometry in Astronomy,”
http://
stjarnhimlen
.se
/comp
/radfaq.html
[last modified April 13, 2010]; also “Planetary Photo Techniques,” a webpage of Galactic Photography,
http://
www
.galacticphotography
.com
/astro
_Planetary
_technique
_3.html
[last modified September 9, 2012]). Venus, Jupiter, Saturn, and Mars become clearly visible around sunset, when the sky's surface brightness is dimming. When the Sun is just 5 degrees above the horizon, a clear sky's brightness at its zenith is +6.5; 15 minutes after sunset it is +13—at this point one can see stars that have an apparent stellar magnitude (note: not “surface brightness”) of +3.5 (Clark,
Visual Astronomy of the Deep Sky
, 16). In ideal dark conditions at night, when the sky's surface brightness is +22, naked-eye observers can generally see stars up to +6.5 apparent stellar magnitude (note: not “surface brightness”). The surface brightness of the middle “star” of Orion's sword, the Orion Nebula, is +17 magnitudes per square arcsecond, and it is visible to the naked eye at night, even in light-polluted skies.

94
 Kronk,
Cometography
, 2:130.

95
 See Yeomans,
Comets
, 11–14.

96
 Sagan and Druyan,
Comet
, 173–187.

97
 See Seargent,
Greatest Comets
, 154.

98
 George F. Chambers,
The Story of the Comets
(London: Clarendon, 1909), 8–9.

99
 See Seargent,
Greatest Comets
, 141; Kronk,
Cometography
, 2:294, 295–296, 298, 299.

100
 Josephus,
J.W.
6.5.3 (§289).

101
 Seneca,
Natural Questions
7.3.2–3.

102
 Ibid., 7.17–18. See Mark E. Bailey, Victor M. Clube, and William M. Napier,
The Origin of Comets
(Oxford: Pergamon, 1990), 10–11; and Yeomans,
Comets
, 8.

103
 Seneca,
Natural Questions
7.

104
 Brandt and Chapman,
Introduction to Comets
, 17. If the inclination is less than 90° to the ecliptic, the comet is prograde; if the inclination is more than 90° the comet is retrograde.

105
 “Eccentricity” is the degree to which a celestial body's orbit deviates from perfect circularity—the eccentricity of a circle is 0; the more stretched the oval is, the higher the eccentricity is, up to 1 (elliptical); an eccentricity of 1 (parabolic) or above (hyperbolic) means that the object is incapable of completing an orbital revolution.

106
 The eccentricity and perihelion distance also determine the range of a comet's velocity.

107
 Cf. Guy Ottewell as cited by Schaaf,
Comet of the Century
, 197.

108
 Steel,
Rogue Asteroids and Doomsday Comets
, 36.

109
 H. F. Levison, A. Morbidelli, L. Domes, R. Jedicke, P. A. Wiegert, and W. F. Bottke, Jr., “The Mass Disruption of Oort Cloud Comets,”
Science
296 (2002): 2212–2215.

110
 Jenniskens,
Meteor Showers
, 72.

111
 For an overview of the greatest sungrazers, see Seargent's
Greatest Comets
, 191–224, and his
Sungrazing Comets: Snowballs in the Furnace
(Kindle Digital book, Amazon Media, 2012).