The Great Christ Comet (19 page)

Curiously, in the months after it was first observed, “rather than flaring in brightness from a single [outburst] or infrequent outbursts, Hale-Bopp seemed to be puffing them
out one after another like a locomotive.”
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In particular, it had a series of outbursts separated by about 2 to 3 weeks.
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Moreover, comets, particularly medium or large dusty ones,
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are subject to brightness boosts when they come close to the imaginary line that cuts through Earth and the Sun, due to the forward-scattering or backscattering of the Sun's light. To put it simply, as a comet moves closer to the zone between the Sun and Earth, the Sun's light hits the small dust particles of the coma and tail and is scattered forward. The result is that the coma and the dust tail are subject to an increasingly large spike in brightness. It is just like when a spider's web is between you and the Sun—the web suddenly becomes stunningly visible as the sunlight strikes it and is scattered forward in your direction. See
fig. 5.18
.

Where lines extending to the comet from the Sun and from Earth converge at an angle (= “phase angle”) of 90 degrees or more, this effect is notable. At 150 degrees the comet is 2.5 magnitudes brighter than at 90 degrees.
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At 166.5 degrees, the brightness boost is 5 magnitudes (a hundredfold boost) greater than at 90 degrees.
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In extreme cases the effect can result in a boost of more than 7 magnitudes, as happened when Comet Skjellerup-Maristany had a phase angle of 173.5 degrees on December 15, 1927.
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Due to forward-scattering, Comet McNaught in January 2007 was boosted by 2–3 magnitudes and so became visible during the daytime
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(
fig. 5.19
) and Comet Tebbutt in 1861 became so bright that it cast shadows on the walls of Athens Observatory at night.
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Where a comet is on the other side of Earth from the perspective of the Sun or the other side of the Sun from the perspective of Earth, the light that it gives off is backscattered, that is, reflected back off the larger dust particles. When such a comet is positioned at an angle close to the Sun-Earth axis (a phase angle of between 30 degrees and 0), it will experience a brightness boost of up to 1 magnitude.
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Another factor affecting a comet's apparent brightness is that the more extended the coma is, the larger the “surface area” is over which its brightness is distributed. All other things being equal, this means that the larger a coma is, the duller it will appear. For any object to be visible in the night sky or the day sky, it must be brighter than the sky.
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Finally, the terrestrial circumstances of the observer, particularly local atmospheric conditions and the lay of the land, are a key factor determining which parts of a cometary apparition may be visible to observers. For example, most Europeans missed daytime sightings of the Great Comet of 1843 because of widespread cloudy conditions.
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Variability

Comets are remarkably varied not only in celestial route, coma and tail size, and brightness, but also in shape and color, and in the duration of their apparitions.

With respect to shape, Pliny the Elder listed ten different types of comets, ranging from the bearded to the sword and from the horn to the burning torch (compare
fig. 5.20
).
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In their book
Comet
, Carl Sagan and Ann Druyan offer what they call a “bestiary” of comets, with images of comets that look like a fan or horse's mane, a fountain, a tall glass, a syringe, an angel, a fetus or rabbit, a lighthouse or ball-point pen, an arrow, and a human.
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A fan-shaped (parabolic) coma, like Tebbutt's Comet of 1861, may look like an angel, and an oval (elliptical) coma, like that of Hale-Bopp, Hyakutake, or Ikeya-Zhang, may look like a fetus in the fetal position or a baby in swaddling clothes.

Regarding color, comets are also diverse—they are often silvery grey, but dustier comets tend to have a yellowish hue (e.g., Comet West in 1975–1976)
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and gassier ones a bluish-green hue
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(e.g., Comet Hyakutake). Single comets may change their color during the course of their apparition: for example, Comet Tebbutt was variously portrayed as white, golden, silver, “bluish-green,” “greenish-blue,” “greenish-yellow.”
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See.
fig. 5.21
.

Concerning the duration of cometary apparitions, a glance at the historical records demonstrates that they may be extremely brief or extraordinarily long (lasting well in excess of a year) or anything in between.
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Comet Orbits

Apollonius of Myndus, who studied with the Bab­ylo­nians,
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the forefathers of modern comet astronomy, claims that they believed that comets were reckoned to be in the same general category as the planets, albeit with eccentric orbits (which, nevertheless, could be calculated).
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Seneca, too, realized that comets were objects orbiting in the highest heavens that were not confined to the zodiac, in contrast to the planets.
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Whereas the orbits of the planets are narrowly inclined to the ecliptic plane and are all prograde (i.e., counterclockwise from the vantage point of Earth's north pole), cometary orbital planes can be inclined at any angle to the ecliptic and hence be either prograde or retrograde (clockwise).
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In addition, comet orbital planes may be oriented at any angle, and the orbit itself may be positioned in any direction within the plane.

How long a comet takes to complete one revolution and how far away from the Sun it goes depend on the comet's eccentricity
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(the shape of its orbit) and how close it gets to the Sun at perihelion.

As regards eccentricity, while some comets, like the major planets, have almost circular orbits, most comets have orbits that are elongated ovals (ellipses), and some have evolved into orbits that are hyperbolic, meaning that they may never return to the inner solar system (
fig. 5.22
). Short-period orbits are less elongated than long-period orbits.

With respect to how near comets come to the Sun, at one extreme we have the sungrazers and at the other extreme we have comets 167P/CINEOS and C/2003 A2 (Gleason), which do not come closer than 11.8 AU and 11.4 AU respectively to the Sun.

As regards the orbital periods of comets, they may be as short as Encke's 3.3 years or in the hundreds of millions of years.
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As far as Earth-dwellers are concerned, the impressiveness of a cometary apparition is heavily dependent on the time at which the comet arrives at perihelion, since that determines where Earth is on its orbit and hence the perspective humans will enjoy. On the one hand, if the comet's orbit is synchronized ideally with Earth's, it is possible for Earthbound observers to have front-row seats for the cometary spectacle both before and after perihelion. However, if a comet is out of sync with Earth, as was Hale-Bopp, humans can only watch the action from their seats near the back row.

Long-Period and Short-Period Comets

Astronomers like to divide comets into two major groups: long-period and short-period ones. Long-period comets take more than 200 years to complete a single orbit, while short-period comets have an orbital period of 200 years or less. In terms of performance and productivity, long-period comets are wild and powerful in contrast to short-period comets, which are tame and weak.
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Long-Period Comets

Many of the individual comets observed over the last few hundred years have been in extraordinarily long orbits with periods in the hundreds, thousands, and even millions of years. Indeed almost all the great comets in history are long-period comets.