by The Great Christ Comet- Revealing the True Star of Bethlehem (retail) (epub)
FIG. 5.10 The Great Comet of 1577 as seen from Prague. A broadside, “Von einem Schrecklichen und Wunderbarlichen Cometen” (Prague: Peter Codicillus, 1577). Image credit: Wickiana Collection, Graphische Sammlung und Fotoarchiv (Department of Prints and Drawings/Photo Archive), Zentralbibliothek Zürich (Zürich Central Library).
FIG. 5.11 An engraving of Coggia’s Comet (C/1874 H1) as seen from the Pont Neuf by Charles La Plante, 1874. Image credit: Patrick Moore Collection, www.patrickmoorecollection.com.
FIG. 5.12 Comet Donati on October 5, 1858. From E. Weiß, Bilderatlas der Sternenwelt (Esslingen: J. F. Schreiber, 1888). Image credit: Wikimedia Commons.
It is surely one of the most remarkable wonders of our solar system that relatively small bodies like comet nuclei may give rise to tails so astonishingly long and wide.
Antitail
Occasionally, comets may have a thin “mini-tail” that points toward the Sun—the so-called antitail.
This happens, from the Earth-dweller’s vantage point, when a comet’s orbit takes it close to the plane of Earth’s orbit. The antitail consists of larger particles of dust recently expelled from the comet that are not so easily pushed back behind the comet and remain in the same orbital plane as the coma. When Earth moves near to the plane of the comet’s orbit, observers can see not just the main part of the tail on the side of the comet away from the Sun, but also some of this larger material still remaining close to its orbital plane, which appears to poke out in front of the coma on the sunward side.52 An antitail may also be seen when Earth’s position relative to a productive comet is such that the rear part of the comet’s tail is seen, in projection, on the sunward side of the coma.53 Antitails may take the form of a fan or a spike, depending on the position of the comet relative to the ecliptic plane.54 Comet Arend-Roland on April 22, 1957, had a fan antitail and on April 25 a spike (fig. 5.13).
FIG. 5.13 Comet Arend-Roland with its long antitail. On April 25, 1957, the antitail was 12 degrees long. Image credit: Armagh Observatory.
The dream scenario for antitail development is a narrowly inclined and very productive comet making one of its first perihelion visits.55 For example, Comet Lulin in 2009 had an inclination of 178.4 degrees, that is, less than 2 degrees from the ecliptic plane, and sported an antitail spike for two whole months,56 although most of the time this was not visible to the naked eye (fig. 5.14).
FIG. 5.14 Comet Lulin: A photograph taken on January 31, 2009. You can see the sunward antitail to the left of the coma. Image credit: Joseph Brimacombe, Coral Towers Observatory, Cairns, Australia/Wikimedia Commons.
Visibility
As comets approach the Sun, they produce more fluorescent gas and sunlight-reflecting dust. The closer a comet gets to the Sun, the larger and brighter the comet becomes in space (though not necessarily to those observing from Earth).57 If the brightness increases sufficiently, a comet may become visible to the naked eye.
Active comets can be seen with the naked eye in a clear, dark sky when their astronomical magnitude attains to approximately +3.4 to +4.58
Comet brightness is dependent on many factors, such as the comet’s size, its productivity, and its proximity to Earth.59
It is very important to differentiate between apparent magnitude, the brightness as it appears to a human observer on Earth at a given point in time, and absolute magnitude, which is a rough measure of the intrinsic brightness of a celestial body. In order to calculate a comet’s intrinsic brightness, astronomers work out how bright it would be if it were exactly 1 AU from both Earth and the Sun.60 A comet may be intrinsically bright, but, owing to its distance from Earth and the Sun, never display its full glory to human observers. Hale-Bopp’s maximum apparent magnitude was -1, but, since its absolute magnitude was around -1, had it arrived at perihelion four months earlier, when it would have been much closer to Earth, its apparent magnitude would have been about -6 to -10, sufficient to make it visible in the daytime.61 Tycho’s comet of 1577 seems to have had an absolute magnitude of -1.8,62 and may have had an apparent brightness greater than magnitude -8.63 Sarabat’s Comet of 1729 had a rather unimpressive maximum apparent magnitude (certainly no greater than +2.6), since it never came closer to the Sun than 4 AU. However, its absolute magnitude was between -3 and -6.64 Had it made a close pass by the Sun and/or Earth, it would almost certainly have been as bright as the full Moon, probably hundreds of times brighter, sufficient to enable Earth-dwellers to read a newspaper at midnight.65 The giant parent of the Kreutz sungrazing family of comets, responsible for many of the greatest historical comets (e.g., 1843, 1882, and 1965), might well have been -5 in absolute magnitude.66
FIG. 5.15 The Great Comet of 1680 over Rotterdam as painted by Dutch artist Lieve Verschuier. Image credit: Collection Museum Rotterdam, inv. no. 11028, http://collectie.museumrotterdam.nl/objecten/11028-A.
The greatest apparent magnitude values among comets belong to the Great Comet of 1680 (see fig. 5.15), which at -18 was 100 times brighter than the full Moon (-12.6), the Great September Comet of 1882, which was -1767 or -15 to -20,68 and the twentieth century’s brightest comet, Ikeya-Seki of 1965, which was -15.69 Other noteworthy apparent magnitudes include the Great Comet of 1577 and the Great Southern Comet of 1865, both of which reached -8.70 The Great Comet of 1744 peaked at magnitude -7, the Great March Comet of 1843 at between -7 and -10, and Comet Skjellerup-Maristany of 1927 at -6 to -9.71 The stunning Comet McNaught in 2007 climaxed at -5.5.72 These are all part of an elite group of daytime comets. Few comets attain to a magnitude more impressive than Venus’s, and most of those that do are too close to the Sun to be easily visible.73 At perihelion Comet Lovejoy, the Great Christmas Comet of 2011 (fig. 5.16), attained to an apparent magnitude akin to that of Venus (-4), but, because of how near it was to the Sun at the time, it was not visible to the naked eye.74
FIG. 5.16 Comet Lovejoy as seen from the International Space Station on December 22, 2011. Image credit: NASA/Dan Burbank/Wikimedia Commons.
Even bright comets usually have to get at least 8–10 degrees from the Sun before their comas are clearly observable (e.g., Kirch’s Comet of 1680). On rare occasions, however, especially bright comets may be detected in broad daylight, even when they are very close to the solar disk, by those who are enjoying clear skies and who block out the Sun, using a wall or their hand. For example, the Great Comet of 1843 and Ikeya-Seki of 1965 could both be seen within a few degrees of the Sun.75 Comet Skjellerup-Maristany was detectable by the naked eye when only 5 degrees from the Sun in 1927, Comet McNaught in 2007 when it was just 5½ degrees away (fig. 5.19),76 and Comet West in 1975 when it was 6½ degrees from the Sun.77
It is from comets that are visiting the outer and inner solar system (i.e., the entire region from Neptune to the Sun) for the first time or, at any rate, one of the first times, and are loaded with volatile chemicals, that the brightest comets tend to be drawn. Comet Hale-Bopp was so rich in volatiles that it became visible to the naked eye (on May 20, 1996, at a magnitude of +6.7) some 10½ months prior to perihelion and remained visible for a total of at least 18 months. Even though its peak magnitude was -1, it remained brighter than magnitude 0 for some 8 weeks, the longest of any comet on record.78
Generally, you can calculate a comet’s apparent magnitude if you know its absolute magnitude (intrinsic brightness) and its location with respect to the Sun and Earth. However, comet brightness is not always predictable.79
Comets may underperform. Some new comets, like Comet Kohoutek of 1973 and Comet ISON of 2013, promise to be magnificent when first observed but end up failing to live up to expectations. When first discovered, many astronomers predicted that at perihelion ISON would be one of the most glorious comets in human history—not only would it have an apparent magnitude of -17, but it would also put on a spectacular, once-in-a-millennium show for Earth’s inhabitants. However, in light of the comet’s failure to brighten as expected over the following months, peak magnitude forecasts were downgraded to -6. Tragically, IS
ON did not attain even to that brightness level or put on any kind of notable display; in fact, half an hour before perihelion, exposed to the full force of the Sun’s powerful gravity and heat, the comet completely disintegrated.
However, sometimes comets end up brighter than simple predictions based on their intrinsic brightness might suggest. Comets, particularly but not only those new to our part of the solar system, may on occasion undergo sudden bursts of brightness that increase their brightness by between 6 and 100 times (= 2–5 magnitudes) or even 1,000 times (= 7.5 magnitudes) for 3–4 weeks or more.80
The comet most famous for its outbursts is Comet Holmes (fig. 5.17). Within 42 hours in October 2007 it grew incredibly and went from being magnitude +17, “a thousand times too faint even to be seen with binoculars,” to magnitude +2.7, which was almost enough to make it one of the top 100 brightest “stars” in the sky.81 That is a half-a-million-fold intensification of brightness.
FIG. 5.17 Comet Holmes during its outburst in 2007. Image credit: John Buonomo/Wikimedia Commons.
Another comet that had an outburst was Halley’s Comet, on December 12, 1991, when it was 14.3 AU from the Sun—it became 300 times brighter. In the year 2000, comets C/1999 S4 (LINEAR) and 73P/Schwassmann 3 also underwent major outbursts.
These outbursts occurred because fresh ice and dust were suddenly being released from the comet nucleus. In the case of the outbursts in the year 2000, what happened marked a key stage in the process of the comets’ splitting into pieces.82 As regards the case of comets Holmes and Halley, the nuclei may have been hit by asteroids, and/or gas pockets within the nuclei may have exploded.83
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.”84 In particular, it had a series of outbursts separated by about 2 to 3 weeks.85
Moreover, comets, particularly medium or large dusty ones,86 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.
FIG. 5.18 Forward-scattering and backscattering. Comets that move into the zone between Earth and the Sun or move “behind” Earth or the Sun are boosted in their brightness. This is because the sunlight is scattered forward and backward (toward Earth) by the comet’s dust. The closer the comet gets to the Earth-Sun axis, the greater the brightness boost is. Image credit: Sirscha Nicholl.
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.87 At 166.5 degrees, the brightness boost is 5 magnitudes (a hundredfold boost) greater than at 90 degrees.88 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, 1927.89 Due to forward-scattering, Comet McNaught in January 2007 was boosted by 2–3 magnitudes and so became visible during the daytime90 (fig. 5.19) and Comet Tebbutt in 1861 became so bright that it cast shadows on the walls of Athens Observatory at night.91
FIG. 5.19 Comet McNaught (C/2006 P1) seen from Lawlers Gold Mine in Western Australia on January 20, 2007. Image credit: Sjbmgrtl/Wikimedia Commons.
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.92
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.93
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.94
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).95 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.96 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.
FIG. 5.20 Images of comets from Cometographia, by Johannes Hevelius of Danzig, 1668. Image credits: NASA/JPL (left); Library of Congress (right). Image enhancement: Sirscha Nicholl.
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)97 and gassier ones a bluish-green hue98 (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.”99 See. fig. 5.21.
FIG. 5.21 The Great Comet of 1861 (C/1861 [Tebbutt]). From E. Weiß, Bilderatlas der Sternenwelt (Esslingen: J. F. Schreiber, 1888). Image credit: Wikimedia Commons.
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.100
Comet Orbits
Apollonius of Myndus, who studied with the Babylonians,101 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).102 Seneca, too, realized that comets were objects orbiting in the highest heavens that were not confined to the zodiac, in contrast to the planets.103
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).104 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 eccentricity105 (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 lon
g-period orbits.
FIG. 5.22 Cometary orbits: elliptical, parabolic, and hyperbolic. Image credit: Sirscha Nicholl.
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.106
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.
