by The Great Christ Comet- Revealing the True Star of Bethlehem (retail) (epub)
With respect to the question of why Jerusalem and its religious establishment would have been rattled by the news that the Messiah had been born, the historical context readily explains this. Judea was ruled by Herod the Great, whom the people of Jerusalem knew to be capable not only of brutal atrocities against his enemies, but even of executing members of his own family. They understood that Herod would feel very threatened by the report that the Messiah had been born, since it would seem to imply that his own dynasty would soon be terminated. Therefore those in Jerusalem would have been certain that the king of Judea would take action against the messianic baby. Moreover, the Jerusalemites would have believed that, when the Messiah was full-grown, he would challenge the dynasty of Herod and the Romans. Such was the power of messianic expectation that the Jerusalemites might well have feared that a new revolutionary movement might be stirred up by the birth of this baby. At any rate, the Messiah’s birth would have seemed to signal impending civil war and even all-out war with the Romans, with the city of Jerusalem being the main focus of the fighting. Therefore, although superficially surprising, the response of the people of Jerusalem to the news that the Messiah had been born makes excellent sense when the historical context is factored in.
Finally, the absence of any ancient astronomical record of the Bethlehem Star would constitute a problem only if we had reason to believe that the surviving astronomical records are comprehensive. Unfortunately, they are patchy at best.126
We maintain, therefore, that there is nothing historically implausible about Matthew’s account of the visit of the Magi.127
Matthew portrays the Star as historical and expects his readers to treat it as such. He regards the Star, together with the journey of the Magi, as a major proof that Jesus was indeed the Messiah.
We have already highlighted a number of factors that favor the Star’s historicity. On a general level, genre considerations, the importance of historical accuracy to the early Christians, the stability of the transmission of the Jesus tradition in the early church, Matthew’s conservative handling of his sources, and the consistency of Matthew’s portrayal of Herod with that of Josephus speak in favor of the account’s historical reliability. With respect to the nativity narrative, the corroboration of many elements of the account by Luke, the lack of any plausible alternative explanation of the origin of the Magi story, and the positive presentation of the Magi argue strongly for historical reliability. So also does the fact that the account of the Magi and the Star has many traits that render it the sort of event that tends to be remembered accurately by eyewitnesses.
Ignatius, who ministered in the first century AD and knew the apostles and other prominent early Christians,128 makes specific mention of the Bethlehem Star in chapter 19 of his letter To the Ephesians:
A star shone in heaven [with a brightness] beyond all the stars; its light was indescribable, and its newness provoked astonishment. And all the other stars, together with the Sun and the Moon, formed a chorus to the star, yet its light far exceeded them all. And there was perplexity regarding from where this new entity came, so unlike anything else [in the heavens] was it.129
This is evidently a tradition independent of Matthew130 and therefore supports the historicity of the Star.
Cullen seeks to undermine the historicity of the Star, asking how the Magi managed to arrive in Bethlehem at the correct, precise moment. He demands to know whether his fellow astronomers are claiming that this was merely a coincidence or something else, such as a miracle or correct astrology. Supposing that these explanations are self-evidently foolish, he pronounces that Matthew’s account is historically implausible.131 However, Cullen tips his hand at this point—he is rejecting the historicity of Matthew’s account because he does not wish to accept the implication of it. Obviously, historical investigation should not operate in this way—one should begin by making historical judgments based on the evidence and then, and only then, ask the questions of why and how.
When one is on a quest for the historical Star of Bethlehem, one is duty-bound to adopt a sympathetic approach to Matthew 1:18–2:23, the only major narrative concerning the Star and the Magi that we have from the first century AD. Indeed the more a hypothesis is forced to diverge from a natural reading of Matthew’s narrative, the less likely it proves itself to be. If Matthew is unreliable, then any hope of identifying the Star is dashed. But if he is reliable, then there is purpose to the quest, and the solution to the mystery will be able to explain naturally every aspect of Matthew’s account.
Conclusion
In this chapter we have considered most of the major hypotheses concerning the Star seen by the Magi in the east and then again in Bethlehem. They have each been found wanting. We must turn next to consider the comet hypothesis.
5
“What Sudden
Radiance from Afar?”
Introducing Comets
Having considered the other major proposals regarding the identity of the Star of Bethlehem, we must now turn to the last and most plausible of the main views, the comet hypothesis.
Unfortunately, much of the academic discussion concerning the comet hypothesis reflects a failure to come to terms with the history of comets and their interpretation, and with cometary astronomy generally. Therefore in this chapter we must pause briefly to become better acquainted with the phenomenon of comets. Armed with this knowledge, we shall in the next chapter be in the position to evaluate the two cometary hypotheses currently on the table, namely that the Bethlehem Star was Halley’s Comet in 12 BC or that it was the 5 BC hui-hsing comet recorded by the Chinese. Also in that chapter we shall develop our own case that the Star was a great comet.
First, then, we need to familiarize ourselves with comets. Along with a meteor storm, a total solar eclipse, and the aurora borealis (Northern Lights), the appearance of a great comet is one of the chief celestial marvels that a human being can witness in his or her lifetime. A comet may be the largest,1 longest, and brightest2 object, not to mention the most unusual3 one, in the sky. A great comet commands the attention of all humans.
The Celestial Jokers
Comets are the celestial jokers,4 mavericks that simply do not follow the normal rules of the heavens. They appear in the sky out of nowhere. They may venture into celestial territory where no planet can ever go, even into the polar regions. Whereas the stars and planets followed set schedules that the ancients were able to identify and on that basis determine seasons and make predictions, comets appeared to be completely random in their movements.5 Sometimes a comet remains in one spot or region of the sky for weeks or indeed for its whole apparition;6 at other times a comet races very quickly across large sections of the heavens.7 Comets may suddenly change their apparent direction and behavior. Moreover, they frequently change their appearance. They can go from being barely visible to being brighter than the full Moon. These strangers may also grow in size to be much larger than the Sun and Moon, and may increase in length to the extent that they stretch across the whole sky.8 Not only is each comet completely individual, but each apparition of a given comet is unique.
What Is a Comet?
A comet near the Sun consists of a coma, that is, a dense cloud of dust around a nucleus, and a tail. When far from the Sun, it is nothing more than a bare nucleus undetectable by the naked eye.
Nucleus
Just as a plum’s flesh surrounds a pit, so a comet coma envelops an inner nucleus. This nucleus is essentially an icy9 ball of dirt, dust,10 and stones. Up close, a nucleus looks like an extraordinarily black and barren giant rock (fig. 5.1).11 Nuclei may have distinctive shapes—for example, those of Comets 1P/Halley, 9P/Tempel 1 (fig 5.1), and 19P/Borrelly look like potatoes; those of Comets 8P/Tuttle and 103P/Hartley 2 (fig. 5.2) have been compared to peanuts; and, from some angles, the nucleus of Comet 67P/Churyumov-Gerasimenko has the appearance of a rubber duck.12
FIG. 5.1 The nucleus of Comet Tempel 1. It measures 7.6 x 4.9 km. Image credit: NASA/JPL/University of Mar
yland/Wikimedia Commons.
Put simply, when a comet nucleus comes to within a certain distance from the Sun, it begins to react, its “ices” being converted directly from solids to gases. These gases rocket off the nucleus in jets toward the Sun and take with them dust and rock particles (fig. 5.2). The Sun’s radiation pressure then pushes the debris back to form a coma and a dust tail (fig. 5.3).
FIG. 5.2 The nucleus of Comet Hartley 2 as it degasses. The hyperactive comet is 2.5 km in diameter. This image was taken by NASA’s EPOXI mission spacecraft during a fly-by on November 4, 2010. Image credit: NASA/JPL-Caltech/UMD/Wikimedia Commons.
FIG. 5.3 How a comet develops a coma and tail. Image credit: Sirscha Nicholl.
How close a freezing cold comet nucleus must come to the Sun before it begins to “degas” varies greatly, based largely on the concoction of chemicals that make up the comet’s “ices.” If we use the distance from Earth to the Sun as a measure (= 1 “Astronomical Unit” or 1 AU), most comet nuclei react to the Sun’s heat when they are within 3 AU (roughly 450 million km) of the Sun. However, some comets, like Hale-Bopp, which have a high proportion of exotic “ices” like carbon monoxide, begin producing fountains of gases some 20 AU from the Sun! When active comets are the same distance from the Sun as Earth, their nuclei have jets and are surrounded by a coma consisting of gas, dust, and rocks and they may have a gas tail and a dust tail. Comets are most productive when closest to the Sun, a point called their “perihelion” (from the Greek peri, meaning “near,” and helios, meaning “Sun”).
As a comet moves away from the Sun, it will react less and less to it and eventually will cease reacting at all.
Most comet nuclei are between 1 and 10 km in diameter,13 although some are considerably larger than this. Hale-Bopp is reckoned to be about 40–70 km in diameter.14 The Great September Comet of 1882 is believed to have had a nucleus of 50 km in diameter.15 However, there are nuclei that are “giant” size. The asteroidal comet and centaur16 95P/Chiron has a nucleus 166–233 km in diameter.17 Some long-period comets are extraordinarily large, 100 km in diameter or greater.18 The sire of the sungrazer comet family is thought to have been 100–120 km in diameter.19 Sarabat’s Comet of 1729 is believed to have been at least 100 km (some say 300 km).20 Bailey et al. estimate that comets larger than 100 km cross Earth’s orbit approximately once every 400 years and come within Jupiter’s orbit once every 70 years or so.21
Coma
FIG. 5.4 A sketch of the remarkable coma of Donati’s Comet, 1858. One side of the comet’s nucleus was especially active whenever the nucleus’s rotation brought it back into sunlight. It is this that gave rise to the strange haloes, or envelopes, on the sunward side of the coma. Image from George F. Chambers, The Story of the Comets (Oxford: Clarendon, 1909), plate 17 fig. 53 (opposite page 137).
A comet’s coma, or head, is the distinctive blotch of bright light on the sunward side of the comet. Usually it is the brightest part of the comet.
A coma is somewhat like an atmosphere around the nucleus, the innermost layer being denser and brighter and the outermost layer less dense and bright.22 Immediately around the nucleus is the brightest region of the coma, known as the pseudonucleus.23
Comas may be anything from very small to mind-bogglingly massive. In space, the diameter of Hale-Bopp’s coma is reckoned to have been twice the diameter of the Sun in 1996,24 and Comet Holmes grew to be more than 5 times the diameter of the Sun in 2007–2008.25
From the perspective of Earth, comas can look extremely large, because they really are huge in space and/or because they are so close to us.26 Due to the fact that they made especially close approaches to Earth, the relatively small comets Hyakutake (with a nucleus about 4 km in diameter) in 1996 and Lexell in 1770 appeared respectively to be 527 and 6–828 times the diameter of the Sun. If the large comet Hale-Bopp had come closer to the Sun and Earth, it would have appeared to be something like 4 or 5 degrees in diameter when 0.1 AU away from Earth29 and considerably larger if nearer still. Astronomers believe that Earth’s skies hosted countless comets with huge, bright comas in the past.30
Generally speaking, with respect to shape, comet comas fall into two major categories. Some are more circular (“globular”), and others are oval (elliptical) (fig. 5.5), such as the comets Hale-Bopp and Ikeya-Zhang, or fan-shaped (parabolic), like Tebbutt’s Comet of 1861.
FIG. 5.5 A comet with a globular coma compared to a comet with an elliptical coma. Image credit: Sirscha Nicholl.
Globular comas often have a blue or green hue, because they are more gassy than dusty.31 Elliptical and parabolic comas often appear more yellowish, because they are dustier. The most spectacular comets in history, heavy dust-producing comets that have made particularly close passes by the Sun (coming within the orbit of Mercury), tend to have elliptical or parabolic comas.32 Because of the Sun’s intense radiation pressure, the dust expelled sunward by the nucleus is immediately forced back behind it, with the result that the nucleus is very close to the sunward side of the coma. These types of comas typically merge into their tails (see fig. 5.6).
FIG. 5.6 A false-color image of Comet Ikeya-Zhang on March 18, 2002. Image credit: Bojan Dintinjana and Herman Mikuz, Črni Vrh Observatory, Slovenia.
Tails
Basically there are two types of tails that comets may (or may not) have: (1) gas (sometimes called “plasma”) tails, which are straight, bluish, structurally fine, and point almost exactly away from the Sun; and (2) dust tails, which are famously yellowish white and thick and may be curved.33
One might imagine that comet tails are like long loose hair flowing out from a motorcyclist’s head as she accelerates along at breakneck speed.34 However, comet tails always point in the direction opposite the Sun, regardless of whether they are moving toward it or away from it (see figs. 5.7–8).
FIG. 5.7 Tail orientation at different stages of a comet’s orbit. Image credit: Sirscha Nicholl.
FIG. 5.8 The orientation of Halley’s Comet relative to the Sun as the Sun makes its way through the constellation of Leo the Lion in 1531. From Petrus Apianus, Practica auff das 1532. Jahr. Image credit: Crawford Library of the Royal Observatory, Edinburgh, Scotland.
Gas tails are formed when the electrically charged gas particles that exploded toward the Sun from the nucleus are pushed by the solar wind directly back behind the nucleus. Because of this, gas tails are generally straight, narrow, and point directly away from the Sun. The sunlight causes them to fluoresce.
Dust tails are formed when small dust particles expelled from the comet’s nucleus toward the Sun are pushed behind the coma by solar radiation pressure. Because dust particles travel more slowly than gas particles, they lag behind the comet, with the result that dust tails tend to be more curved and broad than gas tails and not directly anti-solar, but rather angled back toward the direction from which the comet has just come (fig. 5.7). The section of the dust tail farthest from the coma is less dense, less bright, and more curved. A tail’s curvature and length are typically greatest just after the comet’s closest encounter with the Sun, when it is traveling fastest and is at its most productive. In the case of a comet steeply inclined to the plane of Earth’s orbit around the Sun (the “ecliptic”35; see fig. 5.24 and fig. 7.12), this curvature may be very apparent to Earth-dwellers, and the dust tail may seem wider and be easily distinguishable from the gas tail. However, where a comet’s orbit is angled narrowly to the ecliptic plane, the dust tail will generally appear to Earth-dwellers to be narrower, straighter, brighter, and longer, and will combine with the gas tail to form a single tail.36
Some comet tails are mindbogglingly large. Especially long and wide tails are associated with very productive comets that begin reacting to the Sun when far from it. Some hyperactive small nuclei (like that of Hyakutake) produce extraordinarily long tails. But the dream scenario for a long comet tail is a large, very volatile nucleus.
The longest known comet tails in space are Hyakutake’s at 3.8 AU (570 million km
),37 that of Messier’s Comet of 1769 at 3.5 AU (520 million km),38 and that of the Great March Comet of 1843 at 2.15 AU (330 million km).39 See fig. 5.9.
As regards the apparent lengths of comets in history, the longest is a 300-degree one recorded in the year AD 905 by the Chinese (C/905 K1).40 200-degree comets were seen in AD 89341 and 1618.42 Tebbutt’s Comet of 1861 was 120 degrees long, sufficient to extend two-thirds of the way across the dome of the sky.43 Great comets in 1618 and 1769, as well as Halley’s Comet in AD 837, were longer than 90 degrees, and many other comets in history peaked at 90 degrees (e.g., 1106, 1680, 1843, 1910 [Halley], 1996).44 The apparent length of a comet is determined not just by the actual tail length but also by the comet’s brightness, its distance from Earth, and the angle of the comet relative to the Sun and Earth.
FIG. 5.9 Different comet tail lengths compared. The outermost planet is Jupiter. Image credit: Sirscha Nicholl. Here, as elsewhere, images of the planets are courtesy of NASA.
Comet dust tails may also be very wide. The tail of Donati’s Comet (1858; fig. 5.12) was as wide as the Big Dipper’s handle (16 degrees),45 while that of Tebbutt’s Comet (1861; see fig. 5.21) was as broad as the distance between the top two stars in the Big Dipper’s bowl (10 degrees).46 Halley’s Comet in AD 837 was as wide as four fingers at arm’s length (7–8 degrees), and in 1066 it was as wide as the Big Dipper’s bowl is high (5 degrees), as were comets in 1106 and 1471.47 In 1965 the end of Comet Ikeya-Seki’s tail was as wide as the distance between the 3 stars on Orion’s belt (3 degrees).48 In addition, the Great Comet of 1577 (fig. 5.10) was as wide as the Andromeda Galaxy (2.5 degrees).49 The Great Comet of 1680 (figs. 5.15; 6.7; 10.19) was described as being “like a very wide belt” that stretched from one side of the horizon to the other with little difference in its breadth50 (2 degrees).51
