The Great Christ Comet (43 page)

October 15, 6 BC: Virgo appears to begin active labor.

October 19, 6 BC: A meteor storm radiates from Hydra's tail.

October 20, 6 BC: The cometary baby, having descended, has completely emerged from Virgo's womb and so is regarded as having been born.

Between October 23 and 25, 6 BC: The Magi leave Bab­ylon on their mission to worship the Messiah in Judea.

SCENARIO A

Between November 23 and 25, 6 BC: The Magi arrive in Jerusalem and, that same evening, are ushered by the Star to Bethlehem. Later that night the comet stands as it sets, pinpointing the house where Mary and Jesus are.

Between November 26 and 28, 6 BC: The Magi depart Bethlehem to return home to Bab­ylon.

November 29, 6 BC: Mary, Joseph, and baby Jesus visit the Jerusalem temple on the 40th day to fulfill their religious obligations, and return to Bethlehem.

November 30–December 2, 6 BC: Herod the Great orders the Massacre of the Innocents in the vicinity of Bethlehem.

SCENARIO B

November 29, 6 BC: Mary, Joseph, and baby Jesus visit the Jerusalem temple on
the 40th day to fulfill their religious obligations and return to Bethlehem.

November 29–30, 6 BC: The Magi arrive in Jerusalem and that same evening are ushered by the Star to Bethlehem. Then, later that night, the Star leads them to the house where Mary and Jesus are.

Between December 2 and 6, 6 BC: The Magi depart Bethlehem to return home to Bab­ylon.

Between December 3 and 7, 6 BC: Herod the Great orders the Massacre of the Innocents in the vicinity of Bethlehem.

We must now turn to make some observations about the comet based on our orbital elements.

Long-Period Nearly Isotropic Comet

Within the category of long-period comets, the Christ Comet is a member of the broad family of “Long-period nearly isotropic” (NI) comets.
19
Basically, this is all the long-period comets that are not part of the sungrazer class—sungrazers are reckoned to total about one-third of all comets (if one assumes that recent centuries are representative).
20
The sungrazers have in common that they all approach within approximately 0.01 AU or 0.05 AU of the Sun. Astronomers also use a category of “sunskirters,” which includes all comets that have a perihelion distance that is less than 0.1 AU but greater than that of the sungrazers.
21
According to our orbit, the Magi's Comet came as close as 0.119 AU to the Sun, which is very close, but not quite as near as the sunskirters.

Narrowly Inclined NI Comet

NI comets have a wide variety of inclinations across the full spectrum of 0° to 180°. Strikingly, however, most NI comets whose orbits have been calculated have inclinations between 40° and 160°. Of those between 0° and 40° and between 160° and 180°, the smallest proportion fall within the 170–180° range.
22
Therefore what we have in the Christ Comet may be relatively rare for comets—an essentially ecliptic, retrograde long-period comet. In this respect the Christ Comet, with its 178.3-degree inclination, is like Comet Lulin, the brightest comet in 2009,
23
which had an inclination of 178.4 degrees. It is also reminiscent of Comet Tempel of 1864 (C/1864 N1), which had an inclination of 178.13 degrees and came to within 0.1 AU of Earth on August 8, 1864, moving from the eastern morning sky to the western evening sky at that time.
24
Narrowly inclined comets sport relatively straight dust tails, from Earth's perspective, as opposed to the more curved dust tails of steeply inclined comets. Of course, the tails of narrowly inclined comets are curved in outer space, but the curvature is not apparent to those on Earth because it occurs on the plane on which Earth orbits the Sun.

Reminiscent of Comet Hale-Bopp

The fact that the Christ Comet was seen by the naked eye so long before perihelion is reminiscent of C/1995 O1 (Hale-Bopp). Hale-Bopp was first seen 10½ months before perihelion, when it was still 4.37 AU from the Sun. It has been calculated that Hale-Bopp started form
ing its coma roughly 18 AU from the Sun.
25
The Bethlehem Star comet was first observed on or before December 10–17, 7 BC, about 9½ months before perihelion, when it was 4.71–4.63 AU from the Sun. Its coma probably began to form out beyond the orbit of Uranus (20 AU from the Sun).

Comets such as Hale-Bopp and the Magi's Star begin degassing so long before perihelion because they contain relatively high volumes of extremely volatile materials such as nitrogen, carbon monoxide, and methane.
26
These materials begin reacting to the Sun at long distances from the Sun, whereas water-ice begins to degas only when within 3 AU.
27

Brightness

Having determined an orbit, we must now turn to the matter of the Christ Comet's brightness. A comet's intrinsic brightness is called its absolute magnitude. Its brightness as it appears to observers from Earth at any given point in time is called its apparent magnitude.

The starting point of any investigation of a comet's brightness is to determine the apparent magnitude at first observation. Investigations of ancient cometary apparitions have determined that a tailless comet must attain to an apparent magnitude of about +3.4 to be observed with the naked eye.
28
It seems wisest therefore to assume that the Bethlehem Star comet had a +3.4 apparent magnitude when it was discovered.
29

Next, the pattern of development of the comet's brightness must be estimated. Comets usually brighten exponentially as they approach the Sun. This increase in brightness is directly related to the increase in cometary degassing as the comet nears the Sun. The steepness of the rise in brightness as it nears the Sun and of the decline in brightness as it moves away from it is called the brightness slope, which is expressed as the value of n. The higher this value, the steeper the brightness slope is.

In the absence of adequate data to refine the pattern of cometary brightness development, scientists tend to assume that n=4.
30
This assumption is based on the idea that this is the average value for comets.
31
According to recent analysis by Schmude, long-period comets can have values of n from -2 to over 11, but most are grouped between 2 and 6, with the bulk of them concentrated between about 3 and 5, and the approximate average being 4.
32
For example, while Bennett's Comet of 1970 had a brightness slope of about 5, Hale-Bopp's was about 3. Accordingly, anyone assuming that Hale-Bopp's display in the inner solar system would abide by the average brightness slope n=4 would have been slightly (but not greatly!) disappointed.

In the tables accompanying my discussion of the Christ Comet's apparition, I shall offer brightness estimates based on the following values of n: 3, 4, and 5. It is likely that the bright
ness slope of the long-period Bethlehem Star comet comes within this range of values.

In establishing the correct value of n, we have more to go on than merely estimates based on the comet's probable apparent magnitude at first observation—we have data regarding the comet's performance in the inner solar system. Just as the astronomical community modifies the value of n in response to further observations of the comet after discovery, so also we are in a position to narrow down the brightness slope of the Bethlehem Star comet based on what we know of its behavior around the time of its closest encounter with the Sun.

With our range of possible values of n, the apparent magnitude at first observation, and the orbit, we are in a position to determine the comet's absolute magnitude, that is, the brightness of the coma as a whole if it were precisely 1 AU from both the Sun and Earth. Since we know the range of dates within which the Bethlehem Star was first observed, we can work out a range of possible absolute magnitude values for each brightness slope (i.e., n=3, n=4, and n=5; see
table 9.2
).

If n=3, the absolute magnitude was between -5.2 and -8.1. If n=4, the absolute magnitude was between -6.8 and -10.4. If n=5, the absolute magnitude was between -8.5 and -12.7. Those familiar with comets will immediately grasp how astonishing these values are. The intrinsically brightest comets observed in recent centuries are Sarabat's Comet of 1729, with its absolute magnitude of between -3 and -6, and Hale-Bopp, which had an absolute magnitude of -2.7 in the early stages of its apparition. Even if we adopt a brightness slope (n) of just 3 and assume that the 6 BC comet was first observed at the latest possible time (December 10–17, 7 BC), the Bethlehem Star comet finds itself in an exclusive league with Sarabat's Comet and the progenitor of the Kreutz Sungrazers. If n=4, then the Christ Comet is distinguished as the intrinsically brightest comet in recorded history.

With an estimated value of absolute magnitude, we are in a position to calculate how bright a comet should become over the course of its visit to the inner solar system—that is, what its apparent magnitude will be. With respect to the Christ Comet, if we consider September 30, 6 BC, just a few days after perihelion, the comet's apparent magnitude would have been dramatic (
table 9.3
).

If n=3, then the comet would have been between -10.7 and -13.6 when it was seen in Virgo's belly before dawn on September 30, 6 BC. If n=4, the comet would have attained to between -14.1 and -17.7. If n=5, it would have been between -17.6 and -21.8.

If we remember that the apparent magnitude of the full Moon is -12.6 and that of the Sun -26.7, we can get some idea of how re
markable the comet's brightness would have been according to these statistics.

One major caveat should be mentioned: comets do not always develop in a very neat and orderly manner. With respect to the brightness curve and hence absolute and apparent magnitude values, they can vary within a single apparition. Comets may shift from one pattern of brightness development to another at a certain distance from the Sun or have one pattern before perihelion and quite another one afterwards.
33
Comets with perihelion distances of less than 1 AU, like the Bethlehem Star comet, often display a very different pattern of activity and brightness after perihelion than before. Moreover, sometimes comets have outburst events that dramatically increase their brightness and size. Therefore, as Schaaf points out, “Absolute magnitude and the brightening factor can only be regarded as useful, not perfect, guides for helping to predict and characterize a comet's brightness and brightness behavior during an apparition, or during part of one.”
34

At this point it is important to pull together some of the historical data that speaks to the Bethlehem Star's brightness. Ignatius, in his letter
To the Ephesians
, wrote of the Star in terms that suggested it was especially brilliant:

A star shone in heaven [with a brightness] beyond all the stars; its light was indescribable, and its newness caused astonishment. And all the rest of the 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.

According to Ignatius's authoritative-sounding statement (which was apparently rooted in first-century tradition), the Star was far brighter than all the stars, by which he is evidently including the planets such as Venus, which has a maximum apparent magnitude of -4.89.

Is Ignatius claiming that the Star was brighter than the Moon and even the Sun? That is certainly what commentators believe. If he is claiming this, we must allow for hyperbole but should probably take it to mean that the Star had an apparent magnitude more impressive than that of the full Moon (-12.6).

According to the less reliable
Protevangelium of James
(21:2–3), the Magi reported, “We saw an immense star shining among these stars and causing them to become dim, so that they no longer shone; and we knew that a king had been born in Israel.”
35
This description suggests that the comet's brightness was greater than the Moon's. Moreover, this document portrays the Star as exceptionally large. For a very large comet to be extraordinarily bright means that the apparent magnitude must be most remarkable.
36

Apparent magnitude compares the brightness of
complete
entities to the star Vega (the second brightest star in the northern celestial hemisphere, after Arcturus). It does not compare the entity based on the brightness of a
set portion
of its area, that is, its “surface brightness.” For a large object to be bright enough to bleach out the light of the stars, its apparent magnitude has to be extraordinary, because its overall brightness is distributed over a wider area, which means it is diluted. It is much like the brightness of a beam of light on a wall cast by a flashlight. When the beam is small, the brightness is more concentrated. When it is large, the brightness is more diffuse. If you compare a set area of the beam when small to a set area of it when large, the brightness of the set area of the more compact beam would, of course, be more intense than that of the more extended beam. That, in a nutshell, is surface brightness. On the other hand, do not forget that the same amount of light is being distributed—think of that as the apparent magnitude. The difference between apparent magnitude and surface brightness is the difference between the overall brightness of your whole computer display screen and the brightness of the average pixel on it. In the case of a large comet, the brightness of the whole coma (the apparent magnitude) needs to be high for the brightness of each small section of it (the surface brightness) to be really intense.