The Great Christ Comet (46 page)

During its procession, the comet's motion through the heavens was reminiscent of a bobsleigh from the point that it is approaching its greatest speed until it is almost stationary. All the while, the comet was growing in apparent magnitude. By the end of the journey it had the magnitude of the full Moon! During its procession the comet's coma appeared large, larger than any coma in the last few hundred years. As for the tail, although it briefly decreased in apparent length at the end of July and the start of August, thereafter it grew very long very quickly. By mid-September the whole comet would have stretched halfway across the sky. We shall now take a closer look at a few key stages of the comet's majestic procession.

Water from Aquarius's Water Jar

We consider first the early stages of the comet's journey across the heavens. Ever brightening and growing on its way, the comet revisited the water flowing from Aquarius's water jug. Indeed, our orbit would suggest that, from July 25 to August 8, the appearance of the comet in the sky was radically reoriented, its direction swinging like a pendulum from left to right. It was reminiscent of a shuttlecock or birdie that is hit on the cork side and turns mid-air to fly cork-first. Initially the head was down to the left and the tail pointing up to the right, but by the end of the maneuver the head was down to the right and the tail up to the left.
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As it reoriented itself in this way, the tail was very short. It was shortest on August 4–5, when the comet was close to upright (relative to the constellation figure). As for the coma, it was probably very large at this point (its longest side perhaps something like 5–6 degrees in diameter).

What was happening in outer space was that Earth was crossing the “interchange,” or “crossroads,” between its orbit and that of the comet (see
fig. 10.9
). As a result, from the perspective of Earth the comet's tail seemed to shorten, widen, and brighten dramatically as it shifted from one side to the other. The tail's intensified brightness was due to the fact that more tail debris was concentrated into a small area of the sky in Earth-dwellers' line of sight. Moreover, the geometry of the comet-Earth-Sun at this time would have boosted the brightness by something like 1 magnitude. Essentially, the comet was almost behind Earth from the perspective of the Sun (the phase angle of the nucleus peaked at under 3 degrees on August 4–5!) and the backscattering effect meant that the dust in the coma and tail became slightly brighter. The great size of the coma was because the comet was very close to Earth. Occurring in the context of Aquarius the Water-Bearer (in Bab­ylo­nian thought, the Great One), the comet's large, bright, and probably oval coma may possibly have appeared to be a water-jar and the short tail to be a stream of water issuing from it, on August 2 to 7. Over those nights, the angle of the coma and tail changed, making it look like the water jar was being swung by the constellation figure from his right to his left, rather like a pendulum.
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The constellation figure may well have seemed to observers to have come to life.

On the nights of August 2/3 to 7/8, the comet was between 0.485 AU and 0.384 AU (approximately the same distance as Mercury is from the Sun) from Earth and heading straight toward the crossroads and the Sun, so that during this time humans would have been able to look directly at the sunward and active side of the large, oncoming comet. Had they been able to see what was happening in the coma, they would have caught a glimpse of magnificent fountains of dust jetting out from the nucleus (cf. figs. 10.10–11).
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The comet's apparent magnitude would have been at least -7.1
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at the height of the display on August 5, 6 BC. Of course, the fact that so much dust was now in Earth's line of sight—and the backscattering effect—would have meant that the comet's brightness was greater than the apparent magnitude value would suggest. On the other hand, the large size of the coma would have diluted the intensity of the brightness.

Jews might have been prompted to reflect on Isaiah 12:3—“With joy you will draw water from the wells of salvation” (cf. Isa. 32:1–2; 33:17–21; 35:1, 6; 44:2–3; 51:3)—and Numbers 24:7, where Balaam prophesied concerning Israel that “Water shall flow [or overflow] from his buckets, and his seed shall be in many waters.” In addition, the Feast of Tabernacles in the fall featured a rite in which water from the Pool of Siloam was poured out at the base of the altar of burnt offering in the Jerusalem temple.

In Bab­ylo­nian tradition, the waters of the Great One could be associated with judgment and danger in the form of severe rainstorms and floods, but they were more commonly associated with the coming of the rains and hence with fertility, prosperity, refreshment, purification, and cleansing.
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Occurring in the summer, this celestial phenomenon would more naturally have been interpreted as having a positive meaning.

From early August onward, the comet appeared around sunset and remained visible until it set in the southwest. That meant that everyone could see the comet in all of its glory. It had become a third great light in the heavens.

The Archer's Arrow

As the comet raced through Capricornus, its apparent magnitude would have become increasingly impressive, since the comet was not only getting closer to the Sun but was also coming closer to Earth. At the same time, this was counterbalanced by the fact that the brightness that the comet did have was being spread over a wider surface area.

By the time the comet entered Sagittarius the Archer, on August 14, 6 BC (according to our orbit), its apparent magnitude would have been at least -8.2,
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and it was only 0.334 AU from Earth (1.26 AU from the Sun), the closest it would come to Earth before making its dramatic U-turn around the Sun (
fig. 10.12
). That, incidentally, is closer than most of the historically great comets have ever come to our planet! The comet may have been about 40 degrees long, the coma itself something like 7 degrees in diameter (major axis). Earth now found itself at a perfect vantage point from which to watch the comet as it hurtled toward perihelion at breakneck speed.

Over the next month or so the comet would have become an increasingly bright light shining in the deep darkness. Even as the coma shrank in apparent size as the comet moved away from Earth, its apparent magnitude was steadily becoming greater as it neared the Sun—which meant more brightness concentrated in a smaller area. In addition, more and more of the comet's tail would have been becoming visible to human observers each night. This was because the comet was continuing to race toward the interchange and was degassing more as it neared the Sun, while Earth, after crossing the underpass, was assuming a much better angle from which to see the whole comet. In fact, from the per
spective of Earth, you would almost have thought that the tail was simply stretching as the comet raced across the constellations, because the end of the tail remained firmly fixed in Aquarius, back at the pivot point of the comet's radical pendulum-swing reorientation. When the comet head departed from Aquarius, the whole comet would have been just over 14 degrees in length, but by the time the head reached the far side of Sagittarius, it would have been over 70 degrees long.

The comet must have seemed to observers to have transformed itself into a dramatic celestial arrow! The coma was the arrowhead and the tail was the shaft and fletching. It had become a flaming arrow fired from the Archer's bow! (See
fig. 10.13
.)

To the Bab­ylo­nians and Greeks, the Archer Pabilsag, or Sagittarius, was a winged half-horse and half-human creature known for his great bow and the arrow that is famously aimed right at the heart of the Scorpion (the star Antares). To the Bab­ylo­nians, at least, Scorpius encapsulated the forces of wickedness, darkness, and death.
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Over the nights of August 17–20, it looked like the Archer was sliding the awesome arrow into his bow and then firing it.
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Even by the time the comet set on the 20th, the arrow could already be seen “in flight” toward Scorpius, such was the rapidity of the comet's apparent movement across the starry heavens at that time. The cometary arrow's movement would have been detectable in single observing sessions over the following 1½ weeks.

On August 25 the comet crossed Earth's orbit. At that point Earth was about half the distance from the comet that both Earth and the comet were from the Sun. The comet, both its coma and its tail, would have been brilliant and large. Even as the giant arrow flew through the heavens, it would have grown in length. Sure enough, the cometary arrow, having grown to well over 80 degrees long, and having an apparent magnitude of at least -8.4,
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struck the Scorpion's heart (it
came very close to Antares on August 27–28). The cometary arrow continued on its celestial flight path until it hit the middle of the Scorpion's forehead (
δ
Scorpii) on August 31.
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(See
fig. 10.13
.) By that time the comet would have extended more than halfway across the sky.