The Great Christ Comet (18 page)

The dream scenario for antitail development is a narrowly inclined and very productive comet making one of its first perihelion visits.
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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,
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although most of the time this was not visible to the naked eye (
fig. 5.14
).

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).
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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.
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Comet brightness is dependent on many factors, such as the comet's size, its productivity, and its proximity to Earth.
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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.
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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.
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Tycho's comet of 1577 seems to have had an absolute magnitude of -1.8,
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and may have had an apparent brightness greater than magnitude -8.
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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.
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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.
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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.
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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 -17
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or -15 to -20,
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and the twentieth century's brightest comet, Ikeya-Seki of 1965, which was -15.
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Other noteworthy apparent magnitudes include the Great Comet of 1577 and the Great Southern Comet of 1865, both of which reached -8.
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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.
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The stunning Comet McNaught in 2007 climaxed at -5.5.
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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.
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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.
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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.
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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
),
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and Comet West in 1975 when it was 6½ degrees from the Sun.
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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.
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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.
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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, ISON 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.
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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.
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That is a half-a-million-fold intensification of brightness.

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.
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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.
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