Great Comets in HistoryDonald K. Yeomans (2007)
Jet Propulsion Laboratory/California Institute of Technology
A relative few comets are so visually impressive as to be termed "great comets". Just the right set of circumstances must occur. Far from the sun, the solid portions of comets, which consist mostly of water ice and embedded dust particles, are inactive. They are not large enough to be seen with the naked eye. However, when near the sun, the icy cometary surfaces vaporize and throw off large quantities of gas and dust thus forming the enormous atmosphere and tails that make comets so visually striking. It is the fluorescing of these gases, and particularly the reflection of sunlight from the minute dust particles in the comet's atmosphere and tail, that can make these objects so visually impressive. However, this activity by itself does not insure that a comet will become a great comet. An active comet can only become great by making a particularly close approach to the sun so that it produces enormous quantities of gas and dust or by making a close approach to the Earth so that its tail can be easily viewed. In either case, great comets must be seen in a dark sky.
While applying the appellation "great comet" to a particular cometary return is a subjective process, the following Table is an attempt to list the great naked-eye comets that have been reported. With the single exception of periodic comet Halley, all the tabulated comets have passed through the inner solar system either for the first time or the intervals between their returns are measured in thousands or millions of years.
The first tabular entry gives the approximate date when the comet was first reported as a naked-eye object. The following column gives the approximate observational interval (in days) during which the comet remained a naked eye object. The next two columns give the date and distance in astronomical units when the comet reached its closest point to the sun (perihelion). One astronomical unit is approximately the mean distance between the sun and Earth. The following columns give the date and distance when the comet reached its closest point to the Earth (perigee), and the date and apparent magnitude when the comet reached its brightest in a dark sky. A diffuse cometary image becomes noticeable to the naked eye when it reaches a magnitude of approximately 3.4 in a dark sky. Compared to a comet whose magnitude is 4, a 3rd magnitude comet would appear 2.5 times brighter and a magnitude 2 comet would appear 2.5 x 2.5 = 6.3 times brighter still, etc. The brightest star in the sky (Sirius) has an apparent magnitude of -1.5. At its brightest, the planet Jupiter appears at magnitude -2.7.
- Reported by the Greek historian Ephorus to have split into two pieces.
- The Chinese reported that the tail spanned more than 70 degrees.
- The closest approach to the Earth that comet Halley has ever made. On Apr. 13, the comet's tail was more than 90 degrees in length.
- This comet passed very close to the sun and is perhaps the progenitor of the sungrazing comets of 1882 and 1965 or that of 1843.
- Korean observers reported the comet was visible during the daylight hours on September 9th (probably during twilight only).
- On July 26, Chinese observers reported the tail spanning 100 degrees.
- Chinese observers reported cloudy weather from October 11 until Nov. 9, at which time the comet had passed behind the sun.
- In mid-March, the comet entered solar conjunction and there were reports that it was a daylight object for 8 days.
- The comet was discovered emerging from solar conjunction.
- Last observed on April 20 as it approached solar conjunction.
- This was the first comet discovered with the aid of a telescope (on Nov. 14).
- Visible in daylight only 12 degrees from the Sun on February 27.
- Tail reported as longer than 90 degrees near Earth close approach.
- On the date of perihelion, this sungrazing comet was observed in daylight nearly one degree from the sun.
- Comet observed in southern hemisphere.
- The Great September comet was a brilliant object that was observed very close to the sun, and split into at least four separate pieces near perihelion. This comet and comet Ikeya-Seki in 1965 are believed to be members of the same family of sungrazing comets.
- This comet was easily observed on January 17 only 4.5 degrees from the sun. It is often confused with the later apparition of comet Halley in mid-1910.
- On December 18, this comet was seen in daylight only 5 degrees from the sun. At the end of December, the tail was reported to be nearly 40 degrees in length.
- Sungrazing comet Ikeya-Seki split into two or possibly three pieces near perihelion. Toward the end of October, the impressive tail reached lengths in excess of 45 degrees.
- The tail of comet Bennett reached 10 degrees in mid-March.
- Comet West's impressive broad tail reached a length of 30 degrees on March 8. Near perihelion, the comet split into four pieces.
- The observational interval is based on the time during which the comet had a total magnitude of 3.4 or brighter.
- Naked eye object during January except when too close to the sun near perihelion and a few days thereafter. Maximum brightness in mid-January dramatically enhanced due to forward scattering of sunlight by dust. Impressive dust tail reached 30 degrees and had obvious striae, similar to comet West in 1976
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