The Comets Of Caroline Herschel
By Greg Bryant

Published in the May 1997 issue of Universe

The Herschel family is well known in the history of astronomy. Their observations around the turn of the 19th century resulted in the cataloging of thousands of deep sky objects, as well as the discovery of Saturn's moon Enceladus.

Caroline Herschel, who assisted her brother William in many of his observations, was quite an achiever in her own right, making the original discoveries of many of the clusters and nebulae in the Herschel catalog. Many deep sky observers may not realise, however, that Caroline also discovered eight comets, six of which bear her name.

Comet C/1786 P1 (Herschel)

Herschel's first comet discovery came on the night of 1st August, 1786. She found the comet at a magnitude of about 7.5, though poor sky conditions meant that confirmation of the cometary nature of the suspected intruder had to wait until the following night. The first reported naked-eye sighting of Comet Herschel occurred on 17th August, and Charles Messier observed a tail 1.5 degrees long through his telescope the following night.

On the next night, 19th August, Caroline's brother, William Herschel, described the comet as being "considerably brighter" than the globular cluster M3, indicating a brightness of between 5th and 6th magnitude. Observations continued through to 26th October. Calculations show that the comet had earlier reached perihelion on 8th July at a distance of only 0.41 AU.

Comet 35P/Herschel-Rigollet

The next discovery of Caroline Herschel came on 21st December, 1788, when she found a comet around one degree south of Beta Lyrae. Like her first comet, the brightness was around magnitude 7.5, and brother William described it as "a considerably bright nebula, of an irregular form, very gradually brighter in the middle, and about five or six minutes in diameter". The comet was followed until 5th February, 1789, and its orbit was believed to be parabolic, the comet having reached perihelion on 21st November at a distance of 0.75 AU.

We now move to France, where Roger Rigollet discovered an 8th magnitude comet on 28th July, 1939, nearly 151 years later. Within a few days, L.E. Cunningham used orbital calculations so suggest that this new comet was identical with Herschel's comet of 1788, and this was subsequently confirmed. Periodic Comet Herschel-Rigollet reached a peak brightness of magnitude 7.3 in early August, but after perihelion on 9th August, it slowly faded. The last observation was made on 16th January, 1940, at Lick Observatory.

When the new system of comet nomenclature was introduced at the beginning of 1995, Comet Herschel-Rigollet was given the prefix 35P to indicate that it was the 35th periodic comet to be observed returning to perihelion. Comet Herschel-Rigollet's next return to the inner Solar System is not expected until the end of the 21st Century, something for our descendants to watch for.

Comet C/1790 A1 (Herschel)

Caroline's third discovery came on 7th January 1790, with the new comet being reported at a brightness of 7th magnitude. The comet's apparition was quite poor, being seen only on three other days, the last being 21st January. Two days beforehand, Charles Messier described the comet as nebulous with a bright condensation, and compared the comet's brightness to Pegasus' globular cluster M15. On the basis of the observations available, perihelion occurred on 15th January at 0.76 AU.

Comet C/1790 H1 (Herschel)

1790 was a good year for Caroline. Her fourth discovery, and her second for the year, came on 18th April. The comet was 7th magnitude and showed no tail. By the beginning of May, Comet Herschel had brightened to 5th magnitude and was developing a visible tail. Perihelion came on 21st May, at a distance of 0.8 AU, by which time the tail had lengthened to four degrees. Comet Herschel's closest approach to Earth came in early June, admittedly at a distance of only 0.7 AU. Nevertheless, the comet brightened to 4th magnitude with a one degree tail. The last sighting of Comet Herschel came on 29th June.

Comet C/1791 X1 (Herschel)

On 15th December, 1791, breaking a 20 month drought of comet discoveries by anyone, Herschel came across another comet, which had already brightened to 6th magnitude and was described by Caroline as a "pretty large, telescopic comet". Despite perihelion occurring on 14th January 1792 at 1.29 AU, the closing solar distance was not enough to compensate for the increasing distance of the comet from Earth, and so the comet faded, last being seen on 28th January by Messier.

Comet C/1793 S2 (Messier)

Caroline Herschel discovered another comet on 7th October, 1793, at a brightness of around 5th magnitude. Unknown to her, Charles Messier had earlier sighted this comet on 24th September, when it was 6th magnitude, and thus the comet was named after Messier. Perihelion came on 5th November at 0.4 AU, and Messier was able to recover the comet on 29th December following solar conjunction. The comet was last seen on 8th January 1794, despite still being brighter than 7th magnitude.

Comet 2P/Encke

On the evening of 17th January, 1786, the comet was first sighted in the constellation of Aquarius by the famous French comet hunter Pierre Mechain. At the time, it was likened in brightness to the magnitude 6.3 globular cluster M2.

After notifying Messier of his discovery, both comet hunters and Jean-Dominique Cassini observed the comet two nights later on 19th January. Moving rapidly into evening twilight (future astronomers would determine that perihelion was on 31st January), the comet was not sighted again, and thus an orbit could not be calculated on the basis of two observations.

Nearly ten years were to pass before the comet was sighted again. Caroline Herschel came across it while observing on 7th November, 1795. Her brother, William Herschel, noted that the comet was visible to the naked eye, while Alexis Bouvard compared its brightness to M31, the Andromeda Galaxy.

The comet was observed for a period of three weeks (perihelion was on 21st December), with other notable astronomers to observe the comet including Johann Bode and Heinrich Olbers. An orbit calculation was attempted, but the only conclusion reached was that the orbit was not parabolic.

Yet another ten years passed before the comet was "discovered" again. This time, the comet was first seen on the night of 19th October, 1805 by European observers Jean Louis Pons, Johann Sigismund Huth, and Alexis Bouvard. Huth estimated it to have a brightness of 5th magnitude, with a 5 arc minute wide coma and a tail 3 degrees long.

At this time, Johann Encke entered the picture. Encke was a German astronomer who was to later on become the Director of Berlin Observatory. Encke studied the positions that had been reported of the comet, and suggested that the positions fitted an elliptical orbit with a period of 12.1 years - incorrect, but much closer than other astronomers who were still trying to derive parabolic orbits.

It wasn't until 1818 that the comet's appearance is noted again. Pons sighted the comet on 26th November, and it brightened over the next few weeks to display a similar appearance to the globular cluster M2, just as in 1786.

The comet remained observable for nearly seven weeks, enabling a good set of positions to be determined. Whilst it was Olbers who first suggested that this comet was the same as those observed in 1786, 1795, and 1805, it was left to Encke to mathematically prove that it was the same comet returning with a period of 3.3 years.

In 1819, having obtained an orbital solution for the comet, he calculated its orbit back over time, taking into account the perturbations caused by the known planets with the exception of Uranus, and in six weeks he confirmed that the four comet appearances were in fact the same comet.

Having confirmed the periodic nature of the comet, Encke proceeded to predict the next return of the comet, with a perihelion date of 24th May, 1822. Sure enough, on 2nd June, 1822, Karl Rumker recovered the comet whilst observing at the private observatory of Sir T. M. Brisbane at Parramatta, New South Wales, Australia (the observing circumstances for comet Encke that year were not favourable for the Northern Hemisphere). This was only the second comet for which its return had been successfully predicted, the first being Halley, and in a similar fashion to Halley, the comet was named after Encke.

Thus, were it not for an inadequate number of accurate observations preventing an early reliable orbit calculation, periodic comet Encke could have easily had the alternate name of periodic comet Herschel....or comet Mechain...or comet Pons-Huth-Bouvard.

Comet C/1797 P1 (Bouvard-Herschel)

Caroline's final comet discovery came on 14th August 1797 when Eugene Bouvard and herself independently discovered the 3rd magnitude comet within a few hours of each other. Being so bright and easily visible, numerous other observers found the comet the following night. At the time of discovery, Comet Bouvard-Herschel was only 0.17 AU from Earth, and it moved even closer, passing only 0.0879 AU from Earth on 16th August.

According to reliable records, this is the 13th closest approach of a comet to Earth. The closest approach was the lost periodic comet Lexell, which passed only 0.0151 AU from Earth on 1st July 1770 (the comet of 1491 allegedly missed Earth by a distance of only 0.0094 AU but its orbit calculation is unreliable). More recent close encounters with comets have been IRAS-Araki-Alcock, which made its closest approach on 11th May, 1983 at 0.0312 AU (ranked 3rd in the records), and the Great Comet Of 1996, Hyakutake, which flew by at a distance of 0.1019 AU (equal 19th) on 25th March.

Following the close approach of Comet Bouvard-Herschel, it faded rapidly and was last seen on 31st August. Perihelion had earlier occurred on 9th July at a distance of 0.53 AU.

Caroline's comet discoveries not only established a precedent for female astronomers, but remained a record for comet discoveries by women until the 1980s, when another female astronomer with a similar first name not only beat her record but firmly established herself in the history of comets. Her name....Carolyn Shoemaker.