Published in the May 1999 issue of Universe
and the Summer 2000 issue of Comet Tales
During the last two centuries, there have been many hundreds of comets that have been observed by astronomers. Some have been little more than a minute smudge on a professional photographic plate, but many have been visible to the amateur astronomer. For the amateur astronomer who takes the time to hunt down those comets that are 10th or 11th magnitude, as part of a night's observing program, the appearance of a bright comet is truly a special moment. A rich selection of bright comets have punctuated our skies at irregular intervals, and it is these special visitors that will be the prime focus of this series, accompanied by anecdotal notes on some of the other comets of the era that set the scene.
1801 - 1807
The first comet discovery of the 19th century was made by Jean Louis Pons, from Marseilles in France. Between 1801 and 1827, Pons discovered 37 comets, a record for a single observer which still stands today (excluding satellite observatories and ground-based patrol telescopes).
Pons' first discovery was made on the night of 11th July, 1801. The comet was 7th magnitude at the time of its discovery in the constellation of Ursa Major, and was described as small, round, and without a tail. Independent discoveries of Comet C/1801 N1 (Pons) were made the following night by three other comet hunters of the time : Charles Messier, Pierre Mechain, and Eugene Bouvard. All four discoverers were based in France at the time...must have been the weather.
The comet was lost from sight after 23rd July (perihelion was on 9th August at 0.26 AU), but it introduced Pons to the comet hunting stage. Pons was the first discoverer of several comets over the next five years. One discovery was particularly interesting. On the night of 20th October, 1805, Pons, Huth, and Bouvard independently found a comet that was brighter than 6th magnitude. The comet was followed for a month, at which time it was 4th magnitude. History would show that this was the third sighting of periodic comet Encke, and it was this particular apparition that caught Johanne Encke's interest. He calculated an elliptical orbit for the comet, albeit it one with a period of 12 years. The work was forgotten until the next decade when Encke revisited it. Similarly, we'll revisit the "discovery" of Encke in a forthcoming column.
Pons developed a habit for finding comets that were recognised as periodic years later. Three weeks after observing what would become comet Encke, Pons discovered another comet in the constellation of Andromeda on 10th November, 1805. The comet was brighter than 5th magnitude at the time, and as the comet brightened by another magnitude during the next 12 days, independent discoveries were made by Bouvard and Huth. By 10th December, the comet reached 3rd magnitude as it passed just 0.04 AU from Earth. Shortly afterwards, the comet was lost from sight, but it would be seen again in 1826 when Wilhelm von Biela observed it, and the periodic nature of this comet would be revealed.
In case you feel that Pons has been hard done by, he does in fact have his name on a few periodic comets, as we will see in a future column.
The Great Comet Of 1807
Imagine discovering a comet when it was already shining at 1st magnitude. The Italian astronomer Giovanni did just this when he found a comet in the evening twilight of 9th September, 1807, not far from the 1st magnitude blue-white Spica. It must have been a magnificent setting, as Venus, Mars, and Saturn were all nearby. Comet C/1807 R1 was independently discovered by many observers, including Pons, after it passed perihelion on 19th September at a distance of 0.65 AU from the Sun. By this time, the comet had moved into the midst of the evening planetary gathering.
Initially short in length, the comet's tail grew to some 8 degrees by late September, whilst still maintaining its 1st magnitude brilliance. Amazingly, 27th September was when the comet was at its closest to Earth : a not-so-close 1.15 AU. Only two other Great Comets have never ventured closer than 1 AU from Earth. One, Hale-Bopp, was visible to us just two years ago, and the other will be covered in the next instalment of this series.
As September crossed into October, and the comet moved from Virgo into Serpens, two tails became visible : a long, straight gas tail of 10 degrees in length; and a short, curved dust tail little more than a degree long. Moving into Hercules, the comet faded from 2nd to 4th magnitude by early November, yet the gas tail remained up to 5 degrees long.
By mid-December, as the comet was fading from naked-eye visibility, it was situated not too far from the 1st magnitude star Deneb in Cygnus. During this apparition, the comet was observed by William Herschel for five months, during which time he calculated the "round and well-defined disk" within the comet's coma to have a diameter of 860km - this was not the nucleus but an area of significant condensation in the coma surrounding the nucleus. The comet's last telescopic sighting was on 27th March, 1808. It is a long-period comet, and should return in about 1,500 years.
News of new discoveries and reports on how current comets are performing, can be found in the Autumn 1999 issue of Comet Tales, available from the writer at 2/100-104 Kissing Point Road, Dundas. NSW. 2117. Annual subscriptions are $10, payable to the writer.