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Astronomical Discovery-- a personal view
By David H. Levy
I love the idea of Charles Messier. Using a small telescope, he began his career by missing out on the chance to be the first to spot Halley's comet on its first return after the great Halley himself predicted it. I like to think that Messier, disappointed in his near-miss, decided to get even by starting his own program of comet searching. Whether that was his approach or not, Messier was captivated by comets. Having found his first comet in 1760, he became so well known for his finds that within a few years Louis XV had dubbed him the comet ferret.
As he searched for comets, Messier would from time to time encounter some odd fuzzy objects. He kept a list of them. In the most famous example we have of serendipity in astronomy, more than two hundred years later, Messier's comets are long gone, but the galaxies, clusters, and clouds of gas and dust that he found during his search are still there, still precisely in the sky where he found them, and still easily visible through small telescopes. He wanted to find comets, but it is what he found on the road to those comets that makes him such a popular figure today, and which is relevant to this book. His catalogue is the first list of objects in distant space. Created to give Messier an idea of what roadblocks to avoid on the way to a comet, the catalogue now stands by itself as a way for any new observer to become familiar with what's out there.
I was well on to identifying all the Messier objects when I started my program of comet hunting on December 17, 1965, I had no idea that that the adventure would lead as far as it did, to 8 visual discoveries so far, and 13 photographic finds.
A Background of my comet hunting program
I have been comet hunting for 40 years, and over that time my program has evolved. When it began on the night of December 17, 1965 , (39 years to the day before I write this) I did not list the actual finding of a comet as the program's primary aim. In the program log that night I wrote instead what my program goals were:
1. To become very familiar with the sky through searching for comets and/or novae.
2. To discover either a comet or a nova.
3. To learn as much as possible about comets and/or novae through a research program.
I learned a lot about comet hunting in the months after that chilly December night. The first breakthrough came the following summer, when under the dark sky of the Adirondack Science Camp I was able to spot a faint, large galaxy called M101 during the course of my searching for comets on the night of July 4, 1966.
Even after accomplishing that end-- moving to the dark sky near Tucson , Arizona in 1979-- it was not until November 13, 1984 , that I discovered my first comet (Comet Levy Rudenko C/1984 V1) near NGC 6009, a cluster of stars. After 917 hours, 28 minutes, spread out over nineteen years, the second aim of my program was achieved at last.
Early in January 1987, I found my second comet (C/1987 A1) as a faint fuzzy visitor on a chilly- and rainy-- Tucson morning. The sky was pretty cloudy–in fact within half an hour of the discovery it was pouring rain! I discovered my third comet (C/1987 T1) only 107 observing hours after the second.
A Tale of Two Comets
On March 20, 1988 , I found a comet (C/1988 F1) in the predawn sky only two weeks after I met Gene and Carolyn Shoemaker for the first time. The next month the Shoemakers included the new comet on their list of fields of sky to photograph, but due to a plotting error they photographed the wrong field. The following evening, Carolyn placed the films showing Comet Levy on her stereomicroscope and quickly found what she thought was my comet. Gene looked also, but he wondered why the comet was so far from the field's center. Carolyn then tried to measure the comet's position relative to the surrounding stars. To her frustration, the stars on the field did not match any of those on the field they were supposed to be photographing. They had photographed the wrong field of sky! By trying to observe a known comet, they had discovered a new one (Shoemaker-Holt C/1988 J1).
Using these positions of the new comet, Conrad Bardwell of the Central Bureau for Astronomical Telegrams made a discovery of his own. The orbits of Comet Levy and Comet Shoemaker-Holt, he noticed, were almost identical in every respect except that Comet Shoemaker-Holt arrived at its closest point to the Sun, or perihelion, some three months after Comet Levy's closest approach to the Sun. This was the first case of a pair of related long-period comets being discovered independently. The two comets were one some 12,000 years ago which, for some reason, split apart. They are continuing their separate journeys around the Sun, moving away from each other as they go. The two comets will probably be years apart when they next return.
A series of comets
Of the next few comets that I found, the most interesting were the comet of 1990, which became bright enough to be seen with the naked eye during that summer, and a periodic comet in 1991. That comet turned out to be a new periodic comet that returns to the vicinity of the solar system every half century. For some reason it had never been picked up earlier, with one possible exception: In 1499 Chinese and Korean observers observed a comet pass from Hercules through Draco, and the Little and Big Dippers. The orbit of that comet is so similar to that of Periodic Comet Levy that it could be the same comet, although positive identification will probably have to wait until the comet returns around 2041.
Comet Hunting via Photography
In the fall of 1989 I began a new kind of comet searching. My 1988 meeting with the Shoemakers led to me becoming a partner in their comet and asteroid search which took place for a week each month at the 18-inch telescope at Palomar. Our procedure involved taking a film covering an area of sky, then taking a second of a different area, a third, and then a fourth. We would then repeat these exposures so that each area was exposed twice, the different exposures being about 45 minutes apart. During a typical night we might take 5 or 6 sets, and over a full week we would cover a considerable amount of sky. In November 1990 we discovered the first of 13 Shoemaker-Levy comets. By far the best known of these comets is Comet Shoemaker-Levy 9 (D/1994 F2), which collided with Jupiter in July 1994, producing the most spectacular collisions ever seen on another world.
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i CN-3 Record Book, 1965.
ii Brian Marsden, "The Comet Pair 1988e and 1988g" unpublished paper.
iii Donald K. Yeomans, Comets: A Chronological History of
Observation, Science, Myth, and Folklore (New York: John Wiley and Sons, 1991) 410. |
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