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Specola Guestbook: May 4, 1910 – van de Sande Bakhuyzen — 1 Comment

  1. Comet Biela was an important link in our understanding of the connection between comets and meteors. I wrote a chapter about this in a book published many years ago called “Cosmic Pinball”… here’s the highlights:

    In 1772, the French astronomer Montagne had discovered a faint comet. It never grew bright enough to be seen with the naked eye, so once its position and appearance in the telescope had been recorded, the comet was pretty much forgotten. His countryman Jean-Louis Pons likewise found a comet in 1805, this one much brighter. But it wasn’t until 1826, when the German amateur Wilhelm von Biela discovered a comet, that a good enough orbit was determined to make it clear that all three were the same comet. Indeed, von Biela’s comet had a period of a only bit more than six years, orbiting between Jupiter and the Sun. More often than not it was unseen, as it travelled a path that usually took it far from Earth.

    Usually, but not always. Further calculations indicated that it would pass within 20,000 miles of the Earth on December 3, 1832. This caused a mild panic among those fearing a cosmic catastrophe but, as predicted, the comet just missed the Earth.

    The next pass by, in 1839, carried the comet on the opposite side of the Sun from the Earth, where it could not be observed. The 1845 apparition was visible, however. And Herrick and Bradley, observing at Yale, noticed something new this time. The comet now had a faint companion.

    Over the next three months observers watched, fascinated, as the companion comet grew brighter; developed a tail; then *two* tails. The larger comet split, each piece developing its own tail, with a path of light connecting them. Clearly, something was breaking the comet apart.

    The next apparition was due in 1852. At that time, two comets appeared, but both very faint. They finally grew too faint to see in September of that year. The 1859 apparition again was too far from the Earth to be observed. Which brings us to 1866; and that year, when it should have been visible, Comet Biela could not be found.

    Early December of the following year, a meteor shower called the Andromedids was especially notable. This annual shower had been known since the mid 1700’s. Peaking around December 6 or 7, it was noted that the swarm of meteors were exceptionally notable in 1741, then again in 1798; and again in 1830, and 1838. Only during the 1847 apparition were there good enough records kept to begin to calculate where the meteors were coming from; these records were improved in 1867.

    Note that there was no obvious six-year periodicity to these events, and nothing to directly tie them to any comet. It was only by patiently working out an orbit for this dust, following the newly-published techniques of Schiaparelli, that in 1867 both the Edmund Weiss (an Austrian) and Heinrich d’Arrest (a German of French ancestry who did most of his work in Denmark) announced independently that this meteor stream had the same orbit as Comet Biela.

    This was startling news. Schiaparelli, established as the expert in the field, had proposed that comets were formed when meteor swarms coalesced into a solid object. But here was a meteor stream that existed even after its comet had disappeared.

    Instead, Weiss proposed the opposite idea. Perhaps the meteor streams arose when dust was spun off from the comets. The dust for the meteors would follow roughly the same orbital path as the comet, but it could be easily perturbed and thus travel ahead or behind the comet. Eventually the entire orbit would be filled with the comet’s dust.

    Recall that in December of 1832, Comet Biela had nearly hit the Earth. From that, it was clear that early December was when the Earth regularly crossed Biela’s orbit, whether or not the comet was there at that time. Even if the comet were not there, its dust would be. But the density of dust, and so the number of meteors, should be especially thick near the time of the comet’s passing, or once every six years… with enough spreading so that the year before or after the comet’s passing should also provide a meteor storm visible in some part of the world. Since more often than not these storms would be visible only in remote areas, like over the Pacific Ocean, and could occur a year before or after the sixth-year return of the comet, it was not surprising that you couldn’t see any regularity to the occurrence of meteor swarms in European skies.

    But Weiss did one more bit of calculating. When he included the perturbing effects of the other planets on the orbiting dust, he recognized that in 1872 and 1873, which by the six-year pattern were expected to be especially good years for Andromedids, the planetary perturbations would move the date from early December to around November 28. He was one day off. On the night of November 27, 1872, observers from England to Italy counted as many as 30,000 meteors.

    One of the observers in Germany, Klinkerfues, telegraphed to a colleague, Pogson, in India: “Biela touched Earth November 27. Search near Theta Centauri.” Pogson did, and discovered a faint comet. It was probably a piece of the original Biela’s Comet, thrown off years before. It was the last piece of Biela ever observed.

    The meteor storms lived on, giving remarkable results in November 27, 1885 and November 23, 1892. However, without the comet constantly feeding the orbit with dust, the intensity of the storms eventually began to wane. On November 24, 1899, only about 90 meteors per minute were reported. Since then, the Andromedids have become only a minor shower (centered nowadays in early October), their six-year storms having faded completely from sight… but not before having demonstrated conclusively the connection between comets and meteor storms.

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