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DONATI'S COMET in the evening sky, 1858 October 5, with a tail 33,000,000 miles long.
The comet passed perihelion on Sept. 29, and was closest to Earth on Oct. 10. The star near the comet’s head is Arcturus.

© Chris Chatfield

On June 2, 1858, the Italian astronomer Giovanni Battista Donati, in Florence, saw through his telescope a faint nebulous spot, like a minute white cloud, moving north in the constellation of Leo. It was a comet, falling in towards the Sun, and at that moment about 240 million miles from the Earth, or somewhere between the orbits of Mars and Jupiter.

For almost three months the comet remained unremarkable and invisible save in telescopes. It seemed to be moving so slowly that astronomers had great difficulty in plotting an orbit.

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A telescopic view of Donati’s Comet, 23 August 1858. The comet was ‘almost round’; with a bright 6th-magnitude nucleus.

On August 20, though still only visible telescopically, the comet developed a small tail. Nine days later, it became just visible to a 'keen unaided eye'. Then, in September, things began to happen.

On September 6, a slight curvature of the tail was seen. On September 13, the comet was 120 million miles from Earth, with a tail 15 million miles long, which, from our planet appeared about 10 times the width of the Moon. J. R. Hind, from his observatory in Regent's Park, London, described the comet's nucleus as being as bright as a 2nd magnitude star. A "very faint ray of light...emanated from the nucleus towards the sun...There is a high probability of (the comet) being visible with telescopes in full sunshine about...October".

Mr. Hind was right. From mid-September the comet seemed to grow larger and brighter with every night. It still seemed hardly to move, and hung in the western sky, with a tail sweeping in a majestic and beautiful curve. The Earl of Malmesbury wrote in his diary for September 16, "The largest comet I ever saw became visible with a very broad tail spread perpendicularly over the sky, the weather being very hot. Every one now believes in war."

On September 23, the comet was clearly visible half an hour after sunset. On September 28, astronomers in Cambridge, Massachusetts, using a 15-inch refractor, attempted a historic feat; the first photograph of a comet. After a 6-minute exposure, however, only the nucleus and some nebulosity was registered. A Mr. Usherwood, had more success with a stationary camera and a 7-second exposure, although even his image was 'somewhat imperfect'.

Next day, September 29, the comet passed perihelion, swinging round the Sun at thirty miles a second. The vast but insubstantial tail, curved like a scimitar, would soon be about 50 million miles long and 10 million miles wide at its broadest. On October 2, a telescopic view from the Royal Observatory in Greenwich showed the nucleus as a bright stellar point surrounded by a fainter annulus of light, two-thirds complete and brighter than the general envelope. According to the Astronomer Royal, the nucleus was "well defined, and (looked) very hard...From the nucleus a dark shadow diverged, cutting off the light of the circular discs, and everything except the nucleus itself."

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Donati’s Comet near Arcturus, October 5, 1858.

Three days later, on October 5, the tail of the comet passed over the first-magnitude star Arcturus in the constellation of Bootes. Some observers claimed that the star "twinkled on with undiminished lustre" during the passage, but others disagreed, saying that Arcturus 'lost brightness considerably' while in the tail. The poet Alfred Tennyson was visiting in Swainston on the evening of October 5, "and the comet was grand, with Arcturus shining brightly over the nucleus. At dinner he said he must leave the table to look at it, and they all followed. They saw
Arcturus seemingly dance as if mad when it passed out of the comet's tail. He said of the comet's tail, 'It is like a besom of destruction sweeping the sky.'"

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Donati’s Comet, October 9, 1858.

On October 10, Donati's Comet was at its closest to Earth, although still at least 50 million miles away. It was at maximum brightness, with a tail 30, 40, or, as some said, 60 degrees long, and almost as bright as the Milky Way. Telescopic views showed the nucleus "infinitely small, but intensely bright", seeming to cast a long dark shadow through the bright nebulosity surrounding it. Seven successive luminous envelopes were seen to expand from the nucleus, the first rising to a height of about 18,000 miles before it disappeared.

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A telescopic view of the nucleus and coma of Donati’s Comet.

After October 10, the comet rapidly faded, although it caused alarm in Egypt when it was first seen there; "for several days all business and labour were at a standstill, the inhabitants believing it foreboded some great calamity."

The comet passed into the southern hemisphere and was no longer visible in Europe after the end of October. It was last seen from the Cape of Good Hope Observatory on March 4, 1859.

The orbital period of Donati's Comet was imprecise, figures of 1,879, 2,040 and 2,138 years being calculated. Some thought it might be identical with a great comet recorded by Seneca as having appeared in 146 B.C., dated by the Chinese for August. It is due to return in the 38th or 39th century.

Donati's Comet was remarkable, not only for its beauty, but for the fact that several circumstances combined to provide an almost perfect view for observers in the northern hemisphere. Predictions as to how bright it might become were also more accurate than later efforts for later comets.