Comet Donati

C/1858 L1 (Donati)

Donati's Comet in landscape by William Turner
Discovery
Discovered by	Giovanni Battista Donati
Discovery date	1858
Alternative designations	1858 VI
Orbital characteristics A
Epoch	1858-Oct-08 (JD 2399960.5)
Aphelion	~289 AU
Perihelion	0.578 AU
Semi-major axis	~147 AU
Eccentricity	0.996
Orbital period	~1,739 yr
Inclination	116.9°
Last perihelion	September 30, 1858
Next perihelion	unknown

Comet Donati, or Donati's Comet, formally designated C/1858 L1 and 1858 VI, is a long-period comet named after the Italian astronomer Giovanni Battista Donati who first observed it on June 2, 1858. After the Great Comet of 1811, it was the most brilliant comet that appeared in the 19th century. It was also the first comet to be photographed.

Donati first observed the comet on 2 June from the Florence Observatory: it was initially visible as a small nebula-like object of magnitude 7 near the "head" of Leo. By mid-August it had brightened sufficiently to be visible to the naked eye.

In September it passed into Ursa Major. For much of its apparition it occupied a unique position (among great comets) in the sky and was particularly well placed for Northern Hemisphere viewers.

It was nearest the Earth on October 10, 1858, and for much of October was a brilliant object with a long, scimitar-like dust tail and prominent gas tail. It remained a naked-eye object until November for Southern Hemisphere observers. The final observation was by William Mann, chief assistant at the Royal Observatory, Cape of Good Hope, who detected it as a faint nebulosity on March 4, 1859.

During its apparition the comet was particularly closely studied by the astronomer George Phillips Bond and his father William Cranch Bond. G.P. Bond incorporated these observations and those of many other astronomers into a monograph, "An Account of the Great Comet of 1858", which remains his most important scientific work and for which he was awarded the Gold Medal of the Royal Astronomical Society, being the first American to receive the award.

Donati's Comet was successfully photographed on September 27 by W. Usherwood, a portrait photographer at Walton-on-the-Hill, Surrey, using a 7-second exposure with an f/2.4 portrait lens, the first time a comet had been photographed. Usherwood's photograph, which has not survived, showed the bright region around the comet's nucleus and a part of the tail. G.P. Bond also successfully photographed the comet on September 28 at Harvard College Observatory, the first comet photograph through a telescope. He made several attempts with increasing exposure times, finally achieving a discernible image. He later wrote, "only the nucleus and a little nebulosity 15" in diameter acted on the plate in an exposure of six minutes".