Spectral Lines of Comet II of 1864

ink and pencil on paper, 155 x 121 mm

INAF-Arcetri Astrophysical Observatory

In August 1864, Donati observed the comet C/1864 N1 (discovered by Wilhelm Tempel on July 5) with the same apparatus used for stellar spectra (see Fi. 01), succeeding for the first time in analysing spectroscopically the faint light emitted by a comet. Unlike the spectrum of the Sun and the stars, comet Tempel exhibited three bright bands in the blue-green region of the solar spectrum, separated by two dark bands. The original drawing shown here probably served as a model for the illustration accompanying the observations of comet Tempel, published by Donati in the same year in Astronomische Nachrichten (also shown). In the article, Donati pointed out the similarity between the spectrum of the comet and the spectra produced by metals (i.e. in electrical discharges produced between two metal electrodes in the laboratory). Actually the same bright bands had already been observed in 1856 by the Scottish physicist William Swan (1818-1894) in the spectrum of the light emitted by the flame of various hydrocarbons, and are known today as “Swan bands”. The identification of the emission bands observed in comets with the Swan bands was made in 1868 by the British astronomer William Huggins (1824-1910), although the carbon compound responsible for the band emission (the diatomic carbon C2) was identified only in 1927

We also show Tempel’s notebook  with the position of the comet he discovered at about 3 am on July 5, 1864.

Bibl.: DONATI 1864, HEARNSHAW 2014, CHINNICI 2015b.