C/1864 N1 (Tempel)

C/1864 N1 (Tempel) is a non-periodic comet first spotted by Wilhelm Tempel on 5 July 1864. It holds the distinction of being the first comet to have its spectrum studied, a key advance in astronomical spectroscopy.

Tempel, working at the Marseille Observatory, discovered the comet near the star 54 Arietis, confirming his find the next day. Lorenzo Respighi and Franciszek Karliński independently found it on 6 and 11 July, respectively. Giovanni Schiaparelli observed it on 9 July, noting a round coma 3–4 arcminutes wide and an estimated magnitude of 8–9. As the comet drew closer to both Earth and the Sun, it brightened. Johann Friedrich Julius Schmidt saw it with the naked eye on 28 July, describing a coma 10.5 arcminutes across and a tail 0.3° long. By 30 July he put its magnitude at 4, and by 7 August it had reached magnitude 2–3, with a tail stretching 11.5°. The comet made its closest approach to Earth on 8 August 1864, at a distance of 0.096 astronomical units (about 14.4 million km), and was then at a solar elongation of 12°.

After this approach, the comet appeared in the evening sky. Schmidt recovered it on 9 August, estimating its magnitude at 2–3 the next day. It passed perihelion on 16 August, remaining at magnitude 4–5 through the end of the month. The comet faded through September as it receded from Earth and the Sun, last seen with the naked eye on 6 September. The final observation was made on 5 October 1864.

In early August, Giovanni Battista Donati obtained the comet’s spectrum for the first time, using a prism. He detected three bright lines, now known as the Swan band, which are linked to carbon.

The comet has a minimum orbital intersection distance with Earth of 0.0054 AU (about 810,000 km) and is associated with the weak Delta Piscids meteor shower (#410).