Great Comet of 1807

Lore & Background

The comet's discovery is often credited to Augustinian friar P. Reggente Parisi at Castrogiovanni in Sicily, who recorded it very close to the horizon in early twilight on 9 September 1807, near the star Spica and the planets Venus, Mars, and Saturn. Moonlight interfered with observations for a week, and Parisi's southerly location may have allowed him to discover it several days before other European observers. Orbital data suggest the comet should have been visible in the southern hemisphere weeks earlier, but no such reports exist.

Jean-Louis Pons saw the comet on 21 September at Marseille, and Jacques-Joseph Thulis made the first positional determination. By 20 September the comet reached magnitude 1–2. Over the next ten days it was independently discovered by multiple observers across Europe and in New Granada (Colombia). The comet's tail was initially 7°–8° long, and by early October it split into two tails: a straight, narrow tail up to 10° long and a shorter, curved tail. By late October only a single tail of 2.7° remained. The comet was still visible to the naked eye in November and December, but its brightness steadily diminished. The final telescopic observation was made by Vincent Wisniewsky on 27 March 1808.

Reader's Guide

The Great Comet of 1807 holds particular significance in cometary science as the first long-period comet (other than Halley's Comet) for which an elliptical, rather than parabolic, orbit was empirically verified. Friedrich Wilhelm Bessel used observations spanning 187 days to compute an elongated orbit inclined about 63° to the ecliptic. Dissatisfied with initial parabolic and elliptic calculations, Bessel developed a novel method for determining orbital elements that accounted for changing gravitational influences of the Sun and planets and employed a least squares estimation technique developed by Carl Friedrich Gauss. This made the comet a key test case for improved orbital computation. At its closest approach to Earth on 26 September 1807, the comet was 1.15 AU away—an unusually large distance for a great comet, shared only by the Great Comet of 1811 and Comet Hale-Bopp. Notably, when C/1881 K1 (the Great Comet of 1881) was discovered, John Tebbutt initially considered it might be a return of the 1807 comet due to apparent orbital similarities, though this was not confirmed.

Did You Know?

• The comet was independently discovered by at least nine different observers across Europe and the Americas within weeks of its first sighting.
• Its tail split into two distinct tails in early October 1807, with the longer one reaching about 10° in length.
• The comet made its closest approach to Earth at 1.15 AU, an unusually large distance for a great comet.
• Friedrich Wilhelm Bessel developed a novel method for calculating cometary orbits using this comet's data.