C/1618 W1

Lore & Background

Three comets were visible to the naked eye in 1618, with C/1618 W1 likely the brightest, first observed on 25 November. Chinese texts reported a sighting on 26 November with a 10° tail pointed southeast. It may have been seen earlier in Persia by García de Silva y Figueroa, though his reports are inaccurate. Observations were made across Europe, Asia, and the Philippines. Johannes Kepler observed it from Linz from 29 November to 7 January, and Johann Baptist Cysat reported a tail length of 70° on 9 December. Longomontanus recorded a tail of 104° on 10 December, while Orazio Grassi estimated 60° on 12 December. The comet was last seen by Cysat on 22 January 1619 with a telescope.

The comet was widely interpreted as an ominous harbinger. A Thuringian chronicle linked it to war, rebellion, and famine, and by 1630 it was seen as a harbinger of the Thirty Years' War. The deaths of Archduke Maximilian III, Pope Paul V, and King Philip III were also speculated to have been 'announced' by the comet. In England, some saw it as divine disapproval of King James I's marriage alliance with Spain, prompting the king to write a poem about public credulity.

A colloquium in Ulm in 1619, involving scientists including René Descartes, debated whether the comets were divine signs or natural phenomena. The comet also sparked a controversy between Galileo and Orazio Grassi from 1619 to 1623.

Reader's Guide

C/1618 W1 holds significance as the first comet observed with telescopes, enabling more precise positional measurements that later facilitated orbital calculations. Johannes Kepler published his findings in his 1619 book De Cometis libelli tres, building on Tycho Brahe and Michael Mästlin's earlier parallax measurement of the Great Comet of 1577, which proved comets were celestial bodies, not atmospheric phenomena. Kepler defended rectilinear cometary motion and attributed comet formation to condensations in the luminiferous ether, a now-discredited concept.

The comet's orbit was roughly determined by Friedrich Wilhelm Bessel from 42 observations over 53 days, yielding a parabolic orbit inclined 37° to the ecliptic. It passed Venus on 19 October at about 0.41 AU, Jupiter on 30 October at about 4.67 AU, reached perihelion on 8 November at about 58.3 million km from the Sun, and passed Earth on 6 December at about 54 million km. Due to uncertainty in initial orbital data, it is unknown whether the comet will return to the inner Solar System or was ejected into interstellar space.

The comet's legacy includes its role in the Ulm comet dispute of 1619, where theologians and natural scientists clashed over its meaning, and its mention in literature, including King James I's poem. It remains a key example of early telescopic astronomy and the intersection of science and superstition.

Did You Know?

• Its tail was reported as long as 104° by Longomontanus on 10 December 1618.
• Johannes Kepler observed the comet from Linz and published his findings in De Cometis libelli tres (1619).
• A colloquium in Ulm in 1619 debated whether the comet was a divine sign or a natural phenomenon.