Throughout the ages, comets, those extraordinary, wandering celestial objects that, without warning, shine in the heavens for weeks or months, have impressed mankind. The ancient Greeks called them κομήτης, which meant ‘with long hair’ or ‘hair stars’. In other languages they were known as ‘broom stars’ (in English), or ‘tailed stars’ (‘staartster’ in Dutch). The German word ‘Schweifstern’ (‘stars with tails’) summarizes their diverse appearance.

The debate about these celestial bodies has been beset with errors and confusion, great ideas and amusing anecdotes. Beliefs and superstitions, art and science have all been influenced by the appearance of great comets. They reflect humanity's development and its search for explanations for appearances in the heavens.

The history of this development is described in this book though the descriptions of the 30 greatest comets from early modern times to the twenty-first century. In choosing these events, astronomical points of view were not the only criteria. The aim was to describe the strange ideas that had the greatest influence on people in former centuries.

This book would not have been possible without the help of numerous individuals. In particular, three people have had a great influence: great thanks are due to Hans Gaab, an expert on the astronomical history of Nuremberg, who placed extensive materials, books and copies at my disposal. His help was particularly valuable in the chapters on antiquity, the Middle Ages and early modern times.

Maik Meyer, who holds the record for the most comets discovered with the SOHO solar probe, and co-author with Gary W. Kronk of the multi-volume Cometography series of books, the world's most comprehensive documentation on comets, provided extremely significant help on discovery data and discovery statistics. Burkhard Leitner, the expert on comets on the editorial board of interstellarum magazine, provided the charts of the individual comet apparitions.

This illustrated work would not have appeared in such an opulent edition without the wonderful support from numerous astrophotographers.

Fifteenth and sixteenth centuries

For centuries the nature of comets was not a matter for consideration. Astrological theory was too strongly entrenched. It was only at the beginning of modern times that the fundamental question came to be posed: “Are comets meteorological or astronomical phenomena?” “Are they sublunar, being situated under the Moon, or are they supralunar, lying beyond it?”

These questions should have been answered by the determination of the distance of comets. The principle of distance determination rests on parallax, that is, on the apparent spatial shift in the positions of comets in front of a more distant background (the starry sky), based on viewing the comet from different angles. If comets were closer than the Moon, as Aristotelian doctrine held, then when comets were viewed from two different locations they should appear at different positions on the sky. Similarly, parallax should have been detected with observations from a single, fixed position over a period of some hours, if the comet were closer to the observer than the whole firmament, which was rotating around the Earth.

Before the discovery of the telescope in 1610, people tried to determine the positions of comets with the naked eye. In the Middle Ages, the instruments available were the torquetum and the cross-staff (or Jacob's staff). In early modern times, quadrants were also used, and the larger they were, the more accurate their determinations of positions. Astrolabes and armillary spheres were also employed.

The Italian geographer Paolo Toscanelli (1397–1482), famous for his (inaccurate) chart of the Atlantic Ocean that led Columbus to believe that a shorter route to India lay to the west, was one of the first people to determine the positions of comets. He followed six comets between 1433 and 1472 and recorded their motions across the sky. His notes were not rediscovered until the nineteenth century.