Huygens’ discovery of a ring around Saturn

10 februari 2012

The planet Saturn was a significant problem for astronomers in the first decades of the seventeenth century. Viewed through the first telescopes, the planet displayed a strange, irregular form, a form that also changed with the passing of time. Various explanations were offered for this phenomenon, but it was Huygens who finally found the solution to the puzzle.

While observing Saturn through his telescope, Huygens developed an explanation for the seemingly fluctuating form of this planet. Galileo was the first to observe these constantly changing ‘ears of Saturn’.

In March 1656 it became clear that Huygens had discovered something important, when he published a pamphlet reporting his discovery of a moon near the planet Saturn a year earlier. At the time, Saturn was the furthest planet from the earth known in our solar system.

However, when Huygens published his discovery of the Saturn moon (named Titan), he was not yet absolutely sure that his ring hypothesis was correct. Huygens announced his hypothesis in the same manner as he had announced the discovery of Titan; in the form of an anagram. He could then make the solution known at a later date, when he was sure he was right.

No copies have survived of the original pamphlet that Huygens distributed among his colleagues. However, the publisher Adriaan Vlacq included the text of the pamphlet as an insert in the back of Pierre Borel’s book about the discovery of the telescope (De vero telescopii inventore), which also appeared in March 1656.

In March 1658, three years after his initial observation and two years after his publication, Huygens was sure enough of himself to reveal the meaning of his second anagram. He wrote to his correspondent J. Chapelain that the anagram represented the following sentence in Latin: ‘ANNULO CINGITUR TENUI, PLANO, NUSQUAM COHAERENTE AD ECLIPTICAM INCLINATO’, which means ‘it is surrounded by a ring, thin and flat, never touching, oblique in relation to the ecliptic’. Huygens published this solution to the anagram, which could not possibly have been solved by anybody else, in 1659 in his book Systema Saturnium.