350 years ago -- March 25, 1655:

Christiaan Huygens discovers Saturn's moon Titan using this telescope lens

Christiaan Huygens (1629-1695) discovered Saturn's moon Titan on March 25, 1655, with the telescope lens shown above. The lens was recovered in 1867 in the University of Utrecht's collection of historical physical instruments. It is now in the Utrecht University Museum, where it is treasured as one of the most important objects of the collection.

The plano-convex lens measures 57 millimeters in diameter and has a focal length of 336.7 centimeters. The lens itself is only 3.4 millimeter thick. Unfortunately the telescope's tube and ocular lens have not been preserved. Christiaan Huygens designed and constructed the telescope together with his brother Constantijn Huygens (1628-1697), who later became a statesman and secretary to the Stadtholder-King William III.

Along the rim of the lens the following inscriptions can be read: X [pedem] 3 FEBR. MDCLV and Admovere oculis distantia sidera nostris ("They brought the distant stars closer to our eyes"). The first text gives the date when the lens was finished and ready for use (February 3, 1655), together with its focal length (10 Rhineland feet). The second text is a verse from the Roman poet Ovid and part of the anagram: Admovere oculis distantia sidera nostris vvvvvvv ccc rr h n b q x that Christiaan Huygens sent to some of his scholar friends in the summer of 1655.

By means of this anagram Huygens secured his discovery of Saturn's moon but kept it secret until he was completely sure about his discovery. On March 5, 1656, he revealed the meaning of the anagram in a pamphlet that was printed in The Hague: Saturno luna sua circunducitur diebus sexdecim horis quatuor, which can be translated as: "A moon revolves around Saturn in 16 days and 4 hours” (later observations revealed that the true orbital period of Titan is slightly short of 16 days.)

Huygens observed Saturn for months after his discovery of the moon. He was struck by the curious 'extensions' of the planet (which had puzzled astronomers since they were first observed in 1610 by Galileo Galilei) and concluded that this should be a ring around the planet. Again he secured his discovery by means of an anagram at the end of his pamphlet: aaaaaaa ccccc d eeeee g iiiiiii llll mm nnnnnnnnn oooo pp q rr s ttt uuuuu. The solution which nobody possibly could have found was revealed three years later in his book Systema Saturnium: Annulo cingitur, tenui plano, nusquam cohaerente, ad eclipticam inclinato (encircled by a ring, thin and flat, nowhere touching, inclined to the ecliptic).

Huygens did not name the satellite he discovered. He referred to it as Luna Saturni (Saturn's moon). When more moons around Saturn were discovered by other astronomers he simply referred to it as 'my moon', others usually referred to it as 'Huygens' moon'. The name Titan was proposed in 1847 by the English astronomer John Frederick William Herschel, who supplied mythological names for all the Saturnian moons that were then known, but Herschel's names did not become popular until the end of the 19th century.

During their scientific and political careers, Christiaan and Constantijn Huygens constructed dozens of telescopes. The largest lens had a diameter of 23 centimeters and a focal length of 65 meters. Most of these lenses can still be admired in the Museum Boerhaave collection at Leiden, the Netherlands.

Rob van Gent, Tiemen Cocquyt, and Carl Koppeschaar

More to read:

  • Astronomy Picture of the Day, 2005 March 25: Huygens Discovers Luna Saturni
  • Christaan Huygens and his telescopes
  • De Saturni luna observatio nova (1656)
  • Christiaan Huygens - Systema Saturnium
  • European Southern Observatory: Focus on Titan
  • Dutch:

  • 350 jaar Huygenslens
  • De ringen van Saturnus (anagram)
  • Een rode vlek draait op Jupiter rond (anagram)

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