How did Huygens see Titan?
Christiaan Huygens polished his telescope together with his brother Constantijn and completed it in spring of 1655. The original lens of this telescope (0.32 cm thick, 5.7 cm diameter) is now kept in the Museum Boerhaave at Leiden. The telescope with an objective focal length of 377 cm (twelve feet) and an estimated ocular focal length of 7.5 cm had a magnification of about 50. On 25 March 1655 he directed his telescope towards the planets, first to Venus and Mars, later to Jupiter and Saturn.
Huygens wrote about the observation of Titan in his book Cosmotheoros (original edition in Latin, 1698). He identified a bright celestial body rotating once a few weeks around Saturn. After four rotations, in June 1655, he was sure that it should be a moon of Saturn.
He wrote in Cosmotheoros:
One of Saturn's moons, brighter than the others and rotating on the outermost orbit, flew into my face, and I was the first who saw him in 1655 with my telescope not longer than twelve feet.
He determined the rotation period to 15 days 22 hours 41 minutes 11 seconds, barely differing from the currently accepted value. The distance of Titan from Saturn was indicated as 4 ring units.
He didn't dare to say anything about the properties of Titan, but thought that the moons of Jupiter and Saturn must be of the same nature as the Earth's moon. He had no other choice than to transfer the properties of Earths's moon to the other moons, i.e. Titan should have many craters, but no water and no atmosphere.
Huygens is right in his observation that Titan rotates synchronically and therefore, men on Titan could see Saturn only if they live on the Saturn side. The other inhabitants of Titan could it never, except they travel to the Saturn side.
Huygens rhetorically asks if the moons of Jupiter and Saturn shall rotate "without purpose", i. e. without living being, around the planet. Perhaps plants and animals could live there without water, too.
In his lifetime four other moons of Saturn were detected by Cassini. Huygens saw Rhea and Iapetus, but probably not Tethys and Dione. He identified a quite large gap of about 8 ring units between Titan and Iapetus, so he assumed one more moon between them , but cound not find it himself. This assumption was confirmed 1848 by the detection of Hyperion by Bond.