1655: On 25 March 1655, Christiaan Huygens discovered Titan, a moon of Saturn.
Huygens was born in The Hague in 1629 and was famous amongst his contemporaries for his development of advanced pendulum clocks. He also invented an improved type of dual-lens eyepiece, now named after him, and constructed very long telescopes of up to 80 metres focal length.
With these, he made important discoveries such as Jupiter's equatorial bulge, the polar caps of and a dark surface feature, later named Syrtis Major, on Mars. He discovered Saturn's satellite Titan on 25 March 1655 and was the first to clearly see Saturn's rings and see their appearance over time.
In 1656 he independently discovered the Orion Nebula M42 and made detailed studies and sketches of this object.