Huygens, Christian (1629-1695)
Huygens was a Dutch astronomer who studied and drew the first map of Mars. While drawing the map he noticed a V-shaped mark on the surface of Mars that reappeared in the same place every 24 hours. This was the geographical feature of Mars now known as the Syrtis Major. Huygens deduced that the regular appearance of this terrestrial feature indicates the length of a Martian day.
The Rings of Saturn.
Huygens made was confirming Galileo's assumption that Saturn appeared to be a triple planet. In fact, Huygens showed that his appearance was due to the rings of Saturn which had previously not been discovered. What enabled him to see the rings was a telescope he made himself.