Observations on Mars
To see what isn't there, that is the Illusion.
The first telescopic observation of Mars was probably undertaken by Galileo Galilei around 1608 to 1610, but it was not until 1636 that the first records were taken, this time by Francisco Fontana. Unfortunately, his sketches seem to show more the problems of his telescope than the structure of the planet, with a central black spot and a dark outer ring. Telescope design gradually improved throughout the 17th century, and by 1659, Christian Huygens had made probably the first informative sketch of Mars. His observations (taken over a number of weeks) show a triangular feature which is still recognisable today, known as Syrtis Major. Huygens' results were so reliable that he even used the reappearance of this feature to measure the rotational period of Mars as 24 hours, less than 1 hour away from the value recognised today. Huygens was also able to crudely sketch the presence of a polar ice cap to the south of the planet, although he did not at the time identify it as such.
1666 saw Giovanni Cassini making some more precise measurements of Mars. He calculated the rotational period to be just under 24 hours 40 minutes, almost 37.5 minutes longer than Earth's and very close to the presently accepted value. He is also heralded as the man who discovered and identified the polar ice caps on Mars, which were not described any more fully until 1719 by Giacomo Miraldi. The Martian polar caps in fact bear striking similarities to the Arctic and Antarctic caps on Earth, but almost a century passed before the German born English astronomer William Herschel suggested the caps may be made of ice or snow.
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