Mars was an enigma to ancient astronomers, who were bewildered by its apparently capricious motion across the sky—sometimes in the same direction as the Sun and other celestial objects (direct, or prograde, motion), sometimes in the opposite direction (retrograde motion). In 1609 the German astronomer Johannes Kepler used the superior naked-eye observations of the planet by his Danish colleague Tycho Brahe to empirically deduce its laws of motion and so pave the way for the modern gravitational theory of the solar system. Kepler found that the orbit of Mars was an ellipse along which the planet moved with nonuniform but predictable motion. Earlier astronomers had based their theories on the older Ptolemaic idea of hierarchies of circular orbits and uniform motion.
The earliest telescopic observations of Mars in which the disk of the planet was seen were those of the Italian astronomer Galileo in 1610. The Dutch scientist and mathematician Christiaan Huygens is credited with the first accurate drawings of surface markings. In 1659 Huygens made a drawing of Mars showing a major dark marking on the planet now known as Syrtis Major. The Martian polar caps were first noted by the Italian-born French astronomer Gian Domenico Cassini about 1666.
Visual observers subsequently made many key discoveries. The rotation period of the planet was discovered by Huygens in 1659 and measured by Cassini in 1666 to be 24 hours 40 minutes—in error by only 3 minutes. The tenuous Martian atmosphere was first noted in the 1780s by the German-born British astronomer William Herschel, who also measured the tilt of the planet's rotation axis and first discussed the seasons of Mars. In 1877
Asaph Hall of the U.S. Naval Observatory discovered that Mars has two natural satellites. Telescopic observations also documented many meteorological and seasonal phenomena that occur on Mars, such as various cloud types, the growing and shrinking of the polar caps, and seasonal changes in the colour and extent of the dark areas.
The first known map of Mars was produced in 1830 by Wilhelm Beer and Johann Heinrich von Mädler of Germany. The Italian astronomer Giovanni Virginio Schiaparelli prepared the first modern
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