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1° 134 BC Hipparchus discovers precession, prepares stellar catalog. Hipparchus compared his own observations with earlier ones to discover precession, the slow change in the direction of the Earth's polar axis.
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1600 William Gilbert proposes Earth has dipole magnetic field. Gilbert proposed that the Earth acts like a big magnet whose field aligns the small magnet used as a compass needle.
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1609 Galileo uses telescope for astronomical observations. Galileo didn't invent the telescope but he was among the first to use a telescope to examine the heavens.
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1646 Robert Hooke shows that a central force leads to orbital motion. Hooke used a pendulum to demonstrate to the members of the Royal Society that in order to stay in orbit, the planets must be continually pulled toward the Sun.
