Tycho Brahe

• better scientific instrumentation
• better attention to capabilities and calibration of instruments
• ...which led to better observations of planets

Kepler

• 1st to determine that planets have elliptical orbits w/Sun @ one focus
• 1st to determine that planets move faster when closer to the Sun
• 1st to note that P² is proportional to a³

Galileo

• 1st to make astronomical observations w/a telescope
• noted the phases of Venus

Newton

• discovered the law of universal gravitation
• pioneered the idea that the same physical laws apply on Earth and in the sky

Copernicus

• popularized the idea of heliocentrism
• calculated a detailed mathematical model of a heliocentric system

Ptolemy

• calculated the most detailed mathematical model of a geocentric sysem