On this date in 1687, Isaac Newton published one of the most important books in the history of science.
Its full name is Philosophiae Naturalis Principia Mathematica, or "Mathematical Principles of Natural Philosophy."
It began as a brief tract called "On Motion," in which Newton discussed mathematical theories of planetary motion.
Almost as soon as he'd finished writing it, he began revising and expanding it.
When he'd finished, the Principia contained Newton's three laws of motion, including these two
"Objects in motion tend to remain in motion,"
"For every action, there is an equal and opposite reaction."
Newton also unified celestial and terrestrial mechanics under one umbrella: gravity, which caused planets to orbit the Sun, moons to orbit planets, and earthly objects, like apples, to fall to the ground when dropped.