Why don't you feel the Earth spinning at 1,700 km/h?

You don't feel the Earth's spin because motion at a steady speed leaves no trace you can feel from inside it. At the equator the planet carries you around at about 1,700 kilometres per hour, and you notice nothing, because everything around you is travelling along with you. A spin is not perfectly steady, since its direction keeps turning, so the effects are not quite zero, but they are subtle: the first simple, direct evidence of the rotation only came in 1851, from Foucault's pendulum.

The first person to explain this clearly was Galileo Galilei. In his 1632 book, the Dialogue Concerning the Two Chief World Systems, he asked readers to imagine a ship moving at constant velocity on a smooth sea. A man shut below deck, watching water drip from a bottle, fish swim in a tank and butterflies flutter about, could not tell whether the ship was moving or docked: it all behaves the same either way. That answered a popular argument against a moving Earth, which said that if the ground were really hurtling along, dropped objects would be left behind and drift west.

Galileo paid for his book. In 1633 he was found "vehemently suspect of heresy" over it, and the Dialogue stayed on the Index of Forbidden Books until 1835.

The idea survived him. Newton placed the ship in the Principia and built his laws of motion on it. Today it is called Galilean invariance: the laws of motion are the same in every frame moving at constant velocity relative to another. It was also the seed of Einstein's special relativity. The first of the two postulates of his 1905 paper, that the laws of physics are identical in all such frames, is Galileo's ship, promoted from mechanics to all of physics. The second postulate, a constant speed of light, is the one that would break Newton's picture of space and time.