# When and by whom was the earliest definition of speed given?

Speed is defined to be distance divided by time; when and who by was this definition first put forward?

The obvious guess would be Galileo in early modern physics; is this right, and can it be pushed further back?

• No, and yes it can. Eudoxus wrote a book On Speeds c. 375 BC, that's the book where he introduces homocentric spheres to model planetary motions jstor.org/stable/41133535. The concept itself is likely much older, as even rudimentary astronomical observations suggest it, not to mention watching ships at sea, or planning travel by them. My guess is that ancient Egyptians and Babylonians already had a concept of speed. – Conifold Apr 28 '15 at 21:24

Aristotle (Physics, Book VII, Chapter 5) made use of the concept of speed in his laws of motion written around 330 BC.

He said that a force which moves a mass a distance D in time T will move half the mass twice the distance in the same time.

While the result of his equation is not correct where the force is gravity (he was concerned with preserving proportionality), it and his other rules of motion indicate that he employed the concept of average speed.

See this summary: http://faculty.poly.edu/~jbain/mms/lectures/15.GalileoMotion.pdf

• Also see Aristotle's Physics Book VII Part 5. This clearly describes motion as distance covered in a specified time. – John Rennie Apr 28 '15 at 6:04
• See also e Aristotle's Physics Book VII Part 4 : " two things are of the same velocity if they occupy an equal time in accomplishing a certain equal amount of motion. [...] We may say, therefore, that things are of equal velocity in an equal time they traverse the same magnitude". – Mauro ALLEGRANZA Apr 29 '15 at 10:24

The Italian physicist Galileo Galilei is credited with being the first to measure speed by considering the distance covered and the time it takes. Galileo defined speed as the distance covered per unit of time.

Wikipedia

• Wikipedia isn't a very reliable source. – Geremia Apr 28 '15 at 19:46
• I have to say, Wikipedia is rarely wrong this badly. Especially since they have an article about Oresme and "Merton mean speed theorem", which influenced Galileo. en.wikipedia.org/wiki/Mean_speed_theorem – Conifold Apr 28 '15 at 22:26
• On my opinion, Wikipedia is closer to the truth in this case: this should be credited to Galileo, not Aristotle. – Alexandre Eremenko Apr 28 '15 at 23:30
• @Geremia The accepted answer talks about the fact that Aristotle thought about speed while the question asks about who defined it. So I feel wikipedia is correct in saying that Galileo defined it. – Freddy May 1 '15 at 5:56
• @AlexandreEremenko Since Galileo defined it? As stated in the question? – Freddy May 1 '15 at 5:58

The exact notion of speed, and its precise definition are due to Galileo. But long time before him people had a good intuitive concept of speed, and could use it. (The notions of momentary speed and average speed were confused, of course). One example where the notion of speed was used before Galileo is "ded reckoning" in navigation, which is the computation of the place of the ship using time traveled, speed and direction.

Columbus (who lived roughly a century before Galileo) was a great master of this art, one of his extraordinary achievements was steering his fleet in the second transatlantic voyage exactly to the place where he left a party in his first voyage. So he had a very clear concept of speed, though the means of measuring it were very primitive.