Why were Newton's laws accepted when there exists a counter-example?

Question

Though the planets of Venus, Earth, Mars, Jupiter, and Saturn seemed to obey Kepler's laws, Mercury did not. I just found this page which states that the discrepancy was noted very soon after newton published his laws of motion. Since a law of nature is obviously not true if there exists a counter example, why were Newton's laws accepted despite the obvious counter example? Was science less rigorous during the time of Newton?

Note that this question was translated into English by my father, who is the one typing it. I'm eight years old and I don't use SE unsupervised.

Answer 1

Every scientific theory has some counter-examples or "discrepancies".

But, in general, a "good" theory will not be rejected until a "better" new theory is available.

"Little" discrepancies, like that involving Mercury's orbit were far less relevant that "big" successes like the correct prediction of the existence of a previously unseen planet; see Discovery of Neptune (1846).

You have to take into account that, also after the availability of a new "better" theory, like Relativity, that is able to explain Mercury's orbital anomalies, Newtonian' mechanics is so "reliable" (being an approximation of Relativity that fit very well when the velocities involved are very little when compared to the speed of light) that human being have been able to safely "land" on the Moon (mission: Apollo 11 : July 20, 1969) and come back based on calculation that are not so different in principle from what Newton himself had done in his masterpiece of 1687: Philosophiae Naturalis Principia Mathematica.

Scientifc knowledge is not "perfect": we have to continue to revise and improve it.

So, good luck for your student career: we need future scientists that can contribute to the process of continuously improvement of human knowledge.

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Note : suggestion for your future studies in depth :

• Scientific Progress

• Karl Popper.

Answer 2

Other bodies also do not obey Kepler's laws strictly. Only approximately. The most well-known exception is the Moon, not Mercury. The reasons of this were clear to Newton and to his readers. Kepler's laws describe the motion of TWO bodies. When a third (or more) bodies are taken into account, it is difficult to compute from the law of gravitation and the laws of mechanics how will they move. Newton himself could not fully explain the observed motion of the Moon, its irregularities known at that time. And until this motion was explained (in the second half of 18s century) there were indeed doubts about exactness of Newton's gravitation theory. This was the most serious test.

Another crucial test was his correct prediction of the shape of the Earth. The most famous test came in the 19 century, when a new planet (Neptune) was discovered on the basis of calculations using Newton's theory, and after that Neptune was found in the sky. The small disagreement for Mercury had to wait for the beginning of 20s century and invention of general relativity.