Did Newton ever get hit on the head by an apple?

Before the 17th century, the mainstream view in Europe was that all celestial bodies revolved around the Earth. After Copernicus and Galileo, almost all astronomers were convinced to change their minds: the Moon revolves around the Earth, and the planets revolve around the Sun.

But the original question was still not solved: which one revolves around the other, the Earth or the Sun? The popular modified geocentric theory holds that while the planets revolve around the Sun, the Sun also revolves around the Earth.

Astronomical observations at the time could not solve this problem because motion itself is relative. If we only consider the interior of the solar system, whether the Earth orbits the Sun or the Sun orbits the Earth, the motion is completely equivalent, and we only need to perform a coordinate system transformation.

If the entire universe is taken into account, there is a difference between the two: if the earth is moving, then when the earth is in different positions, the starry sky seen should be slightly different, which is called "parallax". If it is the sun that is moving, there will be no difference. Astronomical observations at the time did not find parallax, which seems to support the idea that the sun is moving. Those who support the idea that the earth is moving believe that this is because the stars are very far away, resulting in parallax that is too small to be observed. Although this statement is possible, there is no evidence after all.

Moreover, if the stars were really that far away, it would mean that some of them must be very large, with estimates at the time suggesting that the radius of some stars was larger than the distance between the Earth and the Sun. Today we know this is possible, but 17th-century astronomers would have thought it was ridiculous.

This fundamental disagreement about which is moving, the Sun or the Earth, could not be resolved by observations in the 17th century, but there are other ways besides observation. For example, we can explore the underlying mechanism: Why do celestial bodies revolve around each other?

The ancient Greeks believed that circular motion was a natural law of celestial bodies, and there was no reason for it. The progress of modern physics has negated this view, especially the first of Newton's three laws, which states that objects should move in a straight line at a uniform speed when not subject to external forces. In other words, celestial bodies that move in a circle must be acted upon by some force.

What force is it? Newton believed that it was the universal gravitation between objects.

There is a famous story that Newton discovered gravity when he was young because he was hit on the head by an apple and thought about why it fell instead of flying up. There are many inaccuracies in this story. For example, it was first told by Newton in his later years. Historians suspect that it may not have happened at all, but was just a matter of arguing for priority. For example, in the original story, Newton only saw the apple fall, not the apple hitting his head. Although the apple hitting the head is not as painful as the durian, it is still quite painful. But the most important thing is that physicists before him had already tried to explain why the apple fell. Newton's breakthrough was not to think about the fall of the apple, but to realize that the apple and the moon obey the same physical laws.

So why doesn't the moon fall to the earth? Newton said, imagine a cannon firing a cannonball at an increasingly faster speed, and the distance the cannonball lands will also become increasingly farther. What will happen if it keeps going farther and farther? The earth is round, and the distance it can travel around a circle is limited, so the cannonball will inevitably start to circle the earth once, twice, three times, or even more times before falling, and finally become a stable circular motion around the earth. It can be understood that although the cannonball is falling towards the earth, the surface of the earth is also sinking because it is round, and the two just cancel each other out.

This idea may not have been the first to come up with Newton. Another physicist at the time, Hooke, also believed that the attraction between celestial bodies was the driving force behind the movement of celestial bodies. The two argued for years over priority. However, Hooke could only calculate circular motion, not elliptical motion, and at the time, the astronomical community generally accepted that the orbits of planets were elliptical. It was Newton who mathematically proved that gravity could indeed lead to the elliptical orbits we observe.

Once the source of power for celestial motion was determined, the heliocentric vs. geocentric debate was naturally resolved. Forces act reciprocally, and the sun and earth exert the same gravitational pull on each other. But Newton's second law states that when subjected to the same force, the greater the mass, the smaller the acceleration. The sun is very large, so it should be almost motionless at the center. In comparison, the earth is very small, so it should be circling around the outside. This argument basically sentenced the modified geocentric theory to death, and when astronomical advances later discovered the existence of parallax, it completely put the lid on the geocentric theory.

Observation is the starting point of all sciences, but many problems cannot be solved by observation alone. We must go deep into the explanation of the underlying mechanism to explore the origin of the world. In fact, many ancient knowledge systems have accumulated a wealth of observations. The difference between them and science lies in whether they can correctly explore the mechanism behind the observed phenomenon.