There is no simple observation that leads you to this conclusion. The only real data that shows that the Earth orbits the Sun is found in a phenomenon called 'stellar aberration' which is very hard to explain even to college students in an introductory astronomy course.

Unlike the motion of a pendulum which swings back and forth and eventually rotates its plane of swinging 360 degrees over the course of a full day, the motion of the Earth around the Sun, without discussing stellar aberration, has to be accepted for other reasons that may make sense to students if they are not too inquisitive and questioning.

The sun has over one million times the mass of the earth. It makes more 'sense' for a puny earth to go around the massive sun, than the other way around. ALso explain the the gravity of the sun is millions of times stronger than earth's gravity so this makes the earth go around the sun not the other way around.

Copernicus felt that the magnificent Sun was the natural candidate to orbit, and from this he created the modern version of heliocentrism.

Once Kepler identified that the orbits of the planets were elliptical not circular this led him to the discovery of three simple laws for planetary motion which could not be uncovered from the Ptolemaic model.

70 years later, Sir Isaac Newton showed how these three laws were a direct result of the 'inverse square law' behavior of the force of universal gravitation from the Sun.

So, Copernicus's assertion that the Earth revolves around the Sun leads directly to a very simple explanation for this behavior in terms of the basic laws of mechanics and gravitation which were discovered 130 years later! Ptolemaic geocentrism would never have been found to be consistent with how gravity operates.

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