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In a university course on the logic & philosophy of science a teacher once asked us to write an essay attempting to prove the idea that the Earth revolves around the Sun from a commonsense point of view. There was to be no appeal to information not immediately evident from our senses, including the fact that scientists have told us that the Sun is much larger than the Earth. In my essay I imagined myself to be an unaging person who lived and roamed the planet from ancient to modern times.
In the whole of my life of several thousand years I have never developed much ability in mathematics (despite an intuitive insight into geometry), so my ability to keep-up with the mathematical astronomers has always been limited.
I have followed the popular views on astronomy and have also followed the authorities in their shift from the Ptolemaic System to the Copernican System. Nonetheless, I have never been mathematically competent enough to judge their conclusions based on their mathematical models. Therefore, the alterations in my views have been very gradual over the centuries and cannot really be said to be the result of a sudden decision of any sort. Rather, I have gradually & continually ascribed less probability to the system of Ptolomy and more probability to the system of Copernicus.
By my first observations it seemed evident that the Sun came up on one side of the Earth and went down on the other. That observation in itself led me to believe that the Sun continued its path "under" the Earth at night to rise again the next day. Of course it could have been a different Sun, and its evening path could have been to roll-around the side, but consistency of behavior has always appealed to me.
Further observation revealed the existence of five Wandering Stars, two of which (Venus & Mercury) are evident only in the early evening and near dawn. These Wandering Stars not only failed to follow the sphere of the other stars, but they had the "habit" of slowing down and changing directions. Claud Ptolomy seemed to have a pretty good theory in his idea that these wanderers made small circuits (or "epicycles") as they circled the Earth. Frankly, I couldn't ignore the intimate association of Venus & Mercury with the Sun, and was rather inclined to agree with Heraclitus Ponticus that those bodies orbited the Sun, which in turn orbited the Earth.
My sailing career added to my observations, and I learned much from travelling with European explorers. I observed that the North Star remain fixed while the other heavenly bodies rotated about it. And when I traveled North or South for considerable distances, many constellations would become visible which had not been evident before, and a proportional amount of stellar horizon would "roll-off" the horizon. This, combined with the fact that I could travel West from Europe and return to Europe by following the Sun (without ever going East) gave me solid evidence that the world is roundish. I could even see the roundness, in a sense, because land becomes visible from the mast of a ship before it becomes visible from the deck. Similarly, when a ship disappears over the horizon, the deck disappears before the mast does. It is evident that since water seeks the lowest point, and that since it doesn't flow "downwards" with the ship, that it is probably clinging uniformly to the spherical Earth.
I did considerable speculation with the evidence I had before me. It seemed to me that since the Earth must be spherical — and "contained" within a sphere of heavenly bodies — that either the Earth could be rotating within the greater sphere or that the greater sphere is rotating around the Earth. Or perhaps both, if it is simply a matter of "point of view". I felt very much like the duck in the bottle speculating as to whether the bottle contains the universe and he is on the outside, or vice-versa. It does appear to me when I am sailing into or out of a port that the port is moving, and not I.
By analogy, it seems odd that the container of the Earth should move rather than the Earth — although until the distance to the stars was determined, this could be debatable. If stars are at great distances and of greater sizes than the Earth, it would seem odd that they would be hurtling about the Earth at enormous velocities. The universe is mysterious, however, and stranger things than this are seen. The thought that the universe must necessarily be boundless (irrespective of the size and distances of the stars) is what caused me to doubt the idea of a sphere of heavenly bodies rotating about the Earth.
But by assuming that the Earth is rotating, it seemed likely that the Sun could be more stationary than it appears. Since the periodicity of the Sun's position with respect to the heavenly sphere is approximately one year, the Sun could be very slowly rotating about the Earth and most of its apparent motion could actually be due to the Earth.
The Moon, though it appears to be of about the same size as the Sun, is in actuality probably closer and smaller in size, since in a total eclipse it appears totally dark. In the day its phases show that its light side is directed toward the Sun. So it seems plausible that its phases at night also indicate the probable position of the Sun at that time. This possibility became especially persuasive to me in the light of observations Galileo showed me through his telescope, in exchange for doing his laundry and various janitorial chores.
The first observation is that Jupiter has four bright satellites (moons?). This could be an instance of a rotating heavenly body with smaller heavenly bodies rotating about it (all of which are spherical). The second observation is that spots on the Sun seem to indicate rotation of that sphere — which could be a good analogy for the rotation of the Earth as a heavenly sphere.
The third observation is that Venus exhibits phases just as does the Moon. This observation indicates either that Venus is rotating around the Earth at a closer orbit than the Sun (but not at a farther orbit, since phases would not be evident) or that it is rotating around the Sun. The former possibility is rejected both because of the aforementioned morning and evening appearance of Venus, as well as because "eclipses" of the Sun by Venus do not appear to occur (these would be more conspicuous than sunspots, in all probability).
Not long afterwards Copernicus announced his theory of a Sun-centered Solar System, which seemed to eliminate the need for most (but not all) of the confusingly complex epicycles of Ptolomy. Hot upon his tail was Kepler, who eliminated all of the epicycles with his theory of elliptical orbits. I must confess that I never understood the mathematics of these gentlemen and was thus less shaken by their evidence than was the scientific world. I was impressed by the attempt of Tycho Brache to see the parallax of the stars' direction of movement with the extreme positions of the Earth in the proposed orbit of the Earth around the Sun. His failure to find a parallax, however, cast a gloom upon me which was not lifted until 1838 when Friedrich Bassel succeeded in finding the parallax.
The determination of the size and distances of the heavenly bodies by careful measurements of their angles from distant points of the Earth — especially the indicated size of the Sun — gave me more reason to accept the system of Copernicus. The idea that the Earth is just one of the Wandering Stars seems very plausible. Nonetheless, I must still acknowledge that much of my personal evidence for my beliefs is based on hearsay, and on trust in great "authorities". None of my personal observations since my time with my last and best good friend Galileo have contributed much more to my acceptance of the idea that a tiny Earth is in orbit around a massive Sun.
In the interest of personal history I am not going to alter the text as I originally
wrote it. But a reader has noted that correlation of the behavior of the tides with the position
of the Sun and the Moon would provide further support for the idea that the Earth revolves
around the Sun.