The relativity of motion: why we cannot feel the Earth turn
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Rafal searches for how the Earth could move without our feeling it. As he stumbles, he sees “the mountains upside down” for an instant and understands: it is his eyes that moved, not the mountains. He immediately generalizes: “If the observer moves, the motionless distant object will seem to move too.” The intuition of the relativity of motion is born.
The subject in depth
Saying that an object “moves” is meaningless on its own: you must specify relative to what [Wikipedia] . That “relative to what” is called a frame of reference. A passenger seated in a train is at rest relative to the carriage, but moving relative to the platform. Both descriptions are equally valid.
The principle of relativity, formulated by Galileo, goes further: in a frame that moves in a straight line at constant speed, the laws of physics are exactly the same as at rest. No experiment done inside, without looking out, can tell whether you are moving. That is why an object dropped in a steadily moving train falls at your feet, not toward the back.
Applied to the Earth, this principle dissolves the key objection against heliocentrism. The Earth carries us with it, atmosphere included, at nearly constant speed. We share its motion, so we do not feel it, exactly as Rafal stops seeing the mountains move once he understands that it was he who moved.
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Heliocentrism
Hubert defends a forbidden idea in Orb: the Sun at the center, the Earth turning. We explain heliocentrism, its history, and why it caused such an uproar.
The Earth's rotation and revolution
Hubert explains it in Orb: the Earth moves twice. We separate rotation (day and night) from revolution (seasons), and why we feel nothing.
Ptolemy's geocentric model
Before heliocentrism, the Earth sat at the center of the world. We explain Ptolemy's geocentric model, its epicycles, and why it lasted over a thousand years.
The constellation Orion
Orion's Belt is a clue in Orb, centered on the star Alnilam. We explain this constellation, its three aligned stars and how to spot it.
Aristotelian physics
Rafal cites Aristotle to prove the Earth is at the center in Orb. We explain Aristotelian physics and its link to geocentrism.