What arguments do flat-Earth proponents use about the horizon to claim a flat planet?

1. How flat-Earth advocates turn everyday sight into proof of a flat world

Flat-Earth rhetoric centers on the everyday visual impression that the world looks flat and that the horizon always meets the eye, which they present as simple, empirical proof. Proponents invoke the law of perspective and ordinary binocular vision to claim that distant objects simply shrink and vanish due to perspective, not because the surface curves; this argument is bolstered by the contention that the horizon “rises to eye level” no matter the observer’s height ^{[2] [4]}. They frequently treat low-altitude observation as definitive, asserting that if human perception does not reveal curvature directly, then the spherical model is suspect. That claim relies on selective use of perception and dismisses controlled measurements and instruments that extend beyond unaided vision.

2. The ship-on-horizon claim and the safety-net of alternate explanations

A signature flat-Earth talking point is that ships “disappear hull-first” only because of perspective, atmospheric distortion, or insufficient visual acuity, not because they move over a curve. Flat-Earthers deploy what has been described as “safety net answers”—multiple alternative explanations (perspective, refraction, limited resolution) that allow them to reinterpret or dismiss counterexamples, preventing a single failed test from falsifying the overall claim ^[5]. This rhetorical strategy creates resilience: any experiment can be reframed as inconclusive due to refraction or equipment error, which shifts the burden onto challengers to eliminate every possible confound, rather than producing predictions that could be decisively disproven.

3. The refraction argument: an internally inconsistent escape hatch

Some proponents claim atmospheric refraction explains why distant objects sometimes appear above or at the horizon when a simple curvature model would hide them. That requires light to bend in ways inconsistent with standard atmospheric profiles—effectively demanding upward bending of rays at scales and directions that conflict with how density gradients normally refract light. Scientific reviews show refraction can alter apparent positions but cannot consistently reproduce the systematic dip-with-altitude and hull-first disappearance predicted by a spherical geometry, making refraction an inadequate universal explanation for the phenomena cited by flat-Earthers ^{[6] [3]}.

4. Measurements and instruments that contradict the ‘eye-level horizon’ claim

Practical instruments—altimeters, theodolites, flight attitude indicators, and simple experimental setups like water-level bottles or horizon dip calculations—show the true horizon lies below eye level and that the angle of dip increases with observer altitude in a predictable manner. These measurements are reproducible and match the geometric predictions of a spherical or oblate Earth, so they systematically contradict the claim that the horizon always rises to meet the eye. Flat-Earth sources often ignore or dispute instrument calibration, or interpret photographic limitations as evidence of systematic manipulation rather than instrument-confirmed geometry ^{[3] [1]}.

5. The broader context: why horizon arguments persist and what’s omitted

Horizon-based flat-Earth arguments succeed rhetorically because they appeal to immediate perception, exploit scientific illiteracy about scale and perspective, and introduce multiple ad hoc explanations that shield the belief from disconfirmation. Critics note that proponents often omit large-scale, independent datasets—satellite imagery, circumnavigation routes, geodetic surveys, and flight logs—that corroborate curvature at many scales. The debate is as much about interpretation and trust as it is about raw observation: flat-Earth advocates frame institutional evidence as conspiratorial while mainstream science emphasizes reproducible measurements and cross-validated instrumentation ^{[7] [8] [4]}.

Conclusion: The horizon arguments used by flat-Earth proponents rely on intuitive impressions, alternative optical explanations, and rhetorical strategies that avoid decisive falsification. Empirical counters—dip-of-horizon measurements, instrument data, and physics of atmospheric refraction—provide consistent, testable evidence for a curved Earth that flat-Earth rebuttals fail to reconcile across the full range of observations ^{[1] [3] [4]}.