Someone made the Earth.

They placed man on the surface and gave them plants, animals, the waters, and the mountains to make their life compete.

The Earth exists within a spherical firmament upon which the Moon, planets, stars and the sun are affixed.

The firmament is, of course, an artificial construct. The Moon, stars, and planets are projections upon its surface. The Sun is a radiation source that travels around the sphere, giving light and heat to the inhabitants.

The Earth has an axial tilt, and the Sun's radiation fluctuates with the seasons. The Moon, as well as the planets and stars is a light source, one that waxes and wanes periodically.

Given a civilisation and history that mirrors our own, at what point in history would astronomers prove the actual existence of the firmament and how would they do so?

Assume that the firmament is roughly twice as far away as the Moon is now. The Sun or Moon do not visibly illuminate other items on the inside surface of the firmament.

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    $\begingroup$ Parallax. The Greek astronomers computed the distance to the Moon measuring its parallax; they also knew that the stars were much much farther away, because they could not measure their parallax, not even their annual parallax -- the first stellar annual parallax was measured in 1838 (about 0.5 arc-seconds for the star 61 Cygni, 10 light-years from our Sun). $\endgroup$
    – AlexP
    Apr 13, 2017 at 15:23

1 Answer 1


Ancient astronomers (at least of the Classical Greek period, maybe earlier) were aware of trigonometry and able to use parallax to measure some distances in the solar system. The complete absence of (daily or yearly) parallax for celestial bodies will let some sharp people conclude that they all are at the same distance. Triangulating with some big triangle (like Erastosthenes did) will give them even the distance of firmament.

A firmament has probably some more side effects (including a greenhouse effect on the climate), but I doubt that such kind of effects were accessible to Ancient astronomers.

The final proof would follow after the development of the laser: With a laser, an artificial star can be projected onto the firmament.

  • $\begingroup$ Why do you think that the objects like stars won't move around firmament same way as Moon or Sun, to make it look exactly as in real? Then your triangulation would show just same data as in real life without firmament. Because it is not about where the source is, but about angle we get the information from. $\endgroup$ Apr 13, 2017 at 10:15
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    $\begingroup$ @AntoineHejlík This would imply different firmaments for each place on the Earth. When there is only one firmament with one projection for all of the Earth, the effects are measurable and the measurement is possible in the Classical Greek period. $\endgroup$ Apr 13, 2017 at 10:22
  • $\begingroup$ I think the proof would be a descrepancy between paralex caused by the moving Earth (which is faked by the display) and no paralex from simultanious observations at different points of Earth, since the screen is flat. $\endgroup$
    – JDługosz
    Apr 13, 2017 at 17:53

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