Astrolabe

Astrolabe is an astronomical instrument for measuring the altitude of the sun or stars, and to determine the solution of various problems in astronomy, time, and navigation. Astrolabe was used from classical antiquity, about 2nd century BC,  until the age of discovery where it was superseded by the more accurate sextant, star charts, and time-keeping devices.

Flat-Earthers claim that astrolabes can only work because the Earth is flat. They are wrong. Astrolabes were designed using the spherical Earth model. To use an astrolabe, a good understanding of the spherical Earth model is required.

The star chart portion of an astrolabe consists of two parts: the mater that represents the view of the sky from the observer, and the rete that represents the celestial sphere.

The rete is a basic star chart, centered on the north celestial pole, representing the celestial sphere usually up to the tropic of Capricorn.  To use an astrolabe south of the equator, a specific southern rete is needed. Stars in rete are laid out using the stereographic projection, which preserves the shape of the constellations but distorts their sizes. Constellations closer to the edge are comparatively larger than those near the celestial pole.

The mater represents the sky as seen from the observer. It marks the boundary in which stars depicted in the rete are visible. Because Earth is spherical, there are specific plates for each specific observer latitude. As most astrolabes describe the celestial sphere up to the far tropic parallel, the mater cannot show the entire view from the observer unless the observer is north of the Arctic circle (or south of the Antarctic Circle for southern astrolabes).

In the flat Earth state of affairs, the rete is analogous to the azimuthal equidistant map, which flat-Earthers mistakenly regard as ‘the flat-Earth map’. While mater is analogous to the day & night boundary in the azimuthal equidistant map. In both cases, objects closer to the edge are distorted more. But in the case of astrolabes, the distortion can be easily seen and confirmed by merely looking at the sky. In a northern astrolabe, constellations toward the north appear smaller than those toward the south.

Astrolabes are designed using the spherical Earth model in mind, and they would not be usable if the Earth were flat. A good understanding of spherical Earth is required to use an astrolabe.

Simulating An Astrolabe Using Stellarium

Follow these step by step instructions to simulate the movement of an astrolabe using Stellarium. This instruction is written for Stellarium 0.18.2 under Linux. Other versions might have minor differences.

  • Set up your location (F6) and time (F5)
  • Open “Sky and viewing options window” (F4)
  • Under the “Sky” tab, change projection to “Stereographic” or “Fish-eye” whether we want to simulate a stereographic astrolabe or an azimuthal equidistant one.
  • Under the “SSO” tab, turn off “solar system objects.”
  • Under the “Markings” tab, turn on “Celestial Sphere”, and turn off all the others, except “azimuthal grid”, “equator (of date)”, “ecliptic (of date)”, “horizon”, “cardinal points”, “celestial poles (of date)”, “zenith and nadir”.
  • Under the “Landscape” tab,  disable “Show ground”.
  • Under the “Starlore” tab, select “Western,” enable “Show constellation lines” and “Show labels.”
  • Open “Search Window” (F3), open the “Position” tab. Set the “coordinate system” to “Equatorial.” Set “Right ascension” to 0° and “Declination” to “+90°” if you are north from the equator, or “-90°” if you are south from the equator.’
  • Zoom out all the way (mouse wheel scroll).

Stellarium should now show how an Astrolabe looks. Use Ctrl-Mouse drag to travel through time, and see the movement of an astrolabe.

Moving parts (the ecliptic line and the stars) belong to the rete. Static parts (azimuthal grid, horizon, and zenith markings) belong to the mater. The equator line can belong to either.

At the time this article was written, Stellarium cannot display the tropic of Capricorn/Cancer lines. We can enable “Equatorial grid (of date) to get an approximation of the positions of both parallels (they are at +23.5° and -23.5°). Like the equator line, the tropic parallels can belong to either the mater or the rete. Most northern hemisphere astrolabes were designed to show stars up to the tropic of Capricorn.

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