Editing Astrolabe (section)

==== The horizon and the measurement of altitude ====
[[File:Proyección estereográfica del horizonte sobre el tímpano de un astrolabio.png|thumb|upright=2.8|Stereographic projection of an observer's horizon at a specific latitude]]
On the right side of the image above:

# {{color box|#0325F1}} The blue arrow indicates the direction of true north (the [[Polaris|North Star]]).
# {{color box|#0325F1}} The central blue point represents Earth (the observer's location).
# {{color box|#001919}} The black arrow represents the [[zenith]] direction for the observer (which would vary depending on the observer's [[latitude]]).
# {{color box|#001919}} The two black circles represent the [[horizon]] surrounding the observer, which is perpendicular to the zenith vector and defines the portion of the [[celestial sphere]] visible to the observer, and its projection on the celestial equatorial plane. 
# {{color box|#001919}} The geographic south of the celestial sphere acts as the [[Stereographic projection|projection pole]].
# {{color box|#B8B8B8}} The celestial equatorial plane serves as the [[projection plane]].

When projecting the [[horizon]] onto the celestial equatorial plane, it transforms into an ellipse upward-shifted relatively to the center of the tympanum (both the observer and the projection of the north-south axis). This implies that a portion of the celestial sphere will fall outside the outer circle of the tympanum (the projection of the celestial [[Tropic of Capricorn]]) and, therefore, won't be represented.

[[File:Proyección estereográfica del horizonte y almucantar sobre el tímpano de un astrolabio.png|thumb|Stereographic projection of the horizon and an almucantar.|upright=2.8]]

Additionally, when drawing circles parallel to the horizon up to the zenith ([[almucantar]]), and projecting them on the celestial equatorial plane, as in the image above, a grid of consecutive ellipses is constructed, allowing for the determination of a [[Horizontal coordinate system|star's altitude]] when its ''rete'' overlaps with the designed tympanum.