Editing Astrolabe (section)

==== The meridians and the measurement of azimuth ====
[[File:Proyección estereográfica del horizonte y azimut sobre el tímpano de un astrolabio.png|thumb|upright=2.8|Stereographic projection of the north-south meridian and a meridian 40° E on the tympanum of an astrolabe]]

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|#D92929}} The five red dots represent the [[zenith]], the [[nadir (astronomy)|nadir]] (the point on the [[celestial sphere]] opposite the zenith with respect to the observer), their projections on the celestial equatorial plane, and the center (with no physical meaning attached) of the circle obtained by projecting the secondary meridian (see below) on the celestial equatorial plane.
# {{color box|#DA5E1B}} The orange circle represents the [[Meridian (astronomy)|celestial meridian]] (or meridian that goes, for the observer, from the north of the horizon to the south of the horizon passing through the zenith).
# {{color box|#D92929}} The two red circles represent a secondary meridian with an [[azimuth]] of 40° East relative to the observer's horizon (which, like all secondary meridians, intersects the principal meridian at the zenith and nadir), 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 [[Meridian (astronomy)|celestial meridian]], it results in a straight line that overlaps with the vertical axis of the tympanum, where the [[zenith]] and [[nadir (astronomy)|nadir]] are located. However, when projecting the 40° E meridian, another circle is obtained that passes through both the zenith and nadir projections, so its center is located on the perpendicular [[bisection]] of the segment connecting both points. In deed, the projection of the celestial meridian can be considered as a circle with an infinite radius (a straight line) whose center is on this bisection and at an infinite distance from these two points.

If successive meridians that divide the celestial sphere into equal sectors (like "orange slices" radiating from the zenith) are projected, a family of curves passing through the zenith projection on the tympanum is obtained. These curves, once overlaid with the ''rete'' containing the major stars, allow for determining the [[azimuth]] of a star located on the ''rete'' and rotated for a specific time of day.