Editing Astrolabe (section)

==== The tropics and the equator define the tympanum ====
[[File:Proyección estereográfica de trópicos y ecuador en un astrolabio.png|thumb|upright=2.8|Stereographic projection of Earth's tropics and equator from the South Pole.]]
On the right side of the image above:

# {{color box|#C4F5F5}} The blue sphere represents the [[celestial sphere]].
# {{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 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]].
# Three parallel circles represent the projection on the celestial sphere of Earth's main [[Circle of latitude|circles of latitude]]: 
#* {{color box|#DA5E1B}} In orange, the celestial [[Tropic of Cancer]]. 
#* {{color box|#8533DB}} In purple, the celestial [[equator]].
#* {{color box|#006713}} In green, the celestial [[Tropic of Capricorn]].

When projecting onto the celestial equatorial plane, three concentric circles correspond to the celestial sphere's three [[Circle of latitude|circles of latitude]] (left side of the image). The largest of these, the projection on the celestial equatorial plane of the celestial [[Tropic of Capricorn]], defines the size of the astrolabe's tympanum. The center of the tympanum (and the center of the three circles) is actually the north-south axis around which Earth rotates, and therefore, the ''rete'' of the astrolabe will rotate around this point as the hours of the day pass (due to [[Earth's rotation|Earth's rotational motion]]).

The three concentric circles on the tympanum are useful for determining the exact moments of [[solstice]]s and [[equinox]]es throughout the year: if the sun's altitude at noon on the ''rete'' is known and coincides with the outer circle of the tympanum (Tropic of Capricorn), it signifies the [[winter solstice]] (the sun will be at the [[zenith]] for an observer at the Tropic of Capricorn, meaning summer in the southern hemisphere and winter in the northern hemisphere). If, on the other hand, its altitude coincides with the inner circle (Tropic of Cancer), it indicates the [[summer solstice]]. If its altitude is on the middle circle (equator), it corresponds to one of the two [[equinox]]es.