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« Astronomiae Instauratae Mechanica » - ARMILLAE  ZODIACALES  i
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i  « ... The "armillae" represent the celestial sphere with its main circles: the meridian, the celestial equator and the ecliptic ... used to measure the celestial co-ordinates of a particular "star" or "errant star"  ...   i  Many problems of mechanical nature [unwanted deformations] and of observational nature [precision in measurements] kept Tycho’s mind very much active and alert ...  i  Finally, he decided to build an "armillae" of gigantic proportions, half buried in the ground, to reduce the impact of the wind. Moreover he built around it a small amphitheatre with a flight of steps, that allowed the observer to find an optimum position for his observations in all conditions ... this is the origin of the "Armillae Aequatoriae Maximae sesquialtero constantes circulo" ...  »  i
«… The circle BCD represents the Meridian, this circle intersects the Zenith, the North and the South Pole ...  i  This circle can be rotated vertically to suit the latitude of the observational site ...  i  The CD axis represents the North-Southern direction; C, in particular, represents the Polar position on the celestial sphere ...  i  If we stand onto the North Pole, C coincides with B, while, if we stand onto the Equator, the axis CD becomes horizontal, as at latitude zero, the Polaris is on the horizon ...  i  The plumb-line falling from B [Zenith] is used to level the whole system... the meridian circle must be North-South oriented...  i  If, at this point, the Ecliptic’s circle is parallel to the astronomical Ecliptic, the measurements can be carried out ... »  i
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