Level horizontal sundials (the spider's web of Eudoxus and the "pelekinos" of Patrokles)

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Level horizontal sundials
(the spider's web of Eudoxus
and the "pelekinos" of Patrokles)


A net with 11 engraved hour lines and three curves indicates, with the help of the shadow trace from the vertical gnomon top (orientated to the South) 10 out of the 12 unequal (depending on the season) daily hours. The closest curve to the gnomon indicates the course of the sun’s shadow at the summer solstice, the most distant at the winter solstice and the middle one (straight line) at the equinoxes. All the net of lines was named "Analemma" and sometimes it included 4 more curves that defined the shade's orbit of the sun for the entry of the remaining zodiacal periods (months). Later an oblique gnomon was used with inclination equal to the geographic width of the place (φ) so that only the orientation of the shadow for the indication of the hours was enough.