Hour Circles are drawn across the celestial poles and perpendicular to the celestial equator, whereas parallels of declination are drawn parallel to the celestial equator. Longitude is the angle formed by this circle at the pole with the meridian passing through any other location. The Zero Hour Circle is the hour circle that passes through the location of the Spring Equinox. The Zero Hour Circle cut the Earth at that spot when the Great Pyramid was erected, much as it cut the position of the Greenwich Observatory when it was established centuries later. The world's time should follow the Equinoxes' precession, but this would need not only the movement of the Observatory every 70 or 700 years, but also the displacement of all time zone meridians - clearly a daunting task.
