the local geographical latitude, denoted Φ. All the hour lines converge to a single centre; the angle θ of a given hour line with the noon hour is given by the formula where t is the time (in hours) before or after noon. However, the vertical gnomon does not always stand at the centre of the hour lines; rather, to show the correct time, the gnomon must be moved daily northwards from the centre by the distance where W is half the width of the ellipse and δ is the Sun's declination at that time of year. The declination measures how far the sun is above the celestial equator; at the equinoxes, δ=0 whereas it equals roughly ±23.5° at the summer and winter solstices. See also Analemma References External links Analemmatic sundial generator Analemmatic and Horizontal Sundials of the Bronze Age History and principle of analemmatic sundials Sun declination tables (mean) Calendars Sundials Sculptures