Sundial time lapse

A sundial is a device that tells the time of day by the position of the Sun. In common designs such as the horizontal sundial, the sun casts a shadow from its style onto a surface marked with lines indicating the hours of the day. The style is the time-telling edge of the gnomon, often a thin rod or a sharp, straight edge. As the sun moves across the sky, the shadow-edge aligns with different hour-lines. All sundials must be aligned with the axis of the Earth's rotation to tell the correct time. In most designs, the style must point towards true celestial north (not the north magnetic pole or south magnetic pole). That is, the style's horizontal angle must equal the sundial's geographical latitude.
It is common for inexpensive decorative sundials to have incorrect hour angles, and these cannot be adjusted to tell correct time.

The principles of sundials can be understood most easily from the Sun's apparent motion. Scientists have proven that the Earth rotates on its axis, and revolves in an elliptical orbit about the Sun; however, meticulous observations and experiments were needed. For the purposes of a sundial, an excellent approximation assumes that the Sun revolves around a stationary Earth on the celestial sphere, which rotates every 24 hours about its celestial axis. The celestial axis is the line connecting the celestial poles. Since the celestial axis is aligned with the axis about which the Earth rotates, the angle of the axis with the local horizontal is the local geographical latitude.

Unlike the fixed stars, the Sun changes its position on the celestial sphere, being at a positive declination in summer, at a negative declination in winter, and having exactly zero declination (i.e., being on the celestial equator) at the equinoxes. The Sun's celestial longitude also varies, changing by one complete revolution per year. The path of the Sun on the celestial sphere is called the ecliptic. The ecliptic passes through the twelve constellations of the zodiac in the course of a year.


Source: Wikipedia

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