What Does a Sundial Prove?

What does a sundial prove?

A sundial is a device that shows the time by indicating the position of the Sun at a given location. It can be a simple device that only uses a line or spot of light, or it can be complex, utilizing both the altitude and the azimuth (direction) of the Sun.

The most common type of sundial is the classical sundial. It has straight hour lines and a conic section of declination lines. This is based on the Sun’s position on the celestial sphere, which changes with the seasons. In particular, the Sun is at a positive declination in spring and summer and negative declination in autumn and winter, with its path on the celestial sphere changing by one complete revolution per year.

In most classical sundials, the gnomon aligned with the celestial axis will cast a shadow that moves uniformly about this axis at about 15deg per hour. The surface-shadow cast by the gnomon likewise rotates uniformly, and the hour-lines on the dial are equally spaced; this is called a “symmetrical” design.

However, the axis of rotation for the Sun is slightly elliptical (in contrast to a perfectly circular orbit) and its tilt about the plane of the Earth’s orbit (called obliquity) causes the Sun’s apparent motion around the Earth to be not as uniform as the symmetrical design suggests. This means that standard clock time and sundial time differ by a small amount, usually a quarter-hour.

To account for this, some sundials have a gnomon that casts a shadow that is different from the other, and the hour-lines on the dial may vary with the time of year. These designs are found in ring dials, analemmatic sundials, and vertical gnomons such as obelisks.

Some sundials also use a refracting medium, such as water to reshape the hour lines. This has been used in various ways, including to recreate the Miracle of Ahaz22,23 and to show the Earth’s longitude on a sundial by reshaping its hour lines.

There are also a number of other types of sundials, such as the equatorial sundial and the armillary sphere. In a classical sundial, the cone of light rays cast by the gnomon intersects the flat surface at a specific point, which is known as the analemmatic point. This analemmatic point is plotted against the equation of time (see below) to give the local solar time at the analemmatic point and on the surface.

Other sundials are designed to follow the movement of a light-spot or a shadow-tip. The cone of light rays cast by the light-spot is a figure-8 curve that is inscribed into a dial, and the analemmatic point has an engraved figure-eight shape corresponding to this. When the edge of the light-spot or shadow-tip touches the part of this curve that represents the current month, it gives the local solar time at the analemmatic sundial and on the dial itself.

In addition, some areas of the world practice daylight saving time, which changes the official clock time by one hour. This shift must be added to the solar time on the dial to make it agree with the official time. This is often done on the dial itself, by numbering the hour-lines with two sets of numbers or swapping them in some designs.