Azimuth Dial - Analemmatic Dial

The azimuth dial, as the name implies, depends mainly on the azimuth of the Sun. The only representative of the azimuth dial is the analemmatic dial.

Characteristics

The analemmatic dial is a pole - style dial. It is not commonly used because its style requires daily setting. The gnomon is a vertical pin or rod which is moved about from place to place according to the sun's declination.

General Appearance

analemma picture 1 analemma picture 2

analemma picture 3 analemma picture 4
Figure 49:Analemmatic dial in the University of Georgia (USA)

general appearance of analemmatic sundial
Figure 50: Dial face of an analemmatic dial

The analemmatic dial consists of hour points which fall along the circumference of a horizontal elliptical dial face. The major axis of the ellipse runs east-west. A date line is set along the minor (north-south) axis. A vertical gnomon, preferably a person with his hands pressed above his head, is placed on the correct date. The time is read from the point where the shadow (or its extension) of the person's finger tips intersects with the ellipse. (See Figure 51.) The size of the dial face should fit a human gnomon.

demonstration of how to use analemmatic sundial
Figure 51

The shape of the analemmatic dial depends on the latitude. On the equator, the ellipse is pinched into a straight line, with a long date line at right angles. Moving away from the equator, the date line shrinks and the minor axis of the ellipse grows. At the poles, the ellipse would become a circle, and we get the equatorial dial.

How it works

derivation of analemmatic sundial
Figure 52

The principles behind the analemmatic dial can be derived from the equatorial dial. We shall consider an equatorial dial of the armillary type. Choose a certain time, for instance 11 o'clock. At 11 o'clock on different days, the shadow of the gnomon always hits the 11 o'clock mark on the hour ring.

We can find out which point of the gnomon causes the shadow by drawing a ray of sunlight 'through' the gnomon to the 11 o'clock mark. On the summer solstice (21st June) the Sun is high in the sky, and it is the green ray that causes the shadow. On the equinoxes (21st March or 22nd September), the Sun is lower and gives the yellow ray. (The hour ring in the picture is closed, so that the front part would block the Sun. Just imagine that the front is open.) On the winter solstice (21st December), the Sun is low, and it is the purple ray that causes the shadow on the hour mark.

In this way a date scale could be constructed along the gnomon, which would run from the green dot (21st June) to the purple dot (21st December) and back again to 21st June.

Now project the hour ring vertically onto the ground. The projection is an ellipse, with the major axis running east-west and the minor axis running north-south. The projections of the hour marks are on the ellipse; some are indicated by blue dots. The point for local noon is at the north side, on the minor axis.

Also project the date scale that we constructed along the gnomon, vertically on the ground. And imagine that the vertical green, yellow and purple lines are real rods. On 21st June, the shadow of the vertical green rod will just hit the 11 o'clock mark on the ellipse. Likewise, on 21st March and 22nd September, the 11 o'clock mark will be hit by the shadow of the yellow vertical rod, and on 21st December by the shadow of the purple rod.

Replace the row of vertical rods by a single, movable rod, a person for instance. When the person sets himself at 11 o'clock on the correct point of the date scale, his shadow will hit the 11 o'clock mark on the ellipse.