created by Carl Sabanski
Types of Sundials
The British Sundial Society Sundial Glossary defines over 100 types of sundials, some of which have more than one name. Some are simple dials while others are complex. The dials discussed in this section are ones that can be designed and built without great difficulty. A lot of the design difficulty can be removed by using the free sundial design software that is available on the internet. These can be found by going to the "Links" page on the site of the North American Sundial Society.
Although there are many types of sundials, they can be grouped by how they tell time. This includes:
Polar-Pointing Gnomon Sundials
These sundials include any type with a gnomon that points to the celestial pole and includes the horizontal, vertical, polar and equatorial sundials. What is the benefit of a gnomon that points to the celestial pole? Figure 1 illustrates two gnomons that will be discussed. Note that both have the same vertical height "H" and project a shadow on a horizontal plane.
Figure 1: Two Gnomons
The vertical gnomon is a rod of height "H" that casts a shadow throughout the day. If you plot the passage of the tip of this shadow on a dial plate, then perhaps you can use it to tell the time. This is illustrated in Figure 2 for a single day each month. Also shown is the position of the shadow tip at four different hours of the day. If you were to use the base of this vertical gnomon as the origin of the dial, problems arise.
Figure 2: Vertical Gnomon
By examining the 1 pm shadow tips it can be seen that the angle "A" made with the noon line varies every day. Every day, during half the year between solstices, the 1 pm shadow tip will point in a different direction relative to the noon line. For the same time period, noon to 1 pm, the shadow tip travels over a different angle each day during this period. This can also be seen from the 10 am and 11 am shadow tips. The angles "X" and "Y" are different for the two days shown as will all the other angles be for this time period. During the other half of the year the shadow will travel back along the same line. All these variations make this a very difficult way to tell the time.
The polar-pointing gnomon in Figure 1 is also a vertical rod of height "H". However, in this case the origin of the dial is located at a point south of the rod such that the style intersects the horizontal plane at an angle equal to the latitude of the dial's location. The style now points to the celestial pole. The distance to the origin from the base of the rod is:
Origin = H / tan (LAT)
Although the gnomon is still the same identical vertical rod, moving the origin to this new location has interesting results. This can be seen in Figure 3, which shows the same shadow tips as above.
Figure 3: Polar Pointing Gnomon
By examining the 1 pm shadow tips it can be seen that the angle "A" made with the noon line is the same every day. Every day, throughout the year, the 1 pm shadow tip will point in the same direction relative to the noon line. For the same time period, noon to 1 pm, the shadow tip travels over the same angle each day. This can also be seen from the 10 am and 11 am shadow tips. The angles "X" and "Y" are identical. This is the case for every hour. It becomes easy to the tell the time using a sundial with a polar pointing gnomon.
Altitude (Elevation) Sundial: any dial that uses the sun's altitude, rather than its azimuth , for indicating time. Usually does not need to be aligned North-South. Examples are ring dials, flag dials, and shepherds' dials.
Altitude (elevation): the angular distance of the (centre of) the sun's disk above the observer's horizon (negative values indicate that the sun is below the horizon).
Azimuthal (Azimuth) Sundial: any dial that uses the sun's azimuth for indicating the time. It usually needs to be aligned North-South., and has a vertical style (if it has no dependence on altitude). An example is the analemmatic dial.
Azimuth: the angle of the sun, measured in the horizontal plane and from true south. Angles to the west are positive, those to the east, negative. Thus due west is 90°, north is ±180°, east -90°.