Capricorn: Dec.22-Jan.29 The Sundial Primer
created by Carl Sabanski
Capricorn: Dec.22-Jan.29

The Sundial Primer Index

DeltaCad Sundial Macros - Valentin Hristov - Spider Sundials

The term "spider" sundial is typically associated with the azimuthal or azimuth sundial. It is a horizontal sundial and uses the sun's azimuth for indicating the time. The gnomon is vertical and located in the middle of a system of circular date lines. This sundial does not depend on the sun's altitude.

Valentin's first spider sundial macro is based on the above description. He also includes a couple of polar pointing sundials that have the "spider" appearance but with a less curved shape of the hour lines for more convenient reading. The sundial plane for all the sundials can have an arbitrary position - declining and inclining. It is required to know the date to determine the time.

Be sure to scan to the bottom of the page! There you will find a newer version of Valentin's spider sundial macro, spiderarb. It will allow you have both an arbitrarily oriented dial plane as well as gnomon. It provides a number of other additional features.

Spider Sundial 1 with Perpendicular Gnomon - spidern

This spider sundial is generated after the user enters the "Latitude", "Longitude", "Central meridian", "Declination of the plane" and "Inclination of the plane" of the sundial's location as shown in Figure 1. A "Place" descriptor can also be included. The hour lines are automatically adjusted for the Equation of Time. Longitude correction can be removed by entering the same value for "Longitude" and "Central meridian".

Figure 1 - Spider Sundial 1 Macro

Figure 1 - Spider Sundial 1 Macro

The sundial drawn with this macro is shown in Figure 2. This is the particular case of a typical spider azimuthal sundial, a horizontal sundial with the gnomon located in the small circle at the centre of the dial and perpendicular to the dial plate, i.e. with a vertical gnomon. The shape of the hour lines is the reason for the name of this type of sundial. The figure shows a dial plate where the top is positioned to the north celestial pole. Normally you would be stand to the south to read the sundial and your shadow could get in the way. By designing a sundial with the "Declination of the plane" set to 180 the sundial would be read from the north and your shadow would create no problems.

Figure 2 - Spider Sundial 1

Figure 2 - Spider Sundial 1

There are two sets of hour numbers. The blue indicate standard time and the red indicate Daylight Saving time.

Spider Sundial 2 with Polar Pointing Gnomon - spiderp

This spider sundial is generated after the user enters the "Latitude", "Longitude", "Central meridian", "Declination of the plane" and "Inclination of the plane" of the sundial's location as shown in Figure 1. A "Place" descriptor can also be included. The hour lines are automatically adjusted for the Equation of Time. Longitude correction can be removed by entering the same value for "Longitude" and "Central meridian".

The sundial drawn with this macro is shown in Figure 3. This is a horizontal sundial with the gnomon originating from the small circle at the centre of the dial and at an angle equal to the latitude. The dial plate is positioned with the vertical line pointing to the celestial pole, in this case true north.

Figure 3 - Spider Sundial 2

Figure 3 - Spider Sundial 2

You can compare the shape of the hour lines produced by the polar gnomon with those on the previous sundial where a perpendicular gnomon was used. Without the EOT correction the hour lines would be straight, as on the standard horizontal sundial with polar gnomon, and the date circles would be not needed.

For declining and/or inclining sundials the gnomon height is calculated and its position is displayed on the dial plate.

A real sundial of this type with a diameter of 70 cm can be seen at http://www.flickr.com/photos/valentin_hristov/. The difference with the picture above is that it is "upside down" (with the declination 180) for convenience of reading. The gnomon is simply a string.

Spider Sundial 3 with Polar Pointing Gnomon - sppolsq

This spider sundial is generated after the user enters the "Latitude", "Longitude", "Central meridian", "Declination of the plane" and "Inclination of the plane" of the sundial's location as shown in Figure 1. A "Place" descriptor can also be included. The hour lines are automatically adjusted for the Equation of Time. Longitude correction can be removed by entering the same value for "Longitude" and "Central meridian".

The sundial drawn with this macro is shown in Figure 4. The date lines for this sundial are not circles as in the previous sundial but squares. This is a horizontal sundial with a polar gnomon originating from the small circle at the centre of the dial and at an angle equal to the latitude. The dial plate in this figure is positioned with the vertical line pointing to the celestial pole.

Figure 4 - Spider Sundial 3

Figure 4 - Spider Sundial 3

For declining and/or inclining sundials the gnomon height is calculated and its position is displayed on the dial plate.

Spider Sundial 4 with Arbitrary Dial Plane and Arbitrary Gnomon - spiderarb

This macro greatly increases the number of features over that provided by spidern and spiderp. This macro incorporates the previous two macros as well as providing additional options. In fact, it gives the maximum possible choice for flat dial plate and straight gnomon that intersects the plate in the centre of concentric date circles.

The inputs for this macro are similar to those in Figure 1. There are additional selections for the "Time interval" as well as selections for "EOT correction" and "Longitude correction". The inner and outer radius of the date circles can also be specified and are used to size the dial plate. These entries also determine whether January 1 is on the inner or outer circle. The dial plane can now also be rotated, "Rotation in the plane", which gives complete control over its orientation. There are 4 selections for "Gnomon type" and the macro calculates the appropriate direction for the gnomon. "Polar" and "Perpendicular" were handled in the previous macros described above. "Vertical" will position the gnomon vertically no matter what the dial plane orientation is. "Arbitrary with gnomon vector" allows the gnomon to be positioned in any direction.

Figure 5: Spider Sundial Macro with Arbitrary Dial Plane and Gnomon

Figure 5: Spider Sundial Macro with Arbitrary Dial Plane and Gnomon

Only the periods when the dial plane illuminated are indicated. If the plane of the sundial is horizontal, it is possible to determine the times of sunrise and sunset on a particular date.

The sundial drawn with this macro is shown in Figure 6. You can see that the drawing includes not only the spider hour lines on the set of concentric circles, but also radial hour lines in the centre that show the local time. At the bottom is presented both graphical and numerical information about the gnomon. This will assist you in the design of the gnomon and is very useful.

Figure 6: Spider Sundial 4 - Horizontal Dial Plane with Polar Gnomon

Figure 6: Spider Sundial 4 - Horizontal Dial Plane with Polar Gnomon

This macro creates a number of different "Layers" that are all turned on when the sundial is drawn. Be sure to check them out and turn off any you do not need.