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Horizontal Sundials: Vertical Gnomon (Azimuth Spider) and Polar Pointing Gnomon.

The gnomon of this horizontal sundial is located in the center of the circular date lines and perpendicular to the dial plane (vertical), or parallel to the Earth's axis (polar).
In the case of a vertical gnomon the term "spider" is applied. The Sun's azimuth is used for indicating the time (azimuthal or azimuth sundial). This kind of sundial does not depend on the Sun's altitude.

Using a polar pointing gnomon without the Equation of Time correction (apparent time) the hour lines are  straight, and the date circles are not needed.

Read the time (standard or apparent) at the intersection point of the shadow of the gnomon with the circle of the current date.

The Sundial Garden of the Deutsches Museum (Munic) is showing a combination of both kinds in action (calculated and constructed Yves Opizzo and Christian Tobis). It can be used to find the true North direction (North Finder).

Northfinder Deutsches

standard time

Check your time zone offset and select the local standard time.


Click the applet first!
Enter latitude in decimal degrees
(southern negative),
enter longitude in decimal degree (western negative),
and press the button Apply input.

gnomon polar
The blue checkbox "POL" is for a gnomon parallel to the Earth's axis (polar),
the red one "VER" for a vertical gnomon.

You may use the keys "y", "m", "d", "h", "n" to increase the year, month, date, hour, minute, or Shift key and "y", "m", "d", "h", "n" to decrease the year, month, date, hour, minute !

Click into the spider web to get the date and the azimuth.
apparent solar
                  timestandard time
Check the box to choose apparent (solar) time, uncheck for standard time.
apparent time
                  standard time
Select from the "Options" menu.
"on/off" items are working in toggle mode.
                  altitude equation of time rise set
The times of transit, sunrise, and sunset are standard time.

Gievenbeck spider

Sundial by Yves Opizzo, polar pointing gnomon,
Muenster-Gievenbeck, Freiherr-vom-Stein-Gymnasium,
51° 57' 40'' N, 7° 34' 06'' E
The gnomon is parallel to the Earth's axis, the diameter of the spider about 14 m.


The shadow on 2010, July 12 at 13:00 CEST

Horizontal Sundials: Vertical Gnomon (Azimuth Spider)

North Finder

My Sundial Applets

Applets: Ancient Sundials and Quadrants

Web Links

The Sundial Primer: Spider Sundials

Horizontal Spider Sundial

Sonnenuhren-Spinne (Wikipedia)

Spinnensonnenuhr / Cadran solaire du type araignée (Yves Opizzo)

The Sundial Garden (Deutsches Museum Munic)

Last Modified: 2013, Feb 05

© 2009-2013 Juergen Giesen