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A self-aligning sundial

This horizontal sundial combines two different principles and is self-aligning to true north.
- The first gnomon is located in the center of the dial and perpendicular to the dial plate. Red lines are representing the azimuth angle of the Sun.
- A second gnomon is ponting from the center of the dial to the north pole (i.e. parallel to the Earth's axis). Blue lines are representing the hour angle angle of the Sun.


The true north-south line is found very easily by turning the dial plate until both sundials read the same time 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 this kind of dial in action ("North Finder" calculated and constructed Yves Opizzo and Christian Tobis).

standard time

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

Click the applet first!
Enter latitude in decimal degrees
and press return key,
enter longitude in decimal degrees
and press return key.

azimuth hour angle Switch between azimuth and hour angle lines.

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 time true 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.
elevation altitude equation of time rise set
The times of transit, sunrise, and sunset are standard time.

The red frame of the applet area is a square (697 x 697 pix.

Aug 18 at 10:00 local standard time:

The angular distance of the shadows reaches a maximum at 3 hours before/after local transit:


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)

Double Foster-Lambert Sundial

Self-aligning sundial

Articles of  H.J. Hollander

Last Modified: 2009, Aug 12

© 2009 Juergen Giesen