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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 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 (Munich) is showing this kind of dial in action, see photo below.

The use will be easy using solar time and separating the time scales.

Check the "Shift" box to separate the azimuth scale (red) and the hour angle scale (blue).
solar or
                  standard time
Use the checkbox "App." to swich between apparent (solar) time and standard time (boy unchecked).
The standard time is taking into account of the equation of time and the difference from the standard meridian

standard time or sppsrent solar time

Check your time zone offset and select the 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 box
Select "Az" or/and "HA" to display the azimuth or/and the hour angle lines.

Select "Half 1" for Dec 21 to June 21, or "Half 2" for Jun 21 to Dec 21, or Jan 1 to Dec 31 (both boxes checked, or none).
Pay attention to the messages.
hour angle
                  azimuth pointers
The magenta circle (or arc) indicates the date.
The red pointer is related to the red lines of the azimuth scale, the magenta pointer is used for the blue lines of the hour angle scale.

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 !

Check the box to choose apparent (solar) time, uncheck for standard time.
interactive Click into the spider web to get the date and the azimuth, if "Shift" is unchecked.

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. Sun's elevation is blue if below horizon.

The red frame of the applet area is a square (700 x 700 pi

The polar gnomon is a triangle (latitude 51.6° N).
Aug 22 at 10:30 apparent time (12:01 CEST):

self aligning
          sundial azimuth hour angle

Aug 18 at 10:00 local standard time:

The azimuth and hour angle lines are quite complex,
and the time scales difficult to read.

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

North Finder
            Deutsches Museum Munich

North Finder in the Sundial Garden of the Deutsches Museum (Munich),
calculated and constructed by Yves Opizzo and Christian Tobis.
(Photo with kind permission of
Yves Opizzo)

Azimuth Spider

The self-aligning Foster-Lambert sundial

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: 2023, Oct 06

© 2009-2023 Juergen Giesen