Quadrans Vetus

This quadrant is a portable altitude sundial for a single latitude indicating the unequal hours (planetary hours), dividing the time intervall from sunrise to sunset into 12 hours.
The instrument is equipped with a simple Sun sight on one edge. A weighted thread with a sliding bead is hanging from the apex.

See instructions for interactive use below

Overview: Dials and Quadrants

Applet: Planetary Hours

Visit my Applet Collection

Details for interactive use:


Enter the latitude (decimal degrees) into the text field and hit Return key.
The latitude is indicated in the lower left.

The interactive regions (light gray scales) are changing the cursor to cross hair.
date month scale
Click into the light gray date scale to set the bead.
scale: Jan - Jun, or outer: Jul - Dec.
Using the declination scale will return two dates. Choose one of them by clicking the inner or outer date scale.
                altitude scale
Click into the light gray altitude degree scale on the limb to direct the sight to the Sun.
unequal hor
The scale of the unequal (planetary) hours:
0 = sunrise, 6 = noon, 12 = sunset.
unequal hour
Read the unequal hour (e.g. between 4 and 5: 4.5 or 10.5).

Using the "Today" button the bead is set to the 6th hour (noon) of the current date.

Accuracy of the Quadrans Vetus

Comparing the quadrant with modern astronomical calculations (my Planet Clock applet) it can be stated that the results of the quadrant become more inaccurate for higher latitudes and higher declination. The error is 0 at rise (unequal hour 0), noon (unequak hour 6), and set (unequal hour 12), and has a maximum in between.
The appoximate mean error in reading the unequal hour is:

Declination 30° N
55° N
0.05 hours
0.1-0.3 hours
+11.7° 0.08 hours 0.15 hours
+23.4° 0.13 hours 0.35 hours

Morrison, James E.: The Astrolabe, Janus. Softcover edition, 2007,
Rehobot Beach, DE USA,
ISBN-10: 0939320304, ISBN-13: 978-0939320301.

Hügin, Johannes: Das Astrolabium und die Uhr, Ulm, 1978,
ISBN 3-921348-23-4.

Rohr, Ren
é R. J.: Die Sonnenuhr. Geschichte, Theorie, Funktion.
Callwey, München 1982.

Meyer, Jörg: Die Sonnenuhr und ihre Theorie.
Harry Deutsch, Frankfurt 2008.

Zenkert, Arnold: Faszination Sonnenuhe (mit CD-ROM),
Harry Deutsch, Frankfurt 2000.
Web Links

Quadrant Sundial

The Astrolabe (James E. Morrison)

Gunter's Quadrant (J. E. Morrison, PDF)

Edmund Gunter biography (MacTutor History of Mathematics archive)

Gunter's quadrant (Navigation Museum)

Gunter quadrant (The Whipple Museum)

A Gunter quadrant and practical knowledge (The Whipple Museum)

Gunter quadrant (National Maritime Museum)

The Electric Astrolabe (J. E. Morrison)
executable file and documentation

Astrolabium (Deutsches Museum)

Der Gunter-Quadrant (Gunter W.)

Das Astrolab (Informatik Uni Erlangen)

Keith's Asterolabe (Java applet)

Literature on astrolabes (M. Brunold)

The Uses of History in Science Education

Hours and hours

© 2009-2023 J. Giesen

Last modified: 2023, Oct 07