Year 2006

Sundial of the month April

Earliest equation-of-time loop
on a sundial

Christopher St.J.H. Daniel, chairman of the British Sundial Society, went to search for the earliest sundial with an equation-of-time loop.
Here is meant a device that will allow corrected time to be read directly, in contrast to the graphs or tables found on many sundials that will only enable the calculation to be made.

Of the examples mentioned, one is an armillary sphere on a 1740 Dutch painting by Nicolaas Verkolje.
In the lower left corner, this painting shows a splendid armillary sphere with an equation-of-time loop.

zw-06-04-01.jpg

The mechanics of the loop represent the sun in its correct place in the sky.
Although armillary spheres were not really meant to be sundials, they do function as such.

zw-06-04-02.jpg

Daniel’s quest has now led to a sundial from approximately 1716 that satisfies his original requirement.
This sundial was made in Germany by Johann Michael Vogler, for Johann Philipp von Wurzelbau.
It is now in the National Maritime Museum in Greenwich.

zw-06-04-03.jpg

It is a portable universal equatorial sundial.
In the centre of the hours and date scale, the equation-of-time loop is visible.
A bead in the centre of the cylinder serves as a shadow caster.

Fer de Vries

Literature:
1. Christofel St. J.H. Daniel, The equation of time: the invention of the Analemma, a brief history of the subject (part 1), bulletin of the BSS vol.17, nr.iii, september 2005.
2. Christofel St. J.H. Daniel, The equation of time: the invention of the Analemma, a brief history of the subject (part 2), bulletin of the BSS vol.17, nr.iv, december 2005.
3. Marinus J. Hagen, Zon & Tijd, a collection of important articles from the Bulletin of the Zonnewijzerkring, 1998. Notably:
- The sundials of David Coster. (Bulletin 12, April 1982)
- armillary sphere with an equation-of-time loop. (Bulletin 22, febr. 1985)
4. http://www.nmm.ac.uk/collections/explore/object.cfm?ID=AST0368&picture=1#content

English translation: RH