Archive for 2006

The Qibla

Friday, December 1st, 2006


For the Moslem, not only the time for prayer is important; knowledge of the direction to Mecca is also necessary. This direction is called Qibla..
Islamic sundials frequently had a Qibla line, but one is possible on sundials anywhere.
The figure below shows a south dial for the Sonnenborgh observatory in Utrecht. The Qibla is shown in red.

To indicate the Qibla, the pole style gnomon has an index. It is square over the small cross in the drawing. The distance to the sundial face is equal to the line at the top of the sundial.

When the shadow of the index falls on the Qibla line, the sun is in the direction of Mecca.

South dial for Utrecht, with Qibla

There are many constructions and formulas to determine the Qibla. Today, the computer makes it even simpler.
Go to and navigate to the desired location. The example shows the Utrecht observatory.
You may select either a map or a satellite photo. In either case, the direction to Mecca is indicated both numerically, and by a red line.

Sonnenborgh Observatory, Utrecht, with the Qibla in red

On two days every year, it is possible to determine the Qibla directly from the position of the sun – if it happens to be out.
On those two days, the Mecca sun zeniths at local true noon.
In that instant, for everywhere else in the world, the sun is in the direction of Mecca.

The dates and times are:
27/28 May, 9:18 UTC
15/16 July,9:27 UTC

If necessary, remember to convert UTC to your local time.

Further information, in English, may be found on the Internet site of our member Robert van Gent.


Fer de Vries

For Islamic prayer lines, see also  Article of the month August 2004.

Last Modified 03/01/2016 13:01:04

English translation: RH


Wednesday, November 1st, 2006


In the book ‘Gnomonik der Araber’ of Karl Schoy, 1923, we find a description of certain horizontal sundials where the time is read using the length of the shadow of a vertical gnomon. Such a sundial is called a Hâfir.

Schoy refers to literature from which it appears that such sundials were known from the middle ages. The time is therefore read in antique (unequal) hours.
For antique hours, see also Article of the month July 2005.

Islamic prayer lines appear on these sundials as well.
For these lines, see also Article of the month August 2004.


Antique hours

Because of the vertical gnomon, the sundial is date dependant. This explains the presence of a date scale, divided according to the signs of the zodiac.
This date scale consists of lines that are 30 degrees apart.
Of course the date scale could be subdivided further, but for the sake of clarity that was not done here.

In order not to overload the figure, the Islamic prayer lines for this sundial are drawn in a separate figure.


Islamic prayer times
From the inside out:
Asr-i evvel
Asr-i sâni

In use, the sundial is placed horizontally and turned so that the shadow of the gnomon falls along the line for the current date.
Time is then read on the terminus of that shadow.

Fer de Vries

Last Modified 03/01/2016 12:56:29

English translation: RH

Armillary sphere sundial with auxiliary scales

Sunday, October 1st, 2006


Several examples of armillary spheres with auxiliary scales exist in The Netherlands.
The specimen shown here is in the garden of Twickel Manor in Delden, Overijssel province.


The sundial sits on a splendid pedestal, the pair really fitting the lush manor gardens.
On top of the large armillary there is a second.

Remarkable are the scales on the outside of the equatorial band, which act as auxiliary sundials.
There are elliptic scales, each with their own polar gnomon, on the four compass points.
In between, three-sided prisms make additional scales, the edges of the elliptical scales and the prisms serving for polar styles.

All these extra scales serve to increase the usefulness of this armillary sphere. Indeed, a single closed equatorial band would not allow any time readout around the equinoxes.


Notice the inscription on a number of rings in the armillary.
The proper designations are the subject of Article of the Month van april 2003.

Fer de Vries

Last Modified 03/01/2016 08:24:20

English translation: RH

Bi-Gnomon Sundial, part 3

Friday, September 1st, 2006


Sundial for Mean Time
An idea by Hendrik Hollander

The two-gnomon sundial game never ceases to surprise.
Earlier publications on this theme are in “Article of the month” for december 2005 and januari 2006.

Here we present a new idea, using a cone gnomon
with the axis of the cone parallel to that of the earth.

The sundial has straight hour lines, but the equation-of-time is incorporated nevertheless.
The date lines are adjusted, but the finished dial looks almost like an ordinary sundial.

And yet, the sundial reads mean time.

For this novel concept, Hendrik Hollander received the

2006 Sawyer Dialing Prize.


Horizontal sundial for a latitude of 55 degrees north …..


….. on a slant for use on a different latitude.

Earlier published examples suffered from the drawback that for some dates, the shadow of the gnomon or cone had to be extended to the appropriate date line in order to read it.
This drawback has been completely removed by switching the cone reading edge in a different way.
Dates by the foot of the cone indicate when to use which side of the cone.


The sundial has date lines for every tenth day.
Observe where the shadow of the proper side of the cone intersects today’s date line, and read the time there.
This particular dial was made for local mean time, without longitude adjustment.


Fer de Vries

Idea: Hendrik Hollander

1) Fred Sawyer, Compressed Gnomonic Sundials,
Compendium, Volume 12, nummer 1, maart 2005.
2) Fer de Vries, Samengedrukte gnomonische zonnewijzers,
bulletin van De Zonnewijzerkring 05.3, september 2005.
3)Hendrik Hollander, Bi-gnomon zonnewijzers,
bulletin van De Zonnewijzerkring 06.1, januari 2006.
4) Hendrik Hollander, Bi-gnomon zonnewijzers 2,
bulletin van De Zonnewijzerkring 06.2, mei 2006.
5) Hendrik Hollander, Bi-gnomon zonnewijzers 3,
bulletin van De Zonnewijzerkring 06.3, september 2006.
6) Hendrik Hollander, Bi Gnomon Sundials,
Compendium, Volume 13, nummer 3, september 2006.
7) Hendrik Hollander, Mean Time Sundial With A Cone Gnomon,
Compendium, Volume 13, nummer 3, september 2006.
8) Fred Sawyer, Equations For Hollander’s Mean Time Sundial,
Compendium, Volume 13, nummer 3, september 2006.

Last Modified 03/01/2016 08:20:02

English translation: RH

An 1808 Sundial Replica

Tuesday, August 1st, 2006


Recently, a wooden sundial from 1808 was discovered in the Drostenhuis museum basement. It is a vertical decliner.
Calculations show that the sundial is designed for a latitude consistent with a location in South Germany.
Possibly, the original Drostenhuis occupants bought it as a souvenir, but forgot about it later.

The dial is no longer fit for outside installation, and besides, the Drostenhuis does not have a suitable wall.
It was decided to preserve the original in the museum, and to make a replica for use outside.
In doing so, some concessions had to made in order to have an acceptable sundial.
Member Bote Holman made the calculations, the design, and the final replica.

The replica was finished in 2006. Honorary member Hans de Rijk unveiled it during the Zonnewijzerking summer excursion. The following photographs give an impression of the unveiling and of both sundials.


Hans de Rijk (left), Bote Holman (right)


The original sundial


Replica. Concessions made are plainly visible.


Fer de Vries

Last Modified 03/01/2016 08:15:07

English translation: RH

Projection for analemmatic and horizontal sundial

Saturday, July 1st, 2006

Both figures below show the celestial globe with its axis. That axis is also the axis of the earth.
Square to the axis, the equator and some hour points are drawn. In reality there are 24 hour points, always 15 degrees apart.
Additionally, two lines on either side of the equator indicate the extreme values for the solar declination: plus and minus 23.5 degrees.
If many solar declinations were plotted, they would trace out, on the celestial axis, a complete date scale.



The equator is now projected onto the horizontal plane.
Left is drawn a parallel projection vertically down, to the right is a parallel projection along the celestial axis (which is really the axis of the earth).

In both drawings we arrive at an ellipse with hour points.
These ellipses are similar, only the scale differs. The ratio of semi-major to semi-minor axis equals the sine of the latitude.

The orientations of the ellipses are different. For the analemmatic sundial, the major axis lays East West; for the horizontal sundial it lays North South.

The date scale on the celestial axis is also projected.
For the analemmatic sundial, it becomes a date scale in the horizontal plane on which a vertical gnomon is placed.
In the horizontal sundial, the date scale collapses into a single point, where the pole style is placed under an angle equal to the latitude. The pole style, therefore, does not need to be adjusted for the date.
Because of this, hour lines may be drawn from the foot of the pole style to the hour points, whereas in an analemmatic sundial, only hour points on the ellipse can be used for readout.

If you have the pattern for an analemmatic sundial, you may construct a pole style sundial directly from it:
Draw hour lines from the centre to the hour points, turn the ellipse 90 degrees, and adjust the hour numerals.


Fer de Vries

Last Modified 03/01/2016 08:08:49

English translation: RH


Analemmatic sundial in a bus shelter

Thursday, June 1st, 2006


A contribution by Niels Welmer

I am a student at Eindhoven University, after having attended a technical college.
Recently I used some information on your web site concerning the design of an analemmatic sundial.
For a project, I designed a bus shelter that doubles as an analemmatic sundial.
I would like to show you the result.


Explanation of the design: 
The reason that one must wait at a bus stop is that one is too early or too late.
Time is therefore essential at a bus stop.
This seemed like an interesting basis for a bus shelter.

Actually turning this into a design proved cumbersome, until I found a reference to a Rietveld bus shelter with a mosaic street map.

This prompted me to design a kind of interactivity with waiting people.

I decided to transform an analemmatic sundial into a practical design for a bus shelter.
An analemmatic sundial is a sundial where the observer provides the reading shadow.


The outline is derived from the rising and setting of the sun. The rear wall goes from low to high and back to low, like the sun does.
The choice of materials represents the use man has made of them through the centuries: from stone to wood and, currently, glass.


Niels Welmer

Last Modified 03/01/2016 08:04:48

English translation: RH

Capucin card dial

Monday, May 1st, 2006


A card dial such as shown here belongs to the family of the altitude dials.
This example shows a Capucin card dial.
Card dials can be made of a variety of materials, such as cardboard, plastic, metal, wood, et cetera.

There is a slit in the card, alongside of which is a date scale, here marked according to the zodiac.
From the slit, at the proper date mark, hangs a weighted string.

With the string at the proper date, it is pulled taut over the XII hour point. A moveable bead on the string is adjusted to this point. The card dial is now ready for use.


The times of sunrise and sunset may be read immediately, by pulling the string straight down and reading the hour scale.

The card dial is provided with a sight. In the example, it is the small pin in the upper left.

Keep the card in the vertical plane, and aim the sight at the sun. The shadow of the pin will then fall over the sight line on top.

Let the string hang free and read the time at the bead on the hour scale.
You will need to know if it is morning or afternoon.


It is not difficult to make a card dial like this. An explanation follows.

Draw a coordinate system and a semicircle on the horizontal axis.
Divide the arc in twelve parts of fifteen degrees each, and draw the hour lines to the horizontal diameter.

Plot the latitude from point 12 to P on the vertical axis. P will be the middle of the date line, which is square to the line 12-P.

Divide the date line according to the zodiac by measuring off, on either side of line 12-P, angles of 11.5, 20, and 23.5 degrees.

Draw a vertical line from end point Q of the date line. This sets the earliest and latest times for which the card dial is useable.

Draw circular arcs from the ends of the date line, through the twelve-hour point, to the vertical through Q. Erase the parts of the hour lines outside these arcs. This will give the hour scale of this card dial its characteristic shape.


Finally, here is a card dial by Eise Eisinga.
It appears in Gnomonica or Sundials, a book that Eisinga wrote in 1762, when he was only eighteen years old. In it, he draws and describes about one hundred sundials on variously oriented planes.


Fer de Vries

Last Modified 03/01/2016 07:59:59

English translation: RH

Spherical Gnomon

Saturday, April 1st, 2006


Place a sphere on a horizontal plane and use it as the gnomon for a sundial.
Here is an idea of what it would look like.


The sundial reads apparent solar time.
The hour lines lie in the same direction as on a normal horizontal sundial, but they are displaced, and do not meet in one point.

The shadow of the sphere on the dial face has the shape of an ellipse.
The time is read at the hour line tangent to the shadow ellipse.
In the drawing, it is three hours after noon.

In the morning hours, read the dial at the left side of the shadow ellipse. In the afternoon hours, read at the other side.

The idea is not new. Below is a photograph of a small model that was made years ago.


Model by Adolf Peitz, Germany.


Fer de Vries

1) Alessandro Gunellea, Italy, A sphere as a gnomon
Compendium, Volume 12, number 4, december 2005.
2) Rolf Wieland, Germay, Letters, notes, e-mail, internet…
Compendium, Volume 13, number 1, januari 2006.

Last Modified 02/29/2016 22:02:13

English translation: RH

Of two make one

Wednesday, March 1st, 2006


In Zuidhorn, in the province Groningen, stood a cube sundial, with the dials on the base. On top was a simple armillary sphere.
The whole looked bare and neglected, and later the armillary sphere disappeared entirely.

In Norg, in the province Drente, were the remnants of a small pillar with a slanting, equatorial, face on top. This appeared to have been carrying a star-shaped sundial.

Two lovers of sundials, Eugène Roebroeck and Wybe Westra, took it upon themselves to combine these remnants into a nice, new sundial.


The pillar with the equatorial face is entirely restored, and a new, metal, star-shaped sundial placed on it.
The side faces of the star are as many sundials, while the edges act as their pole styles.
On the top face of the star is an equatorial sundial, using a rod for a style.


The municipality of Zuidhorn offered the sundial to the Wierenga – van Hamsterborg Foundation at Den Ham, Zuidhorn.
This foundation has its seat in Piloersema House, Den Ham, also known as the Hamsterborg.
The sundial stands proudly in the beautiful Hamsterborg garden since October 2005.

Simple as it may sound, the whole process leading to this desirable result, from inception to completion, took all of approximately 15 years.

Fer de Vries

Idea and execution: Eugène Roebroeck and Wybe Westra.
Address Hamsterborg: S. Veldstraweg 25, Den Ham, Groningen.

Last Modified 02/29/2016 21:31:13

English translation: RH

Sun paths

Wednesday, February 1st, 2006

How high is the sun in the sky, and in which direction?
A graphic representation of the sun path can answer these questions.
This link to Sun Charts will let you download sun path graphs for any desired latitude.
Below are some examples showing how the time system may be chosen as well.


Sun path for 52 deg latitude; apparent solar time.


Sun path for 52 deg latitude; civil time, 21 June to 22 December.
Longitude correction and equation of time are accounted for.


Sun path for 0 deg latitude; apparent solar time.
The sun can reach an altitude of 90 degrees.


Another Sun Chart graphic representation.
It looks somewhat similar to an Oughtred dial, but the projection method is different.

Fer de Vries

Link naar Sun Charts

Last Modified 02/29/2016 21:24:49

English translation: RH

Bi-gnomon sundial 2

Sunday, January 1st, 2006


Sundial for clock time
An idea of Hendrik Hollander

Last month, December 2005, we showed a horizontal sundial incorporating, in a special way, both longitude correction and the equation of time.
That sundial used two gnomons, one for each half year, but the hour line pattern was the same for both periods.
We will now discuss a further development of this sundial.


Latitude 52 N, longitude correction 40 minutes, summer or standard time.

Instead of two gnomons, one centrally placed vertical cone is used. The circle represents the base of the cone, the cross its top.
On lengthening days we use the shadow of the right-hand side of the cone, and on shortening days that of the left-hand side . (looking north with the sun in the back.)

The principle of the sundial readout remains the same. Again, one should know the date.
The time is read at the intersection of the appropriate date line and cone shadow.
In this example, the hour lines are numbered both for summer and for standard time.

Sometimes, the cone shadow does not quite reach the appropriate date line.
In the previous model, the gnomon could be lengthened; no such solution is possible here. We can use a ruler to extend the shadow line.


An advantage of this implementation of the bi-gnomon sundial is that some early morning and late evening hours may be read.

There is more that you can do with the cone. You could use it with an elliptical sundial face, or have it lean over. We will not discuss these possibilities now.

The height of the cone in the example is three times the radius of its base.

Fer de Vries

Idea: Hendrik Hollander

1) Fred Sawyer, Compressed Gnomonic Sundials,
Compendium, Volume 12, number 1, March 2005.
2) Fer de Vries, Samengedrukte gnomonische zonnewijzers,
bulletin van De Zonnewijzerkring 05.3, September 2005.
3)Hendrik Hollander, Bi-gnomon zonnewijzers,
bulletin van De Zonnewijzerkring 06.1, Januari 2006.

4) To be published:
Hendrik Hollander, Bi-gnomon zonnewijzers vervolg,
bulletin van De Zonnewijzerkring 06.2, mei 2006.

Last Modified 02/29/2016 21:15:02

English translation: RH