The motion of the Sun – the change of day to night – is one of the earliest natural phenomena observed by mankind and makes the passage of time perceivable. Examples of the first determinations of time using the position of the Sun or the movement of the shadow are already known to us from antiquity. Many examples of sundials exist of various design.
The horizontal sundial on Halde Hoheward with its obelisk as a shadow caster is modelled after the sundial of the emperor Augustus on the Marsfield in Rome. If the ancient sundial was complete, i.e. including hour and date lines on the morning and evening side, or only consisting of a meridian to measure the altitude of the Sun at midday, is unknown. The recent scientific finding point only towards a meridian.
The observation of the progression of a shadow on a surface marked with a grid enables the measurement of date and local time. Therefore, not only does time become perceivable, but also laws governing celestial mechanics. The observer can now understand his relation to the cosmos.
The first step towards realising a astronomical park was made on 17th May 2005 with the inauguration of the obelisk. The obelisk is located on the south-east plateau of the slag heap at a height of 140 above sea level. The area covered by its shadow is 62 m in diameter.
Further images and information in the Gallery
The tip of the obelisk has an additional ball on a pole mounted above actual tip of the obelisk. This construction considerably increases the precision of measuring the location of the shadow of the tip. The relating detailed analysis and calculations are provided as a download (PDF-format 2.2 MB) by the private initiative Horizontastronomie in the document Gestaltung einer Gnomonspitze (German). This and further documents can be found on the downloadpage. Additional images and animations concerning the obelisk, e.g. the daily motion of the shadow, can be found in the gallery. Images and articles of the erection of the obelisk can be found in the press review.
The image of a model shows the gridlines and the cast shadow of the obelisk. The tip of the shadow moves during a day along a hyperbola. For specific days, e.g. the solstices or equinoxes, the hyperbolic shadow path is marked on the area on which the obelisk is standing.
The hour lines, converging towards a point, are also visible in the model. An observer sitting at this special point is located as it were at a place of temporal rest. From his perspective the ball is located at the celestial pole around which the stars rotate. The ball covers the polestar.
If located at a location from where the ball is covering the Sun, an artificial solar eclipse can be witnessed. Through appropriate movement across the shadow area the Sun can be kept covered by the ball and one can experience the speed at which the Sun and shadow location moves.
The sundial gives the local time which is differs from the common standard time for central Europe (CET). How measure the time with the sundial and then convert real local time into CET, is explained in the following document: Die Horizontalsonnenuhr auf der Halde Hoheward (German) (PDF format, 1.6 MB)
Further information as well as cut-out sheets for constructing your own sundial can be found in the download area.
A detailed interactive depiction of the motion of the obelisk shadow on the Halde Hoheward has been provided for us by Jürgen Giesen as a Java script applet on the following page: http://www.geoastro.de/obelisk
Last Update: 10th February 2009