Less than perfect weather conditions forced the mind behind the huge installation, Burkard Steinrücken (with the stick), and Ulrich Carow (in red) of the Regionalverband Ruhr, which had made it all possible, to rely on local amateur astronomers to provide an artificial sunset ...
As should be obvious, the two giant arcs represent the celestial equator and the meridian for someone standing in the exact center of the observatory (with the eyes as high above a central block as the stick is long). There are many other markers in the installation, however, explained by numerous tables and on the website.
In the International Year of Astronomy 2009 in the Ruhr area there will be many more guided tours at the unique facility, in particular around the equinoxes and solstices.
Here are an article & picture gallery from a local news site.
For comparision I: pictures of an earlier celebration at the site with clear skies can be seen here and here.
For comparision II: aerial and artist's views of the complete arrangement on the Halde are here.
For comparision III: finally, here are some impressions of the construction site in May 2008 - with a slightly drier Steinrücken ... :-)
All pictures by Daniel Fischer (all numbers are identical to the original full-res pics, available on request)