Why are some star maps slightly oval

Basic knowledge: the rotatable star map

The best way to learn how to use a planisphere is to hold it with your outstretched arm in the direction indicated at the bottom of the map. Now you can compare the stars above the horizon with the stars at the bottom of the map. You can find stars towards the inside edge of the map if you look further up.

Another problem is distortion. On a rotatable star map designed for the northern celestial hemisphere, constellations of the southern celestial hemisphere appear squashed, making it harder to compare them with the real constellations. This problem does not occur with maps that are only created for a specific point in time, such as those in the editions of "Stars and Space" or "Ahnert's Astronomical Yearbook".

A few more subtleties

Rotatable star maps are drawn specifically for a specific latitude area. For Central Europe or Canada this is 50 degrees north. For a vacation in southern Europe, the United States or even the Caribbean, maps for this width are then only partially suitable. Fortunately, there are some star maps, such as Chandler's, that are published in different editions for different latitudes.

And then there is Central European Summer Time (CEST), which usually runs from the last Sunday in March to the last Sunday in October. During this time you have to subtract an hour in your mind in order to set the time correctly on the star map. If you take it very precisely, the time that you have to set on the rotatable star map is not that of our time zone (i.e. for us Central European Time for 15 degrees east longitude), but the mean local time. The correction that still needs to be added to CET is shown in the following table for various cities in Central Europe.

Time correction against CETCities
-5 min.Brno, Vienna
0 min.Frankfurt / O., Görlitz (15.0 ° East), Graz, Prague
5 min.Berlin, Dresden, Klagenfurt, Linz / D.
10 min.Chemnitz, Halle, Leipzig, Potsdam, Regensburg, Rostock, Salzburg, Stralsund
15 minutes.Augsburg, Bozen, Coburg, Erfurt, Innsbruck, Lübeck, Magdeburg, Munich, Nuremberg, Schwerin
20 min.Braunschweig, Bregenz, Eisenach, Hamburg, Hanover, Kassel, Kiel, Liechtenstein, Ulm, Würzburg
25 min.Baden-Baden, Bielefeld, Bremen, Darmstadt, Frankfurt / M., Heidelberg, Heilbronn, Karlsruhe, Constance, Ludwigshafen / Rh., Milan, Mainz, Mannheim, Reutlingen, Stuttgart, Tübingen, Wiesbaden, Worms, Zurich
30 min.Basel, Bern, Bochum, Bonn, Dortmund, Emden, Essen, Freiburg / Br., Kaiserslautern, Koblenz, Cologne, Münster, Osnabrück, Saarbrücken, Strasbourg, Wilhelmshaven, Wuppertal
35 min.Aachen, Düsseldorf, Duisburg, Geneva, Krefeld, Luxembourg, Mönchengladbach, Trier
40 min.Amsterdam, Antwerp, Liege, Rotterdam
45 min.Brussels

Alan MacRobert is an editor at Sky & Telescope and an avid amateur astronomer.