Some notes on the sun and the longitude

When you calculate shadows or generate rendered images with sunlight, the calculated position of the shadows might not exactly match the real position, even if you have defined the location and time parameters correctly.

These imprecisions are caused by the difference between the local time, which is based on the position of the sun, and the time based on the time zone valid for the relevant location.

For example, if you set all stationary clocks to the local time, so that they show 12 noon when the sun is at its highest point, the times indicated by the clocks differ by 4 minutes per longitude. The eastern clocks are fast. So people agreed to use a system of 24 time zones of 15 longitudes each.

The coordinated universal time (UTC) – that is the former Greenwich Mean Time (GMT) – roughly corresponds to the mean solar time at the prim meridian (longitude 0) in Greenwich (Great Britain). At Greenwich the local time and the time valid in this time zone are the same.

But as soon as you leave Greenwich and go to a different location, imprecisions will arise.

Mannheim in Germany uses the Central European Time (CET = UTC + 1 hour).
The 15th meridian east of Greenwich is the central axis for CET.
In general, German cities are west of the 15th meridian.
The coordinates of Mannheim are 8° 28′ 9″ E (east of Greenwich), which means that you need to subtract the difference between 15° and 8° 28', which is approximately 6.5°. This corresponds to about 26 minutes (4 minutes each).
As a result, the local time in Mannheim is not 9:00 CET but 8:34.
However, the difference between the time and the local time is not constant but changes during the course of a year. Due to the obliquity of the ecliptic and the eccentricity of the Earth's orbit around the sun, the actual day length varies. Dependent on the type of calculation, you get the mean solar time or the apparent solar time. You can find detailed information on this topic on the Internet.
For example, you can look for information on equation of time or analemma.

To obtain more accurate results, you need to take this difference between local time and time zone into account.

Solutions

Open the Sun dialog box (by clicking the Sun... button in the Photo and Light Settings tools) and correct the value for the time. You need to enter the difference between the local time and the time zone.

Using http://www.stadtklima-stuttgart.de/index.php?klima_sonnenstand,
you can quickly and easily determine the sun's position.

Use the value given for Noon (sun's highest position).

If the sun reaches its highest position at 12:20 CET, there is a difference of 20 minutes. So you need to subtract 20 minutes.

Examples

Some values for locations in Germany:

Aachen - 0:40

Berlin-Charlottenburg - 0:11

Chemnitz - 0:12

Düsseldorf - 0:37

Essen - 0:36

Frankfurt am Main - 0:29

Garmisch-Partenk. - 0:20

Hamburg - 0:24

Heidelberg - 0:29

Kaiserslautern - 0:33

Kempten - 0:23

Leipzig - 0:14

Mannheim - 0:30

Munich - 0:18

Regensburg - 0:15

Traunstein - 0:13

Ulm (Schwaben) - 0:24

Waren (Müritz) - 0:13