Terrain Editing Tutorial: Terrain Sun Light Configuration for Farming Simulator 17 (FS17) by PMC.
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On large terrains you get darkness at far regions of the terrain unless you configure sun light properly. These configuration values are in meters, so 1000 is 1 kilometer, etc.
Sun / star light is not enough to cover whole massive 16384 x 16384 terrain area, so you must increase the sun settings. Open your terrain in Giants Editor FS17, click the sun and change range to 16384. All done. For PMC 32768 Terrain I made the range to 32768.