Sun light in gazebo world

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This is the part of my code that I use to add sun to the scene to keep it all lit up. But it doesn't work, it's still all dark every time. Is it possible to enter the sun somehow by typing other commands than <include><uri>model://sun</uri></include> ? I know you can set one of the three types of light already in the gazebo, but I would like to have the sun set when turning on the world. Thanks for the advice and best regards.