Indeed, but this is no different from "north up in Northern hemisphere / south-up in Southern hemisphere" proposed above. Since we're talking interactive maps,

Indeed, but this is no different from "north up in Northern hemisphere / south-up in Southern hemisphere" proposed above. Since we're talking interactive maps, you can probably just go by the center point (barycenter) of the viewport. W/r/t Sun angles, things are pretty weird anywhere between the Tropic of Cancer and the Tropic of Capricorn.