根据当前时间和地球上的位置计算阳光方向

How can I calculate the position of the sun according to the current time and position on the earth? I am currently adding light-sources to an Augmented Reality framework on Android and it would be great to know where to set the "sun". I think there are a lot of factors but an approximation would be enough for now I think.How can I calculate the position of the sun acc