Basics of visualization is [u][i]pure geometrics[/i][/u] .[br][br]In the [u][b]1.step[/b][/u] it's calculated, how the angle between the direction from earth to sun is changing according to inclination of the axis of the earth depending on dato. The result gives a vector connecting the centres of earth and sun. [br]In the [b][u]2. step[/u][/b] this vector is used as normalvector of a [u][b]plane E[/b][/u] containing centre of the earth. [b]E[/b] divides earth in two parts representing day and night. For each [u][b]circle of latitude b[/b][/u][sub][u][b]g[/b][/u][/sub] the ratio between archlength lieing in daypart and length of circle is exactly the same as the percentage of daylength from 24 h.[br]In the [b][u]3. step[/u][/b] we follow an [u][b]observer B[sub]g[/sub][/b][/u] rotating on its circle of latitude togehter with earth, From the postionvector from geocenter to B[sub]g[/sub] and the normalvector of E at every time the angle between ray of sunlight and horizon is calculated.