The world's surface is represented with satellite imagery from the NASA Goddard Spaces Flight Centre enhanced with 3D terrain data to give a colourful portrayal of the physical geography of the planet.  The area of the globe where it is night-time is shown in shadow. The darkness of the shadow can be set by the user. Where the sun and the moon are both below the horizon, the night time shadow is darker still.  As an area moves into the night-time shadow, town and city lights are turned on.