This project is dedicated to ultra long exposures, images taken within merely minutes but hours or days. All images can be used freely in a non-commercial context (more below) and the complete source code is available via github.
Elsewhere in the web:
All images are licensed using the Creative Commons Attribution NonCommercial (CC BY-NC) license. This means you are free to use everything under the following terms:
Attribution — You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use.
NonCommercial — You may not use the material for commercial purposes.