Seasonally changing landscape

[alex-w]

Original description: https://blueprints.launchpad.net/stellarium/+spec/seasonal-landscape

Current landscape panoramas cannot reflect the changing appearance of the landscape during the year, with dry summer grass or snowy winter landscapes. Depending on location and solar longitude a new landscape "supertype" should automatically switch (when crossing a trigger longitude) or blend between different landscapes. The sub-landscapes may be any of the existing types whose location section is then ignored. Optimally, number of sub-landscapes is not restricted, but 4 seasons may do for a start.

[alex-w]

This is a good task for the community to participate in the contribution into Stellarium. Who wants to help us?