1. Apparatus comprising: a processor coupled to process respective sequences of sky images acquired at respective spaced apart locations, the processor comprising: an image alignment module configured to spatially relate respective views of at least one object visible in the respective sequences of the sky images based on homography of at least one astronomical image acquired at each spaced apart location, wherein said at least one astronomical image includes a number of spatial references corresponding to respective astronomical body positions located practically at infinity relative to a respective distance between said spaced apart locations.