Milan at night as seen from the International Space Station just a few years apart.
These images were taken 400 km above Earth by André Kuipers in 2012 (left) and by Samantha Cristoforetti in 2015 (right), before and after the transition to LED technology in the centre. Milan was the first city in Europe to do a total conversion of its street lighting to white LEDs.
Both astronauts used the same lenses (180 mm) but not the same angle or camera. These images have been colour-calibrated by the International Astronomical Union.
Colour pictures taken from the International Space Station are the best source for scientists to map artificial light at night.
Over the last two decades astronauts are witnessing from space how cities shine whiter at night as new street lighting technologies were introduced.
Researchers highlight how invasive night lights are and their negative effects for the environment in the paper Environmental risks from artificial nighttime lighting widespread and increasing across Europe.