To perform its measurement, Mars must be installed outdoors and have a clear view of the sky.

The Mars™ Soiling Sensor performs its measurement each day around sunset. Its measurement sequence begins slightly before sunset and ends once the sky is dark. 

Mars™ automatically detects sunset based on light levels in the sky and therefore its exact time of measurement adjusts with the seasons and weather.

To determine the time of sunset for a particular location, please consult a sunrise and sunset calculator, such as this one.