Horizontal axis - Time axis in GMT
The horizontal axis show the time in GMT (Greenwich Mean Time). The GMT has been chosen to prevent hour shifts during the nights changing from winter time to summertime and visa versa. Locale time is GMT + 1 hour in winter and GMT + 2 hours in summer.
Clear night
An example of the measurements that are being performed by NachtMeetnet see the graph below. From 18:15 (GMT) it has become dark with a night sky brightness of 0,7 mcd/m² or 21,6 mag/arcsec². Later in the evening at about 23:00 it becomes slightly darker with a  night sky brightness of 0,55 mcd/m² or 21,4 mag/arcsec².
Vertical axes - Night sky brightness
De vertical axes show the brightness of the night sky (each axis a different unit).
The left vertical axis is expressed in mcd/m² (millicandela per square meter). Without light pollution or moon the night sky has a natural occuring brightness of about 0,25 mcd/m². As a comparison: a screen of a TV, PC, tablet or mobile phone has a brightness of roughly 300.000.000 mcd/m² (300 cd/m²). That is about 1 billion times brighter.
The right axis is expressed in an astronomical unit mag/arcsec² (magnitude per square arcsecond).
Night with clouds and clear periods with moon
In the graph below a cloud covers arrives at about 18:00. At about 21:00 and 22:00 there is a short clear period. The night sky brightness then decreases. At 0:30 there is a longer clear period till about 01:30. The night sky brightness decreases during the clear period, because the moon sets. At about 1:30 again a cloud cover arrives till about 2:00. The sky stays clear until 03:15. Later every now and then clouds pass over.


Darkness in the Netherlands online
NachtMeetnet is a tradename under Sotto le Stelle (Registered Utrecht Nr. 30167055)