Recommended equipment : Lhires III, Lisa , Alpy
Time : about 1h (at sunset)
80km above our head is an atmospheric layer with sodium excited by our Sun. During the day, it is not visible as the solar spectrum is too bright. During the night, this layer is not excited by solar rays.
But there is a transition phase when the Sun is ~6° below horizon. The sky spectrum switch from solar spectrum to night spectrum with human light pollution and atmospheric oxygen emission lines. And for some time, the sodium appears in emission from this layer.
This experiment is very easy to do (an idea from Peter Schlatter) with a LISA or ALPY spectrograph aither with a simple lens or attached to a telescope. Here is the result with a LISA and a 85mm refractor:
Experience has been done again at OHP in 2013 with an Alpy 600 spectrograph – 5min exposure time (this time, time goes from top to bottom) :