In the course notes something is made of making sure the the sky isn’t so dark that the definition of a building line is lost.
I did see the sense in that train of thought but I did feel sure that I had seen plenty of night sky images where the sky was darker then the course notes suggested it should be.
In this months British Journal of Photography were two images that demonstrate buildings against a very dark night sky and one more image that is illuminated by the moon.
In the first image on page 12, Ahmed Mater has captured a city skyline against a very dark sky. Because all the buildings are illuminated sufficiently to highlight the building line, it matters not that the sky is so dark.
Unfortunately I am unable to find the image online to link to.
In the next image on page 24 John Stanmeyer has photographed people silhouetted against a night sky. This time it is important that the sky is illuminated somewhat as the outline of the people, who are not illuminated in anyway, would have been completely lost.
This image can be seen on BJP online:
http://www.bjp-online.com/2014/02/vii-photos-john-stanmeyer-wins-57th-world-press-photo-of-the-year/ Last accessed 07/03/2014
Unknown. (2014). Houston FotoFest. British Journal of Photography. n/a (n/a), 12 & 24.
http://www.bjp-online.com/2014/02/vii-photos-john-stanmeyer-wins-57th-world-press-photo-of-the-year Last accessed on o7/03/2014