Merhabalar! I have always wondered about this, but I can´t seem to find a lesson that specifically addresses it. I understand constructions like the one in this sentence:

"Para o kitabın altındadır."

"The money is under that book."

But then I see sentences like this:

"Yıldızlar altında, açıkta uyuduk."

"We slept in the open, under the stars."

Why does one "altında" phrase use a possessive construction and the other does not? 

Hope this isn´t a dumb question. Teşekkür ederim!

"Para o kitabın altındadır." (definite possession)

"Yıldızlar altında, açıkta uyuduk." (indefinite possession)

http://www.turkishlanguage.co.uk/possessiveforms.htm