It appears the concept of orbits had just been proposed (150 B.C.) without much success towards the end of the time frame the Qumran scrolls were copied (200-150 B.C.)
Do you find any of the other astronomical observations in Enoch curious given its antiquity?
There seems to be somewhat of a mystery about when orbits were really proposed. Ptolemy clearly used the idea, which says no later than 150 BC, but I gather it's not clear how much older the idea was. I'm at work at the moment, and most of my history books are at home.
I did find this at NASA.
http://image.gsfc.nasa.gov/poetry/ask/a11846.html