Hi Karen,
I've not been to Nazca, but I have been to many Inca sites. In addition, while visiting Chile, I saw many geoglyphs on hillsides, which are possibly similar in function to the Nazca lines. Apparenly holy men (probably not many women, unfortunately) drew figures on the hillsides by scraping away the dirt. At the time they did this, they would have been in a "trance". Most likely the Nazca lines were not an observatory, but had ritualistic meaning and may have been tied to a pilgrimage to the mountains (considered holy) undertaken by a shaman. Interestingly, new Nazca lines have recently (~ August) been discovered.
This website outlines an interesting possible correlation between water sources and geoglyphs and has a good bibliography of other work on the subject:
http://people.umass.edu/p...a_Lines_Project.html