Hmmm... I'm not an astronomer, and I wondered if they might have once been more-exact. It seems that while Earth's orbit has long term variations (eccentricity, obliquity, precession) the shortest of those cycles is still quite a lot longer than the estimated age of the Nazca lines. (Precession with a 26ky cycle, Nazca lines at 1.5 to 2.5ky.)