That last sentence was really badly phrased: `I meant moving across the astronomical signs by about a days worth every 72 years'. The Gregorian calendar is a tropical calendar which fixes the dates of the solistices, and lets the astrological signs drift. Another truly solar calendar, the bengali secular calendar, is a siderial calendar that fixes the astronomical signs and lets the seasons drift. see
http://tanmoy.tripod.com/bengcal.html if interested in such issues.
Originlly the last sentence was immediately after the mention of astrology, and as I edited the post, the sentence became completely garbled and misplaced, sorry.