Well, so like, there are the constellations that the signs are based on, but there's also the segments the year is cut into with the same names. I assumed they were still (at least mostly) in line with the calendar, unless they had to inscribe a new astrolabe every year which isn't impossible. So the full moon aligning with the Capricorn constellation during the 29th degree of Capricorn, which I'll assumed she got the date right; I don't have an astrolabe on hand. EDIT: I think they misread, it appears to be the 29⁰ of Cancer, but maybe location has something to do with it?