Down Boulevard St Michel, around the corner and into La Poste for stamps to send postcards to friends. Then on to Ste. Sulpice to see how the date for Easter used to be determined before we had calendars. In the south rose window, there is a small opening to allow a ray of sunshine in to shine across the floor through the chancel following along an inlaid brass line. As the sun climbed higher and higher towards noon, the spot of light climbed an obelisk on the wall opposite the light hole. On the day of the spring solstice the spot was at it's highest point there. Easter was the first sunday after the next full moon. Read about it in the book, Sun in the Cathedral.