"The vesica piscis is a shape which is the intersection of two circles with the same radius, intersecting in such a way that the center of each circle lies on the circumference of the other."
Vesica Piscis - Wikipedia
"The mathematical ratio of the width of the vesica piscis to its height is the square root of 3."
Here's a technical drawing I put together as a bridge between the illustration above and the worked over scanned image below. Notice here that the succeeding arcs all build upon the same point (either A or B). On the innermost arcs I let them continue lightly to complete a full vesica piscis.
Now that we have established the mechanics, lets apply this to the main entrance of Saint Mary's Church. To better illustrate I have allowed the arcs to continue all around and darkened the middle arc area. It's all quite on the mark. Sacred geometry at work.
These past few days of visiting and photographing the church and drawing and studying this aspect of sacred geometry has left me sensitive to curves. I begin to notice curves everywhere. Squared off doors and windows of "regular" buildings feel plain and disappointing.