The Geometry of the Anglo-Norman Plan,  c.1110 to 1170

 (Figs. 115-126)


The geometry originates in a circle of 149-150 English feet radius, which represents 153 of the Norman Feet in general use at the beginning of the century. The right square and the turned square inscribed within this circle create the inscribed octagram (Fig. 115). Within this octagram the core octagram is developed (Fig. 116).  This diagram is then developed into four tangential circles in the form of a Greek cross centred on the central crossing.  When squares are inscribed within them they form a cross of contiguous squares (Fig. 118).  A central circle is added to complete the contiguity.

Turned squares are developed in the five contiguous squares to form five octagrams (Fig. 119).  From these an octagon is drawn within each of the five circles (Fig. 120). Within each tangential square a circle is drawn (Fig. 121), and within each circle a smaller octagram is formed (Fig. 122).

The western arm is extended by the development of two circles providing two contiguous squares (Fig. 123).  These additional elements are developed similarly to the first five geometrical elements. Vertical lines are drawn tangential to all the larger circles.  All the vertical and horizontal lines joining the angles of all of the octagons are drawn.   This completes the geometry of the Anglo-Norman building with the exception of the sanctuary area which will be examined separately. (Fig. 124)

It will be seen that the vertical sides of the octagons determine the inside line of the nave and choir walls. The east sides of the transept octagons determine the inside line of their eastern walls.  The interior line of the north and south walls of the transepts are determined by the sides of their contiguous squares.  

It will be seen that the division of each of the transepts into three chapels, is produced by the interior verticals of their octagons.  This result is possible only from the octagon development. It will also be observed that the central tower and the two planned western towers are precisely determined by the geometry (Fig.136), as also are the axial centres and N-S dimensions of the nave and crossing piers (Fig. 247).

The Window to Window Dimension

In making this analysis one problem seemed to present itself.  The major dimension, the diameter of the larger circles and the vertical lines tangential to them that determine the external width of the nave and choir do not run along the exterior face of the wall, but about 2/3 of the way through the thickness of the wall.  Any attempt to enlarge or lessen the diameters of the circles to make these lines conform either to the exterior or interior line of the wall resulted in a disparity in the east–west dimension between the geometry and the elements of the plan that is unresolvable.

Quite fortuitously I was introduced to a set of excellent engravings of Kettering Church that included finely measured and drawn sections through several windows. (R.W. Billings and G.Winter 1843) (Fig 125, Fig 126).  The very accurately recorded measurements show that the window glass in the ca.1300 windows stand at exactly two thirds of the depth of the wall.  Later walls, c.1450-1500, in the Kettering nave, are thinner than those of the chancel but the glass in their windows stands at exactly the same distance from the external face of the wall as it does in the earlier window of the chancel, being. almost exactly half.-way through the thinner wall.

At Peterborough, all the nave windows are mid-fourteenth century replacements for the much smaller windows of the twelfth century church, but the walls are unchanged and we can be sure that the fourteenth century glass stands exactly where the twelfth century glass stood.  Certainly this is so in the transepts where the twelfth century window openings remain in their original form with only the tracery a fourteenth century introduction.  The glass there is undoubtedly in the original twelfth century position.

At Peterborough the walls of the New Building, (the single story ambulatory  c.1500 at the east end of the building,) are thinner than the walls of the twelfth century church.  But here again, as at Kettering, the glass in the thinner walls stands at almost exactly half-way through the thickness, exactly in line with the glass in the older church.

Thus we have to realise that the fundamental dimension of the geometry, the distance from one side of the nave and choir to the other, is the distance between the glass of the windows.  And this is exactly what one should have expected.  It is strange that so little attention seems to have been given to the glass in all the measurements that have been made over the past decades.  The surface of the glass is exactly where the inside of the church begins: there and nowhere else, separating the holy world of the interior from the unholy world outside.  The glass is where sacredness, and the sacramental geometry that creates that sacredness, begins.

The surface of the glass is moreover where the light enters and destroys the darkness.  It is well-known that Light was a fundamental element in the theology of the Middle Ages, being not only the earthly embodiment of heavenly grace but also the manifestation of the spiritual enlightenment that comes to mankind from the powers of Heaven.  The appreciation of the divine nature of light was central to medieval theology.

Von Simson (1956 pp.50-52 ) reports appositely on the importance of light in Christian thought:

Luminosity, the second characteristic (after proportion ) of Gothic.  The affinity between luminosity and the metaphysical trend of the time is perhaps even more striking than in the case of proportion.

Hugh of St.Victoire and Thomas Aquinas both ascribed to the beautiful two main characteristics: consonance of parts (proportion) and luminosity.

Light and luminous objects, no less than musical consonance, conveyed an insight into the perfection of the cosmos, and a divination of the Creator.

For a thinker like Grosseteste, [the Bishop of Lincoln and dedicator of the completed Abbey at Peterborough in 1238] light is actually the mediator between bodiless and bodily substances, a spiritual body, “an embodied Spirit”, he calls it.

Plato:  In the sixth book of the “Republic”, the good is defined as the cause of knowledge as well as of being and essence, and then compared to sunlight, which is “not only the author of visibility in all visible things, but generation and nourishment in growth.”

For the Neoplatonists. this brief sentence was the seed out of which they developed an entire epistemological system.  Light was now conceived as the transcendental reality that engenders the universe and illuminates our intellect for the perception of truth.  These ideas were adopted by Christianity.  St.Augustine developed the notion that intellectual perception results from an act of illumination in which the divine intellect enlightens the human mind

(p.55).   Christian theology is centred on the mystery of the Incarnation, which in the Gospel of St. John is perceived as light illuminating the world.

(p.100). Suger’s description of St. Denis: “the entire sanctuary is thus pervaded by a wonderful and continuous light entering through the most sacred windows.”

“The most sacred windows.” That is where the sacramental geometry at Peterborough begins.