Recently quite a few of my walkers have asked me about the deviating alignments of the plans of new and “old” St Paul’s, as depicted in the Churchyard.
The alignment of the modern cathedral (built by Christopher Wren between 1675-1710 after the Medieval one was burnt down in the Great Fire of 1666), picked out in grey Purbeck Marble, is toward the direction of the sunrise on the Easter Sunday of the year in which the foundations were laid, April 14th, 1675, at approximately 75deg, a full 15deg north of true geographic east.
In contrast, the alignment of the ancient, Medieval cathedral (itself far from the first, and indeed actually the fourth, on the site), picked out in black and white, is 10deg closer to true geographic east, at approximately 85deg, and may have been toward Medieval magnetic east, which may in turn have just happened to more or less coincide with modern magnetic east.
Note in this context that because of variations in the earth’s magnetic field, the locations of magnetic north, south, east and west with respect to true geographic north, south, east and west have actually varied considerably through time! Measurements acquired in London indicate that the angle between magnetic and true north, or “magnetic declination”, here was +10deg (i.e., magnetic north was 10deg east of true north) in the mid-sixteenth century; then fell to -25deg (i.e., magnetic north was 25deg west of true north) at the turn of the eighteenth and nineteenth; and has since risen again to <-5deg, which is the present value. There are no measurements from before the post-Medieval period.
To see a graph showing the variation over time of magnetic north in London, on a different website, click here