It’s quite easy to miss for such a large object, but high up on the wall of the Stock Exchange building is a giant sundial. It’s however a curious device, for it tells the time by the day and month, not the hour.
If you are there around noon on one of those rare sunny days, then a shadow of the arm, topped with a round disc will form on the carved analemma in the stone, and show you what month you are in.
For about 36 years (not far short of the design life of the building) the declination at noon on a particular date will correspond to a line drawn somewhere through the strip for that date. If the building lasts longer than that, thanks to the precession of the sun, the sundial will start to lose a tiny bit of its accuracy.
If you look exceptionally closely, and have very good eyesight, you might notice that there’s a really thin strip at the end of the February month — for those occasions when the month contains 29 days.
Spring and Autumn equinoxes are also here, marked by lines in March and September; Summer and Winter Solstices by vertical lines in June and December.
The sundial was made by the Lida Cardozo Kindersley Workshop, and the sundial maker Frank King.