Visitors to Sir Christopher Wren's greatest work, St. Paul's Cathedral (and to  Temple Bar, another of his designs, to its north of the cathedral), often forget that Wren was a scientist and mathematician first and an architect second.

Amongst his many achievements, Wren held the post of Professor of Astronomy at Gresham College, a job intimately involving him in the measurement of time. Wren personally designed more than one meteorological recording clock.

A visitor passing under Temple Bar into Paternoster Square (illustrated left), will see above his or her head, a statue of King Charles I. As it happens, it was Charles who in 1631, granted the founding Charter to the Worshipful Company of Clockmakers of London. His action led to the flowering of horology in the City and an outpouring of mechanical ingenuity unmatched for some 200 years.

Immediately ahead, the visitor will see a new building, housing the London Stock Exchange. High on its wall and carved deeply into the stone, is one of the most accurate noon-marks in the world, calculated by Dr. Frank King of Cambridge University.

A pylon, jutting out from the top of the building, carries a small disc at its extremity. This casts a round shadow on the wall of the building as noon approaches, with a bright sun-spot in its centre. Exact noon can be read, when the sun-spot touches the figure of eight line, marked with the days of the month.

If the visitor leaves the Stock Exchange to the right and proceeds straight ahead down the lane to Newgate, the Newgate Tower (carrying the Newgate Street Clock on its further side) will appear on the right.

To the north of the Newgate Street Clock, can be seen the ruins of Christchurch. This was a church designed by Wren and engineered by  a second great 17th century polymath Robert Hooke. Hooke, who devised the balance-spring for watches c. 1663, visited and oversaw work on the church in August and November 1677 and February, April and June 1679. Finally, to the east of the clock, stands the massive British Telecom office-block. It was from this site, at the end of the 19th century, that Greenwich Mean Time was distributed via the electric telegraph, to cities throughout the United Kingdom.