In the news this week you may have read that the Monument near London Bridge has just reopened after restoration work. This large column was erected next to the Thames as a memorial to the Great Fire of London of 1666, and was completed in 1677. But what is not so well known is that its designers, Christopher Wren and Robert Hooke, intended it to be used as a telescope.

The Monument was designed as a type of telescope known as a zenith telescope, used to observe stars that pass directly overhead. Wren and Hooke hoped that by looking at one star in particular they might be able to detect stellar parallax - a change in the position of an observed object caused by a change in the observer's position. This was something that should be observed if the Earth was moving around the Sun, but astronomers had not yet been able to detect it. In fact, it was only in the nineteenth century that stellar parallax was finally observed.

Here, the whole structure was the telescope. The observer sat in a room in the basement and looked up through the 'tube' created by the spiral staircase. The flaming urn on top had a hinged lid that opened for viewing. Sadly, it didn't prove to be up to the job because it expanded and contracted in different temperatures and swayed in the wind. Try not to think about that if you go up it!

I'll be blogging about telescopes regularly throughout the International Year of Astronomy 2009, revealing some of the stories behind real and imaginary instruments from the National Maritime Museum collections and elsewhere.

The Monument, from John Stow

The Monument, from John Stow, A survey of the cities of London and Westminster