The Monument to the Great Fire of London that stands near the northern end of London Bridge is a pretty well known landmark. It’s a tall Doric column decorated with dragons near the base and topped with a golden orb. Its height—202 feet—corresponds to the distance from its base to the bakery in Pudding Lane where the fire started. On the inside of the hollow column is a spiraling staircase that stretches all the way to the top, and out on to a viewing platform.

Completed in 1677, the Monument was designed by the celebrated British architect Christopher Wren, and the famous scientist Robert Hooke. At that time, Wren was the Surveyor of the King's Works, and as such he was widely involved in rebuilding the city. Wren was personally responsible for the rebuilding of 51 churches including the St Paul’s Cathedral. Naturally, the responsibility for designing the Monument fell upon Wren.

A massive wooden replica model of 17th century London was set ablaze on the River Thames on September 4, 2016 to mark the 350th anniversary of the Great Fire of London. Photo credit: andrew/Flickr

One of Wren’s childhood friend was the brilliant scientist Robert Hooke, best known for coining the term “cell” after observing these minuscule honeycomb structures in plant tissues through a microscope. Hooke was a genius whose name has largely been forgotten, but his contributions have endured. Among countless other things, Hooke also discovered the law of elasticity, predicted the presence of a substance in air that supports combustion—which we all know today as oxygen, was one of the first to suggest that fossils were petrified remains of once living creatures, and laid down many of the theories of gravitation twenty years before Newton published his “Mathematical Principles of Natural Philosophy”.

Hooke was also the Surveyor for the City of London.

Wren and Hooke together began to draw out plans for the new city, staking out wide boulevards, homes, churches and business. They also collaborated on the monument, although it was designed largely by Hooke alone. There are documents that testify that Hooke made the drawings of possible designs, while Wren signed them to indicate his approval of the drawings.

There is a reason why Hooke took the responsibility of designing the monument. He had a hidden motive.

Hooke was an astronomer too, and one of the problems that had been challenging him for some time was how to measure the distance to a distant star. Hooke knew about the principles of astronomical parallax—the apparent shift in the position of an object caused by a change in the observer's position—and how it can be utilized to measure the distance of distant objects such as stars. The issue for Hooke was that stars are located trillions of kilometers away while the observer's position changes, as the earth moves around the sun, by only a few millions of kilometers. This produces a parallax far too small for telescopes of Hooke’s time to detect. What Hooke needed was a large telescope, as large as the Monument to the Great Fire of London.

Photo credit: Tom Flemming/Flickr

Hooke initially built an 11-meter telescope in his lodgings at Gresham College, where he was a professor of geometry. But the structure was not stable enough and kept spoiling his alignment of the lenses. Hooke decided that stone structure of the Monument would be sturdy enough for his plans.

Christopher Wren was an astronomer and physicist himself, and it didn’t take much convincing to get Wren to agree on a design that would satisfy both the citizens of London as well as the scientific community.

Hooke made the stone tower hollow. At the apex, he planned to fix a lens and observe stars as they pass directly overhead from a room in the basement. The flaming urn on top had a hinged lid that could be opened for viewing. At one point during construction, King Charles II insisted on having a statue of himself or that of a sword-yielding female at the top, which would have ruined Hooke’s plan to use the monument as a telescope. Luckily, Hooke’s design of the flaming urn was chosen instead.

The Monument took six years to build and required so many stone blocks—more than 28,000 cubic feet—that they kept running out of it. Eventually the king issued a proclamation, forbidding anyone from transporting rocks from the Isle of Portland without first consulting Wren.

Sketches of the Monument published on “The Graphic”, 1891.

Once the monument was completed, in 1677, Hooke got down to business. He attached two lenses on either end of the column, and was trying to get them aligned when he realized his folly—it was not easy to keep two lenses aligned, 200 feet apart, with only limited ways to anchor them to the telescope. Worse still, the Monument stood right next to London’s busiest street and vibrations from traffic made accurate measurements impossible to take.

All was not lost for Hooke though. Hooke needed a tall building to conduct experiments that required height. So far he was using London’s Westminster Abbey or St Paul’s Cathedral. Now he had his very own laboratory. In 1678, he finally conducted a successful experiment at the Monument, using a barometer to confirm that air pressure decreases with altitude.

Also see: The Telescope That Was Too Big To Use

The base of the monument. Photo credit: carmen_seaby/Flickr

The spiraling staircase looking up. Photo credit: Judith/Flickr

The crown of fire. Photo credit: Chris Beckett/Flickr