Gamma Draconis

Many of you will know the tower called The Monument. You may not know that its design was inspired by an object 150 light years away.

The Monument commemorated the Great Fire of 1666 and was built by Christopher Wren. The architect, however, was not Wren but his assistant Robert Hooke who had a particular plan for the building.

Like Wren he was also an astronomer. At this time some scientists still believed that the sun rotated around the earth. Hooke was determined to prove that the opposite was true.

A star called Gamma Draconis passes over the City every night and Hooke reasoned that by taking observations of the star at different times of the year he could prove that the earth went around the sun.

To do this he needed a telescope around 200’ long and, obviously somewhere to put it. The tower, therefore, was also an observatory.

Hooke’s astronomy was perfect, his calculations were flawless, the telescope was built with absolute precision and positioned perfectly within the building.

He had, however, failed to take two factors into consideration. His observations could involve fractions of a millimetre but every time a vehicle rolled down nearby Fish Street Hill, the tower vibrated slightly and upset his readings. Also, surprisingly for an architect, he’d failed to consider that a 200’ tall building would sway in the wind!


How would Hooke’s experiment have worked? He was using something called parallax.

Here’s an example. Stand at the end of a room and choose something, say a picture, on the opposite wall. Take 3 paces left and you’ll be looking at the picture from a certain angle. Take 6 paces right and the angle changes. This is parallex.

He theorised that if the earth went around the sun then in, say, June it would be one side of the sun and in December on the other. and there would be quite a long way between the two.

If he took observations of Gamma Draconis in June and then again in December and the star was shown to be at a different angle (albeit a tiny one) then this would prove that the earth was indeed circling the sun.