Why is the cannon there?

I don't know for certain but the text of the plaque seen in the background of this photograph gives one plausible explanation. The plaque is not a primary historical source and must not be assumed to be correct.  However, the text makes good sense in the context given below. 

Reverse view

The British Parliament had passed a Longitude Act in 1714 which established the 'British Board of Longitude'.  This body offered a valuable cash prize for the invention of a practical means of establishing the longitude of a ship at sea.  One of the proposed solutions was to calculate a set of lunar tables that would enable the Moon to be used as a celestial clock.  Newton had attempted this around 50 years earlier, publishing his results in the Principa Mathematica.  However, in the later editions of this work, he also conceded to having got his sums wrong.  The big prize was eventually awarded to John Harrison for his revolutionary chronometer but not before interest in the motion of the Moon had been re-aroused.

The speculation that I am currently using to guide my researches is as follows.  Both the Greenwich and Paris observatories were places where the moon was observed.  Consequently it is likely that the accuracy of the new predictive lunar tables was continuing to be checked at both institutions.  To compare the results from these observatories would require the measurement of their exact geographical separation so that the effects of parallax could be eliminated from the calculations.  This then could be the motivation of the exercise referred to on the plaque. 

For more of my ideas and notes about the cannon and its siting please see my project notes.

If you can point me towards any good sources that will help me to discover more about this monument, please e-mail me at bobharve@cix.co.uk.