Astronomers noticed some peculiarities in the way uranus moved through the sky and , after painstaking observation , concluded it must have been the gravitational tug from another object causing its curious motion .
Before that it was the irish referendum on the fiscal compact ...... then it was the greek election ...... and the artists now have to stand ready to renegotiate the bailout terms to give greece breathing room ( after giving a tug to the merkel plate to prevent her falling off in pique ) .
Today 's best watt balances are extraordinarily sensitive : metrologists have found they can pick up earthquakes , magnetic interference from passing trains , the gravitational tug exerted by nearby snow cover , and even subtle shifts in ground level created when wind passes through nearby trees .