Even a broken clock is right twice a day.  Some time ago a man in my church died and we discovered that he had for years wound and set the clock at the back of the meeting room.  Few noticed when the clock stopped and even fewer understood what it meant.  In a similar way in the past, many have looked up to the heavens but did not understand what they told us.  The movements of planets and moons in our solar system seems to obey and invisible, clock-like mechanism.  So how does one check the precision of this celestial clock?  One method for checking if the solar system is running as predicted can only be used once in most people's lifetime.  Uplook Magazine, April 2012, p.30.  References:  www.transitofvenus.org;  www.astrosociety.org/uitc;  The Universe in the Classroom No. 78, Fall 2011;  William Sheehan and John Westfall, The Transit of Venus (New York, NY: Prometheus Books, 2004).