

Galileo first got the concept of a pendulum whilst watching a swinging lamp in a Pisa cathedral. He then timed its period (how long each swing is) by measuring it against his pulse. Initially it was thought that the period of swing depended solely on the length of the pendulum rod (isochronous), and not on the arc it swang through. However a Dutchman called Christiaan Huygens discovered that as the arc increases a slight increase in the rate occurs as well. For a pendulum to be isochronous it would need to swing in a cycloidal path, see here for an external site explaining a cycloid. You can see the difference between cycloid and circular above. Attempts to get a pendulum swinging in a cycloid arc have been achieved, but with limited success, so a circular arc is usually adopted. The resultant small error that this introduces is therefore called circular error These errors can be dismissed though  so long as the arc never changes. If the arc causes a loss of 5 seconds a day, you simply adjust the clock to run 5 seconds faster to offset the error. If the arc varies however, you start to get problems, as the allowance you have made for 5 seconds no longer applies for the new arc. To find the size of the change in seconds per day that an altered arc would cause you need to know 2 things (1) the angle size of the original semi arc* and (2) the angle of the semi arc* that it has changed to. *semi arc = from vertical @ mid swing to furthest point of swing
You then work out the circular error for each and subtract one from another. Use the formula Error in day = (secs in a day/16) multiplied by the arc in radians (squared) This equates to Radians and degrees can be converted here
So a 2 degree arc that changes to a 2.5 degree arc would work out as follows. 2 degrees = 0.03490658503989 radians 5400 x (0.03490658503989 x 0.03490658503989) = 6.579736 circular error in secs 2.5 degrees = 0.04363323129986 radians 5400 x (0.04363323129986 x 0.04363323129986) = 10.280837 circular error in secs 10.280837  6.579736 = 3.701101 So the 2 degree arc that changes to a 2.5 degree arc will cause a loss of 3.7 seconds daily.
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