The second first became accurately measurable with the development of pendulum clocks keeping mean time -LRB- as opposed to the apparent time displayed by sundials -RRB- , specifically in NUMBER@1 when PERSON@1 added a seconds pendulum to the original pendulum clock of MISC@1 .

Nevertheless , with the development of pendulum clocks keeping mean time -LRB- as opposed to the apparent time displayed by sundials -RRB- , the second became measurable .
