This paper aims, on the one hand, to determine the valid span of the diagram of unequal hours and, on the other, to find a mathematical expression for the error. It is found that the diagram is valid for the two days of the equinoxes and for the times when the sun is on the horizon or on the meridian. This subject has previously been treated by Delambre in 1819 and Drecker in 1925, but not comprehensively.