Although the usual methods of determining local civil time from the time diagram are entirely satisfactory, the better-known methods for finding the corresponding date are not simple and are capable of improvement. It is the object of this note to show how the date is readily determined from the time diagram by employing the lower branch of the hour circle of the (mean) sun.
In the time diagram (Fig. 1) the circle represents the equinoctial; G represents
the Greenwich meridian, with g its lower branch; M and M' represent arbitrary meridians, with m and m’ their lower branches; and S represents the hour circle of the mean sun, with s its lower branch. At the instant indicated by the time diagram (and at any instant), there are two and only two (half-) meridians on the earth