Since 1978 at the Calern Observatory-France and from 1996 at the Observatório Nacional (O.N.)-Brasil solar observations have been carried out, aiming primarily to study the solar diameter. The ensemble of these two series is noteworthy by the length of Calern’s, which spans about two solar cycles, and by the high density of O.N.’s, with typically 20 observations per day, on each side of the meridian, all year around. The precision of a single measure is 015 for the northern hemisphere observations and 02 for the southern hemisphere observations.
The characteristics of the combined series enabled detailed searches for anomalous trends (e.g., from atmospheric origin) and systematic bias (e.g., from instrumental origin). The analysis show the series robust for either. Thus there is a possiblity to investigate the variable time of transit of the solar disc across the almucantars of observation.
The apparent solar time, defined by the actual diurnal motion of the Sun, shows variations because of the varying rate of motion of the Sun in hour angle. That is, due to the inequalities in the annual motion along the ecliptic and to the inclination of the ecliptic to the equator.
In this work we detail the series characteristics and discuss the determination of the hour angle of the true Sun, as exemplified by the equation of time. The accuracy of the determination varies with the solar declination and the length of the observed apparent orbit. Typical precisions follow those for the semi-diameter determinations.