  A sunset over the sea surface offers the possibility to chronometrate a solar
transit across the horizon. The vertical solar diameter is proportional to the
duration of the sunset, the cosine of the azimuth and the cosine of the
latitude of the observing site. The same formula applies to every circle of
equal height, called in arabic almucantarat, and it is exploited in the
measurements of the solar diameter made with the Danjon's solar astrolabes. The
analogies between sunsets and astrolabes observations are presented, showing
advantages and sources of errors of these methods of solar astrometry.
