IN SEARCH OF THE <italic>PROMONTORIUM SOMNII</italic>

Abstract

In 1863-1866, the French writer Victor Hugo wrote a long text in which he related a visit he made in 1834 at Paris Observatory, where François Arago showed him the Sun rising on the Moon. Amongst the lunar features progressively illuminated by the Sun, he noticed <italic>Promontorium Somnii</italic> and, impressed by this poetic name, he used it as the title of his text. We show that <italic>Promontorium Somnii</italic>, which has since disappeared from lunar terminology, was introduced in 1686 by Jean-Dominique Cassini as one of the benchmarks used for determination of longitudes by the observation of lunar eclipses. There have been questions about the location of <italic>Promontorium Somnii</italic> on the surface of the Moon, and we show that it is a corner of a better known feature mapped in 1651 by Riccioli, <italic>Palus Somni</italic>. We give a translation of the 'astronomical' part of Hugo's text and some comments about the determination of longitudes using lunar eclipses.