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 Promontorium Somnii and, impressed by this poetic name, he used it as the title of his text. We show that Promontorium Somnii, 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 Promontorium Somnii on the surface of the Moon, and we show that it is a corner of a better known feature mapped in 1651 by Riccioli, Palus Somni. We give a translation of the 'astronomical' part of Hugo's text and some comments about the determination of longitudes using lunar eclipses.