twenty-two day moon

The westward-moving terminator has now reached the eastern edge of the Mare Imbrium, roughly ¹/₄ of the way down from the northern cusp. The low angle of illumination shortly before sunset enhances most features. It makes the Alps, which define the north-eastern rim of Imbrium, and the Apennines, which run along its south-east, look especially three-dimensional.

Within Imbrium, and slightly to the south-west of its centre, lies the large class 5 Archimedes. To its east is Autolycus, and to the north of that, and the north-east of Archimedes, is Aristullus. Whilst the flooded floor of Archimedes is almost entirely in light, the interiors of its smaller class 1 neighbours are totally black, indicating their much greater depth. The area bounded by these three craters has been named the Sinus Lunicus, to commemorate the probe Luna 2 which crashed into the lunar surface here in 1959 and became the first human artefact to reach the Moon.

To the north of Aristullus by roughly ²/₃ of the distance to the Alps stands the unusual, isolated mountain peak Piton.

Beyond the northern end of Imbrium, the thin ribbon of the Mare Frigoris is now more than half in darkness. As far again north and close to the terminator lies the ancient Goldschmidt, with the small and young Anaxagoras interrupting its western wall. Anaxagoras is so deep, and so close to the lunar equivalent of the arctic circle, that parts of its floor are in permanent darkness.

To the south of Imbrium, the terminator bisects the Mare Vaporum, but has not quite reached the Sinus Medii, just beyond it. Continuing south from the south-west corner of Medii is class 1 Herschel, a small, black circle very similar to Autolycus. Its southern edge touches the large class 5 walled plain Ptolemaeus (Ptolemy), which in turn shares its southern wall with the smaller class 5 ring mountain Alphonsus. This distinctive string of well-defined craters ends with class 3 ring mountain Arzachel, almost tangent with the south-eastern edge of Alphonsus.