twenty-three day moon

When Galileo first looked at the Moon through a telescope four centuries ago, it is quite possible that he saw it as it appears tonight. Certainly, this is how he drew it in three of the five illustrations in his Siderius Nuncius (Celestial Messenger) of 1610, clearly showing the bright arcs of the Alps and the Apennines reaching out onto the dark side from the northern and southern edges of the Mare Imbrium, ¹/₄ of the way down from the northern cusp.

At the south-west end of the Apennines, class 1 Eratosthenes is particularly striking, and to its north by one Humorum length, the small but bright-walled class 1 Timocharis shows well against the Mare, although this is considerably less dark than it was earlier in the lunation. Further north, on the opposite shore of Imbrium, the crisp oval of class 2 Plato remains distinctively dark-floored.

To the south of Plato at a distance of one diameter, the isolated mountain Pico juts out from below the lava plain. Because of its massive size it is far more conspicuous than the Teneriffe Mountains to its north-west and Plato's south-west. To the north of Plato, the narrow band of the Mare Frigoris has now mostly disappeared.

Level with the mid-point of the terminator and due south-west of Eratosthenes, the glorious class 1 ring mountain Copernicus can be seen for the last time, showing strong relief in the low angle of light. Traces of its ray system can be still be found against the Oceanus Procellarum, even under this illumination.

Near the terminator and about ¹/₄ of the way north from the southern cusp is the irregular Mare Nubium. In its south-east quarter a light hairline may be seen running north-east to south-west. This is Rupes Recta (Straight Wall), a 75 mile long fault. Despite its sharp appearance, it is not a precipitous cliff but a moderate-to-steep slope. It faces towards the west, and consequently is at its brightest in this light.

Due south by about one Humorum length lies class 1 ring mountain Tycho. Its deep interior shows strong contrast, but the massive ray system which made it the focal point of the full moon has all but disappeared. To the south-east of Tycho, at a separation a little more than its diameter, the large class 3 walled plain Maginus shows well, having reversed its full moon vanishing trick. To the south-west at a similar distance again lies the enormous walled plain Clavius, the second-largest crater on the visible side of the Moon. Within it, a series of six diminishing craters runs in an attractive arc from the south to the west wall.