At the beginning of April many casual observers will spend a while admiring the crescent Moon, faintly tinged with earthshine blue, high in the evening sky. Each spring, the Moon in its waxing crescent phases appears high in the western skies, and telescopic observers take advantage of the Moon's altitude to observe features on its eastern hemisphere.
On 1 April most Moon watchers will concentrate their attentions along the terminator, the dividing line between the Moon's illuminated and unilluminated hemispheres. A wealth of topographic detail is on offer this evening, from the fine details in the lunar seas Maria Nectaris, Tranquillitatis and Frigoris, the fascinating plain of Lacus Mortis within which resides the crater Burg surrounded by ridges and rilles, to the crater-filled southern uplands dominated by the spectacular 125 km Hommel.
If your eye wanders from this spectacle up to the northeast quadrant of the Moon, you'll notice a dark elongated patch close to the edge. This is Mare Humboldtianum (Humboldt's Sea) presented excellently, having been brought into view by a whopping libration in longitude and latitude favouring the Moon's northeastern libration zone. This region is favoured for the first week of April. Humboldtianum is a dark patch of lava filling an ancient impact basin whose extreme eastern floor extends onto the Moon's far-side. Later impacts have scarred Humboldtianum, a mass of light coloured eject from the crater Hayn overlies the western floor and the huge 200 km diameter crater Belkovich intrudes upon Humboldtianum's northeast flank. From above, Humboldtianum is shaped like a wide crescent, and the region in which it lies was first pictured by the Soviet spaceprobe Luna 3 in October 1959.
The German astronomer Johann Madler named this feature to honour his compatriot, the naturalist Alexander von Humboldt. Humboldt's explorations of unfamiliar terrestrial continents in the late 18th and early 19th centuries formed a symbolic analogy to Madler's own lunar surveys, so Mare Humboldtianum represented a physical link between the known and (then) unknown hemispheres of the Moon.Later this month, seekers of near-limb features will be treated to a good view of the Mare Orientale region on the opposite limb of the Moon to Humboldtianum. Orientale is a more elusive feature since most of it lies on the Moon's far-side. It was named Mare Orientale - the Eastern Sea - because at the time this was the convention for lunar east-west. In 1961 the International Astronomical Union reversed these directions to the so-called astronautical convention, so that nowadays the Eastern Sea lies in the west! The Orientale basin consists of several fractured mountain rings that encircle the small central sea, and was formed 3.8 billion years ago by an asteroidal impact. It is the youngest of the Moon's major maria. Unlike the large basins on the near-side of the Moon which have all been filled with lava, the Orientale basin is only partly flooded, and Mare Orientale itself is only 300 km across. The outer regions of the Orientale basin are stained by lava flows, and of these Lacus Veris and Lacus Autumni (the Lakes of Spring and Autumn) are situated just on the near-side and may sometimes be glimpsed even if Mare Orientale is not on the disc. The true majesty of the whole formation only came to light when the US Lunar Orbiter photographed the area in the 1960's. In all, the Orientale basin occupies a surface area equal to the Earth's Caspian Sea.
During this apparition, from 17 to 21 April, observers will be able to discern the central lava-filled plain of Orientale right on the lunar limb, surrounded by the narrow lava tracts of the Lacus Autumni and Veris. Earth's Caspian Sea. During this apparition, from 17 to 21 April, observers will be able to discern the central lava-filled plain of Orientale right on the lunar limb, surrounded by the narrow lava tracts of the Lacus Autumni and Veris.
