Mare Orientale is the youngest impact basin on the Moon. From orbit it is impressive and shows us for example how the Imbrium basin would look like if it was stripped of its thick lava cover. Unfortunately the basin is centred at 95°W making it very difficult to observe from Earth. Librations must be favourable, terminator must be in the right position (if you want to appreciate the topography) and weather must be co-operative (which is a big problem in Belgium). Two such occasions are illustrated here. And although the view from Earth is not optimal it remains one of the more awesome ones illustrating the violent past of the Moon and the importance of impact basin for the Moon’s geology. The Montes Cordillera define the outer ring and are easily visible. Notice the scarp facing the basin centre. The Montes Rook (actually the Outer Rook Mountains) are another ring of mountains. The Inner Rook Mountains and even the centre of Mare Orientale may be visible with very favourable librations. Lacus Autumni and Lacus Veris are two dark patches of lava leaked to the surface along deep fractures associated with the basin ring formation. South of the Orientale impact basin some very nice valleys can be seen: Vallis Bouvard, Vallis Baade and Vallis Inghirami. They are pointing away from the impact basin and are formed by its immense ejecta blanket.

Using some ‘landmarks’ the Orientale impact basin is easily found. The dark material filling the craters Grimaldi and Riccioli can be used to find the northern part of the impact basin. The similarly dark coloured Crüger and the bright ray system of Byrgius A are just east of it. The great Schickard is situated southwest of Mare Orientale.


040404 (next picture) A rare occurrence of favourable librations, the right terminator position and good seeing conditions shows the main features of the Orienale impact basin.

030317 Drawings of Wright and Shaler (left) and part of Montes Rook and the floor between this mountains Cordillera (right), indicated with the arrow in 040404.

031209 When the terminator is not revealing details of Mare Orientale its position can be found by looking at the dark mare patches of Lacus Autumni and Lacus Veris.