L80: Mare Orientale

This mare can be glimpsed on the Moon’s western limb. It is rather small, in size comparable to Mare Humorum. In images from lunar spacecrafts three concentric rings can be seen, the innermost nesting the mare, giving the feature the look of a bulls eye.

At the eyepiece I found this to be quite a challenging object. The angle of view made it difficult to observe, and it took some time before I could separate the rim from other nearby features.

L80_Orientale basin
Sketch made on 3 April 2015 using a SkyWatcher 10″ and a TeleVue Nagler zoom 3-6 mm. North is towards the right.

Below is a 2010 image of Mare Orientale from the Lunar Reconnaissance Orbiter. The crater Schlüter, to the right in the sketch, is the large crater in the top right hand corner of the image, just outside the outermost ring.

Image by Lunar Reconnaissance Orbiter, NASA/GSFC/Arizona State University.

Data from Lunar Reconnaissance Orbiter has also been used to create a beautiful fly over video.

But how come a mare situated on the Moon’s western limb is named the Eastern Sea (orientale meaning eastern i latin)? Apparently the mare was named according to a previous standard when east and west was defined by terrestrial cardinal direction. When looking at the full moon Mare Crisium appears on the side of the moon facing the terrestrial west, while Mare Orientale are on the limb towards the terrestrial east. Thus the eastern sea. A consequence of this standard was that the lunar sun rose from its western side, obviously creating some confusion. In 1961 and with the advent of the space age this nomenclature was replaced, now putting the eastern sea on the western limb.