Located in the far northeast quadrant, Mare Humboltianum straddles the limb, showing itself only at times of favorable libration. Humboltianum is a beautiful example of a multi-ring impact basin. Two of these mountainous rings are visible in my sketch, the 650 km ring just above lava floored Endymion at the bottom center, and the 300 km ring that encompasses the Mare. Chuck Wood pointed out in "Modern Moon" (Sky Publishing) that these rings would have hastened our recognition of the importance of basins if Humboltianum was placed more central on the visible face. Nectarian in age, it's formation occurred about 3.8 billion years ago, preceded by farside basin Moscoviense and followed by nearside Humorum. The mare fills only a partial area approximately 160 km wide of the inner 300 km ring. To the north, huge 204 km Belkovich has obliterated a large area of the basin floor, it's brightly glowing rim leads off to the left in my sketch. Long linear valleys, radial to the basin center in the zone between the two rings, are sobering reminders of the energy of the collision that creates large muti-ring impact basins. To the left, the outer ring seems to fail to continue the shape of the ring, presumably this is the result to some kind of post-impact modification.
Subject: Mare Humboltianum Rukl: 7
Time: 12:41am - 2:34am PDT Date: May 10, 2009
Seeing: Antoniadi II-III Weather: clear, ocassional light breezes
Lunation: 15.18 days
Colongitude: 99.6 deg.
Lib. in Lat.: +06 deg. 25 min.
Lib. in Long.: +03 deg. 42 min.
Phase: 347.0 deg.
Telescope: 12" Meade SCT f/10
Binoviewer: Denkmeier BV-25 with 2X nosepiece
45 deg. W.O. erect image diagonal
Eyepieces: 26mm Meade Super Plossls
Sketch Medium: White and black Conte' on Black 400 Strathmore Artagain paper
Sketch size: 8" x 11"