Another giant impact-generated fault scarp is visible as the inward face of the Rupes Altai, a mountain arc that rims the Nectaris basin
and lies about 200 km southwest of Mare Nectaris itself.
Other fine lunar mountain ranges for telescopic inspection include the Altai Scarp, which is a part of the outer rim of the Nectaris basin
, and the Jura Mountains, which hold Sinus Iridum, the Bay of Rainbows.
Similarly, the 90[degrees] arc of Rupes Altai along the southwest rim of the Nectaris basin
weakens and disappears in both directions.
Fracastorius is another obviously tilted crater; it partly formed on the Nectaris Basin
's inner ring and partly on the mare.
To the east, beyond Deluc and Lilius, are more overlapping craters 30 to 40 km wide that may be secondaries from the Nectaris Basin
, nearly 1,500 km to the northeast.
Nectaris has almost exactly the same area of dark, mare lava as Humorum, but the main rim of the Nectaris basin
is twice as large--535 miles (860 km) in diameter versus 265 miles.
Like the Altai Scarp that partially surrounds the Nectaris basin
, this elevated chain rises little above the terrain outside it but drops steeply to the inside.
For example, relatively little lava erupted inside the Nectaris basin
, so you can still see much of its original multiple-ring structure.
Fracastorius formed right on the main rim of the Nectaris basin
. The crater is tilted toward the basin's center, and its walls are progressively lower as they grade toward the mare floor.
For example, the Nectaris basin
is partly defined by the striking Altai Mountains, and a portion of the main Imbrium rim is described by the conspicuous Apennine range.
But it is at the Nectaris basin
(described in the June issue, page 68) where mare fill is minimal enough that we can see most of the basin floor.
The main rim of the Nectaris basin
is well seen only along the Altai Scarp (L7 in the Lunar 100, introduced in the April issue, page 113, and available online at SkyandTelescope.com/observing/objects/ moon/article_1199_1.asp), a 500-km-long curved fracture in the lunar crust.