The last region to be covered in this book is the West Limb, the eastern boundary of the far side dominated by the Orientale Basin (Figure 12.1). This completes the photographic coverage of the entire Moon that was begun in the companion volume, "Lunar Orbiter Photographic Atlas of the Near Side of the Moon," which started with the near side portion n of the same area. Orientale, the youngest of the large basins, oo the one that ended the era of the basins, is on the leading 0 edge of the Moon as it rotates around the Earth. This large basin and its ejecta establish the boundary in time between the Early Imbrian Period and the Late Imbrian Period.

The eastern edge of the Orientale Basin can be seen from Earth, but only when the Moon's libration is favorable. Its -§ eastern ejecta pattern was detected first, and called the Hev-r Si elius formation after the seventeenth century astronomer. FarHevelius started the practice of using names with roots J! meaning "eastern" for features near the western edge of the T near side because he was observing through a telescope whose optics inverted images.

At about 25° north latitude along the limb, the Orientale ejecta blanket terminates, exposing the ejecta of the near side Imbrium Basin. The Lorentz Basin (pre-Nectarian, 325 km) lies beneath that. The shore of Oceanus Procellarum comes to within 10° of the far side in the range of 20°-50° north latitude.

Near the limb south of Orientale are the Mendel-Rydberg and Pingre-Hausen Basins. First came Mendel-Rydberg (630 km) in the pre-Nectarian Period, followed by Pingre-Hausen (300 km) in the Nectarian Period. These two overlapping basins were then covered with Orientale ejecta.

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