Ancient Thebit and Huygens’s Sword

The Straight Wall at last quarter
One of the Moon's most striking features is the Straight Wall or Rupes Recta, as it is officially known. This 120-kilometer-long fault, seen here slightly before last quarter as a thin, bright line, is a striking sight in just about any telescope.
Courtesy Thierry Legault.
The absolute best example of a lunar fault is found along the eastern shore of Mare Nubium — the Straight Wall. This well-known lunar feature is a long thin line that never fails to impress; even casual Moongazers enjoy this magnificent sight. The view is best when the Sun has risen over it and the terminator is a little to the west in central Nubium. Under these conditions the wall casts a shadow that stretches along the entire 120 kilometers of its length. This dramatically demonstrates that the lunar surface must be lower on the west side of the fault than to its east.

Various authors report that the scarp is 250 to 300 meters high, but my measurements of its shadow length suggest that it may rise as much as 450 m above the basin's western floor. In spite of appearances, the Straight Wall is not a sheer cliff, though it is relatively steep — rising above the mare plain at an angle greater than 20°.

The Straight Wall (traditionally called the Railway by British observers, and now officially known as Rupes Recta or the Straight Fault) terminates in the south against a jumble of short ridge segments that the 17th-century selenographer Christiaan Huygens likened to the handle of a sword, with the Straight Wall being the blade.

Straight Wall after first quarter
Shortly after first-quarter phase the shadow of the Straight Wall is visible on the plains of Mare Nubium. At last quarter the setting Sun directly illuminates the face of the Straight Wall, causing the feature to turn white.
Courtesy António Cidadão.
If you widen your view you will notice that the Wall slices through the lava-flooded floor of an old, unnamed ruined crater that I call "Ancient Thebit," after the 57-km-wide crater Thebit on its rim. To the east, the rim of 200-km-wide Ancient Thebit is well defined, but its west rim is marked only by arc-shaped wrinkle ridges.

Although some researchers have noted that the Straight Wall is roughly radial to the Imbrium basin, and thus perhaps related, it clearly has a much closer relation to Ancient Thebit. I believe that Ancient Thebit formed on the edge of the Nubium impact basin and that subsidence of the basin lowered the crater's western wall, which was ultimately buried by the lava flows that flooded the basin. This is essentially the same sequence as at Sinus Iridum on the Imbrium basin, and Fracastorius on the rim of the Nectaris basin. The basinward portion of the floor of Ancient Thebit faulted downward to accommodate the basin's sinking. This is again very similar to Fracastorius, which is cut by a delicate crack where the mare sunk.

Moon with Straight Wall area highlighted
Straight Wall highlighted.
Courtesy António Cidadão.
Finally, if you look very closely under good seeing conditions, you should see a tiny crack roughly parallel to the Straight Wall just west and north of the bright crater Birt. Each end of this rille, known as Rima Birt, terminates in a tiny pit. The pit on the northern end sits on top of a dome that is noticeably darker than nearby mare. Rima Birt is a challenge for telescopic observers and for scientists who try to explain why it is there. Because the tiny pit at the rille's northern end is located near the rim of Ancient Thebit, we can speculate that fractures associated with the rim provided an easy path for lavas to erupt to the lunar surface, producing a dome, collapse pits, a lava channel, and perhaps a small deposit of gas-propelled ash and debris.