Rupes Recta and Thebit

Aloha,

I send to you a sketch created last year of a favorite region of the lunar landscape. Rupes Recta, latin for “Straight Fault” or “Straight Wall” is a lunar fault at the southeastern region of Mare Nubium. This sketch was created at ~8 days lunation when the angle of the Suns light casts deep shadows across the linear fault giving it the appearance of a very steep cliff.

In spite of appearances, Rupes Recta is not a sheer cliff, though still relatively steep. It rises above the mare plain ~250 to 300 meters high with ~20 degree grade as reported by some authors. The fault runs NW to SE for ~120 km terminating at the south end as a jumble of rubble and then a smooth curved wall. This creates the appearance of a sword as likened by 17th century Huygens, the curved region being the sword handle with the cliff edge being the sword blade.

Seeing was very good on this night but the angle of light at the terminator did not illuminate the “handle” as well as on other nights of observing. When this region is observed days later the steepness of the fault vanishes and appears as a step transition instead. The light plays amazing tricks on the eye.

Sketched in relief to the west of the fault is crater Birt just becoming lit. Thebit and Thebit A are sketched to the SE of Rupes Recta.

Cindy (Thia) Krach
Maui, Hawaii
4,000 el.
12.5” Portaball
14mm 109x, 9mm 169x