October 23, 2017

Astro-Challenge: Scouting Out the Lunar Straight Wall.

The waxing gibbous Moon with the Mare Nubium/Rupes Recta region circled. (Photo by Author)

An interesting lunar feature comes into view this week, right around 1st Quarter phase. Located in the Mare Nubium just before the start of the Lunar Highlands sits a feature known as the Lunar Straight Wall. 120 kilometers long and about 400 meters high, this scarp is hard to miss as a long shadow slice along the surface of the Moon. Visible in even small telescopes at moderate magnification, the Lunar Straight Wall is generally visible within 24 hours of 1st Quarter, which occurs this week at 04:56 AM EDT/ 08:56 AM GMT. Formally known as Rupes Recta, the Lunar Straight Wall was first noted by Christiaan Huygens in the 17th century. He described it aptly as “sword-like”. The Lunar Straight Wall is one of the best examples of a fault on the surface of our Moon, and makes the number 15 entry on Charles Wood’s Lunar 100 list of outstanding lunar features.

The Lunar Straight Wall region. (Photo by Author).

Although it appears to be shear, photographs taken by Apollo astronauts and the Lunar Reconnaissance Orbiter reveal that the Lunar Straight Wall is actually a gradual scarp rising at a respectable 20° degree plus inclination above the surrounding plain. The “hilt” of the sword is made up of the half-buried rim of an ancient crater, and the Wall also slices through an ancient buried crater-rim referred to informally as “ancient Thebit”. The modern 53 km in diameter crater of Thebit sits to the east of the Lunar Straight Wall, with tinier20 km Birt crater to the west of the feature. If the region looks familiar, that because it’s just to the northwest of the Purbach cross, aka the Lunar X, another elusive feature that presents itself around 1st Quarter phase.

The region surrounding Birt crater also hosts series of rilles that run parallel to the Lunar Straight Wall but are much more elusive. The Apollo 16 astronauts and JAXA’s Kaguya spacecraft also made some low passes over the region, producing some stunning imagery.

 

Current thinking is that the formation of the Lunar Straight Wall occurred as part of the filling in and formation of the Mare Nubium; the area has obviously seen a lot of action, both geologic and meteoritic in the ancient past. The Surveyor 7 spacecraft landed some distance away from the Lunar Straight Wall near the crater Tycho in early 1968. I always make an effort to show folks the Lunar Straight Wall around First Quarter, as it never fails to impress. What would it look like to stand atop this feature and watch the lunar sunrise? Incidentally, About 3 days past New to 1st quarter phase is also our personal favorite time to host a star party, as it throws the Moon into the mix. A crescent Moon also shows mountains and craters in profile, but is not over-powering in terms of hunting down deep sky favorites.

Speaking of which, the southern Delta Aquarid meteor shower also peaks around the morning of July 28th, although the Moon will be waxing gibbous and at an unfavorable 75% illumination at this point. Still, with a radiant just south of the equator at -17° in the constellation Aquarius, the Southern Delta Aquarids are one of the better southern hemisphere meteor showers. Expect a maximum Zenithal Hourly Rate of 20 emanating from a region just north of the bright star Fomalhaut. The Southern Delta Aquarids also make a good prelude to the Perseid meteors in mid-August and the start of “meteor season”. More on that to come!

 

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  1. [...] provide a wealth of candidates for the 'Lunar O,' and straight line features such as the Rupes Recta lunar straight wall feature in the Mare Nubium could easily pass for the 'Lunar I'. Veteran lunar observer [...]

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