Sunset at the Straight Wall reveals amazing detail

Hi all,

This is my second lunar sketch done using eyeglasses.  I’ve adapted a headband magnifier so it can carry a pair of spectacles suited to my eyes.  I’m getting used to the new way of sketching as there is now an additional step in my eyepiece-to-sketch-pad routine.  But the difference using these specs makes is well worth any initial discomfort.

I got up very early on Sunday morning for the chance to sketch something during the ¾ phase of the Moon.  As it turned out, seeing wasn’t the best despite the wee hour and cool temperature, but the occasional glimpses of remarkable clarity made the effort worthwhile all the same.

With all the lunar features on view tonight, it was an old favourite that most caught my attention, Rupes Recta, the Straight Wall.  I’ve sketch the Straight Wall during the ¼ phase as the Sun rose over this escarpment (see below), but this particular phase had the Sun setting, and the wall instead of being a dark shadow, was brilliant white, and the very low angle the sun was at cast the most remarkable long shadows and revealed a textured surface and details of this area of Mare Nubium that were jaw-dropping remarkable.

From the above sketch of the Straight Wall, the higher angle of the Sun leads one to think that the flooded ghost crater the Wall sits in is relatively smooth and not particularly textured.

Now, move the phase forward some 14 days, with the Sun casting its last rays across this same area at a glancing angle, and a staggering textured and tortured surface is revealed.  The Straight Wall and Rima Birt are not the only geological formations here.  Just west and running parallel to the Straight Wall is a long straight “depression” or fold that is roughly as long at the Straight Wall itself.  This long depression is crossed by Rima Birt, and continues on straight northward.

Rima Birt itself is remarkable in that its official selenologic origins are somewhat uncertain.  Most likely it is a rift fault that then allowed lava to erupt up along through the fault.  And as my sketch progressed, I picked up two volcanic domes (Birt 1 & 2) at the northern end of Rima Birt, the larger of the two sitting right on top of the rile!

The glancing angle of the fast setting sun also reveals so many other obscure features.  The oh-so-faint rims of several ghost craters are just barely visible – one being outlined by the chain of mountains, west of the Straight Wall, that circumvents and suggest the basin in which the Straight Wall sits in.  A highly pock-marked field of craterlets lies scattered south of Birt.  The faded ray system that extends southward belonging to Birt.  Several winding folds of old lava flows form wrinkles across the moonscape.  Long string-like streaks of light are cast eastward from the northern end of the Straight Wall across a jet black otherwise invisible plain.  And of course the curious twin peaks of dark shadows cast by the crater Birt across the brilliant white escarpment.

Object:  Sunset along the Straight Wall and surrounds.

Scope:  8” SCT

Gear:  10mm Pentax XW, 200X

Date:  5^th August, 2018

Location:  Sydney, Australia

Media:   White and grey soft pastels, charcoal and white gel ink on A5 black paper

This was a remarkable session for me.   From starting not wanting to repeat a feature I had sketched before, a little time spent with it showed an astounding amount of invisible details revealed only by having the sun at as low an angle as possible before it dips below the horizon.   Quite superb!

Alex.