The partially visible rim of an unexpected basin, or just an arc-shaped arrangement of several low-albedo spots?

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It was once the topic of several messages in Charles Taylor's LUNAR OBSERVING GROUP (Yahoo), and it was also the spark to start an investigation performed by Charles A.Wood (somewhere in 2002, iirc). The source of this topic was... me. In those days I detected a peculiar semi-circle shaped arrangement of several low-albedo spots southwest of Mare Humorum. The best time to observe this semi-circle shaped arrangement is during and after Full Moon (from Full Moon to Last Quarter Moon).
This page is some sort of "revival" of the days when this lunar peculiarity was discovered and described by me in Chuck Taylor's LUNAR OBSERVING GROUP.
I hope you are about to enjoy a new investigation of a lunar surface formation which could be a true discovery (an unexpected basin) or just an arc-shaped arrangement of several low-albedo spots...
- DannyCaes Feb 19, 2017


Now, when you observe it in the Equidistant Cylindrical Projection of the moon's albedo formations (Full Moon appearance, so to say) of the LROC ACT-REACT QUICK MAP, you can't deny the presence of a dark geometrically perfect circle-shaped appearance southwest of Mare Humorum (more-or-less the same diameter as the dark part of Mare Humorum or Mare Nectaris). Although there's only one half of it (the south-southeastern half), it's obvious we have something here which could not immediately be catalogued as a "coincidence".
So... what is it?
See shortlink to LROC's albedo map of the region southwest of Mare Humorum:

Can you see it?

If you can't, I'll help you to detect this obscure semi-circle shaped appearance. Fasten your seatbelts...
We start clockwise at the west-southwestern part of Mare Humorum (at the elongated dark part near the WSW coastline of that sea) and continue our tour southward, toward the irregular dark spot at Doppelmayer C. From there we travel southwest toward another irregular dark spot slightly east of both Doppelmayer Y and Doppelmayer Z. Now we continue our tour slightly southward at the most western section of Lacus Excellentiae and at an elongated southwestern dark "appendix" of it near Drebbel B and Drebbel D. From there on it's due westward to the dark spot Drebbel E (which is the most southern part of our semi-circle). Our next stop is the dark spot Lehmann E (west-northwest of Drebbel E), and our final goal is the system of very dark spots slightly west of Fourier N (north-northeast of Lehmann E). This system of dark spots (west of Fourier N) was once called the Basaltic Lava Areas by the dedicated British observer of the moon Harold Hill (see his superb book A Portfolio of Lunar Drawings), and also Honey Lake by several lunar observers from the golden era of William Radcliffe Birt (I wish both H.Hill and W.R.Birt could be alive today, to have a little chat with them).
Now, it could be that the elongated dark spot near De Gasparis and Palmieri (with the dark Palmieri attached to it) is also part of this possible basin, but I'm not sure about that. I also discovered a third elongated dark presence at Doppelmayer and Puiseux, which means there are three elongated dark spots in a row: the first spot inside the semi-circle, the second spot AT the northern part of the semi-circle, and a third one outside the semi-circle.
In some way these three dark regions remind me the repetitive appearances of the so-called Brecher clouds in earth's atmosphere (aka Kelvin-Helmholtz clouds), see also: Kelvin-Helmholtz instability.

The centre of the suspected basin, as it should be marked on the Quick Map's Equidistant Cylindrical Projection

When you draw a vertical line upward from the southernmost part of the semi-circle (at the low-albedo spot Drebbel E), and a horizontal line leftward from the easternmost part of it (at Doppelmayer C), you have the cross-point of both lines at a location between Fourier and the low-albedo crater Fourier A (somewhat nearer to Fourier, in fact a little bit to the west of an unnamed low-albedo spot immediately west of Fourier A).

Dark spot Drebbel E, once called Nobelarii?

According to Ewen A. Whitaker, the dark spot Drebbel E was once called Nobelarii by the Dutch/Belgian cartographer Michel Florent van Langren (also known as Langrenus). M.F.v.Langren was the first one in the history of selenography who made a real serious map of the moon, full of names for the craters and albedo formations. Who knows, perhaps he too detected this semi-circle shaped arc of dark spots...
For the name Nobelarii (Drebbel E), see page 196 in E.A.Whitaker's splendid book Mapping and Naming the Moon; a history of lunar cartography and nomenclature.

Dark elongated spot near De Gasparis and Palmieri, once called Arabiae paludes?

According to Ewen A. Whitaker, the elongated dark spot near De Gasparis and Palmieri was once called Arabiae paludes by the Polish selenographer Johann Hewelcke (also known as Hevelius). See page 202 in E.A.Whitaker's Mapping and Naming the Moon.

Antonin Rukl's Atlas of the Moon

The semi-circle shaped arc of dark spots is depicted on charts 51, 52, 61, 62 (the region southwest of Mare Humorum).

21st Century Atlas of the Moon

Page 82 of C.A.Wood's and M.J.S.Collins's 21st Century Atlas of the Moon shows a photograph of the southwestern quadrant of the Full Moon. The semi-circle shaped arc of dark spots SW of Mare Humorum is very well depicted in this photograph! Note also the appearance of C.A.Wood's Doppelmayer pyroclastics. This pyroclastics-region is part of the semi-circle shaped arc of dark spots.

Clementine Atlas of the Moon

LAC 93 (page 186) shows the northern part of the semi-circle shaped arc (at the southwestern part of Mare Humorum).
LAC 110 (page 220) shows almost the whole of the semi-circle (you can't miss it!) (north-northeast of Schickard).

Times Atlas of the Moon (1969)

Charts 89 and 90 of this great atlas show almost the whole of the semi-circle shaped arc (except the most northern part of it, which is depicted on chart 79), but... on all of these yellowish colored charts you should add the grey shades of the low-albedo spots! (a grey pencil type H-7 is perfect).
See also LAC 93 and LAC 110 (Lunar and Planetary Institute - Lunar Map Catalog).

Consolidated Lunar Atlas

Full Moon Plate F-V (northern part of the semi-circle shaped arc).
Full Moon Plate G-IV (southern part of the semi-circle shaped arc).

Danny Caes,
Manual labourer and dedicated explorer of the moon's surface,
Ghent - Belgium