Ptolemy’s Lunar Neighborhood

Ptolemy’s neighborhood is a popular one for moon gazers, which is as it should be; for it is populated by some of the most famous names in astronomical history and is home to some of the most beautiful and fascinating formations found on the Moon as well. Be sure to catch its monthly appearance and have a closer look at this fascinating and historic area

The Old Astronomer’s Club

Many lunar features have been named after astronomers or persons in astronomy to recognize their achievements, such as 20th-century astronomer William Pickering and the Pickering crater. However, few Areas of the Moon have a “who’s who” neighborhood like this one.

Famous people? They’re here! “Ptolemaeus” is the Latinized crater name memorializing the better-known Ptolemy. The most famed astronomer and astrologer (often the same thing in those long ago ages) during the first 1500 years of the Christian era. His heavenly interpretations were not supplanted until Copernicus’ contrary heliocentric ideas were eventually confirmed as a more accurate interpretation of reality when Galileo finally observed the phases of Venus through his telescope.

Claudius Ptolemaeus’ shabby neighbor to the east, the ruined walled plain of Hipparchus, is the memorial of perhaps the greatest astronomer & observer of all the ancients. He invented trigonometry, invented the astrolabe, discovered precession and created the first star atlas of the western world. Ptolemy and Hipparchus were collaborators across time as well as neighbors on the Moon; Ptolemy based much of his later work on that of his mentor, Hipparchus.

Albategnius, or Al-Battani as he is otherwise known, was an Arab astronomer whose work was later developed by Copernicus. Al-Battani determined the most accurate period for the solar year: within three minutes of the modern value before the dawn of the second millennium.(Trekkies may recognize his name too–as it was the name also of Kathryn Janeway’s first starship, the USS Al-Batani. )

Arzachel (or Abu Ishaq Ibrahim ibn Yahya Al-Zarqali ) was another Arabic astronomer of the middle ages to whom Copernicus acknowledged a debt. His crater may overlay Alphonsus’ to the north, but his work underlies that crater’s namesake; for his astronomical tables, the Toledan Tables, were later supplanted by the more accurate Alphonsine tables commissioned by Alphonsus, a medieval King of Spain, otherwise known as Alphonso the Tenth. We, however, can revere old King Alphonso most for establishing one of the first European centers for the translation of many essential ancient documents, last preserved in Arabic, back to the Latin language in Toledo, Spain. This act responsibly promulgated the ancient wisdom of the Greeks throughout a Latinized but darkened medieval Europe.

So we have a large portion of the history of astronomy memorialized here, but a consiiderable portion of the Moon’s history is as well, as we shall see…

Geologic Overview

Below is a graphic that illustrates the main features of the Ptolmaeus region, along with a geological map depicting its history.

An image of Ptolemaeus Region of the Moon with a colored map outlining the different sections within.
Ptolemaeus Region

This area shows scars from two great basin forming impacts; Imbrium and Orientale. Nearer Imbrium gave the most scars here; the liniments (radial sculpture), indicated by dashed lines, rip like claws across all but the later (Imbrium era ) craters, scarring all the major features here save the Erastosthenian era crater Hershel and Lower Imbrium era crater Arzachel. Note that the longest such liniment on the face of the Moon is noted as beginning at Lalande C and extending all the way to the Rim of Alphonsus for a total distance of 235km. 

Some lunar scientists believe that the relatively far away Orientale impact could have contributed some minor secondary craters to this area, and notable among the possibilities are Catena Davy (reference) and craterlets Ptolemaeus H and HA (Wilhelms). The Davy chain is a notable feature with various explanations offered as to its cause, ranging from the foregoing to a comet impact ala Shoemaker-levy on Jupiter. Nevertheless, even more of a mystery than the Davy chain is the larger, more plainly defined but unnamed crater chain beginning at Ptolemaeus R and ending at Muller A that crosses the northern rim of Ptolemaeus– which points to absolutely nothing at all. 

You will notice on the colored geologic map that something called the “Cayley” figures prominently in this area. The Cayley formation is a geologic grouping of terrae that is thought to consist primarily of a thick flow of pulverized crustal material. This material is cast out by the ferocity of the Imbrium basin forming impact and later at least parially covered by a thinner layer of similar ejecta originating from the Orientale basin forming event. These layers are grouped as megaregolith, in other words crushed and broken regolith but spread over moon wide scales instead of locally created by more minor impacts. Its almost liquidized flow of impact debris blankets the entire area, overlain only by later deposits from the later Imbrium, Eratosthenian, and Copernican ages. Notice how a few earlier features from the Pre-Imbrium poke out, mainly in the form of older, Nectarian, and Pre-Nectarian crater rims.

Famous Crash Sites

Other points of interest shown in the larger view are a trio of Nectarian-era craters, Albategnius, and its oddly darker floored sibling, Klein. We will take a closer look at it later, along with the very odd crater Alpatragius with its peculiar, oversized, dome-shaped central peak– for which no one seems to have a cogent explanation.

A Galilean Re-visitation

Another bit of history resides here. Probably the most reproduced view of the Moon in history is Galileo’s famous rendering shown below:

A drawing of the moon created by Galileo.

It is easy to assume (and most do) that this is a rendition of the whole Moon—but could it really be just a tiny portion of the Moon, en eyepiece drawing in fact? One made at low power and a very narrow Galilean telescopic field of view? (Only about 15 minutes of arc in diameter at 30x) What is that colossal crater that’s featured so prominently? Nothing like it seems to exist at such a position. Historians have been puzzling over what exactly Galileo was trying to portray here for centuries. Several experts have concluded that it is none other than Albategnius but curiously enlarged way out of proportion. Perhaps the mystery will never be solved to everyone’s satisfaction. But don’t let that stop you from contemplating the greatest ever popularizer of telescopic observation of the heavens, and his revolutionary drawing, when you observe this magnificent feature.

A Closer Look

Now for a closer look at the prominent craters in this famous trio, Ptolemaeus, Alphonsus, and Arzachel, and some of the lunar mysteries that surround them:

Features marked (a) in the graphic above denote a larger-scale feature of interest in this area: a significant Imbrium basin radial subsurface fault. At least this is what researchers think is the cause of the confluence of the odd “braided” ridgeline involving Alphonsus’ central peak (see inset for a better view), the identically aligned oddly elongate peak of Arzachel and the third fault-like depression aligned with the others on the SW floor of Ptolemaeus. Connected all these anomalous elongate features can be seen to have formed along a line pointing directly back to the center of the Imbrium basin, thus indicating that a deep crustal fracture probably had some controlling influence on the shapes of these surface features as we see them today.

(b) Marks a somewhat anonymous crater called Klein, lying within the confines of the walled plain Abategnius. The oddity here is the albedo or the color of this crater’s floor–you will notice it is much darker than the floor of the plain it lies within–obviously, its composition is different, but what and how? One explanation is that it is an isolated volcanic filling as differentiated from the light plains which fills Albategnius next door. A closer look lends support to this explanation as not only is this in an established volcanic area (Alphonsus), but disturbed features on its floor (the rough-looking area on the western side) as well as within a crater form immediately adjacent (d) which contains a plain & suspiciously volcanic looking feature (inset) indicate a strong possibility of a history of volcanic activity here.

Albatgenius and Laser Beams

Albategnius is of note for another bit of history. It was here that two MIT researchers in 1962 became the very first to successfully fire a laser beam that actually returned from the Moon back to the Earth, a feat that would lead to establishing the most accurate distances and orbital relationships between those bodies yet measured.

Alphonsus and the splash seen around the world

A lot is written about the crater Alphonsus, famous for its Lunar Transient Phenomena sightings and its six notable dark spots consisting of material spewed from small volcanic vents, which can be seen as tiny pits under ideal seeing conditions. How many can you see? Other less well-known but intriguing (if somewhat difficult to see) features on the eastern floor of this crater are the system of arculate and straight fracture formed fault/rilles, the more prominent of which are marked in the graphic above. Note also the landing site (or, more accurately, crashsite) of the famous Ranger 9 mission. This was the first of the scientific missions rather than simply reconnaissance for potential Apollo landing sites. It was also the first Moon video broadcast live on television in 1965. Talk about making a splash!

Alphonsus, how old are you, really??

The last feature of interest of Alphonsus’ is the very little noticed crater Alphonsus B, marked (c) on the graphic and partially lying on Alponsus’ eastern rim. What is odd about this crater is its form and orientation and that it is there at all. The form is that of an oblique impact, coming in low and fast, creating the triangular extension of its western wall. Where did this crater come from and when did it hit? Well, looking at it’s shape gives a clue to the direction of impact:

So the impact which created Alphonsus B came from the east. But where? Drawing a line back to the east, we notice that it intersects exactly in the middle of the Nectarin basin. So could this be a Nectarian basin secondary impact? 

Not if we accept that the age of Alphonsus is indeed of Nectarian age! Since Alphonsus B overlies the rim of Alphonsus, the law of superposition dictates that it could not have formed earlier than Alphonsus itself but only afterward. This would preclude the Nectarin basin event as a source for this crater–as a Nectarian crater, by definition, is *post* Nectarian impact formation. (The Nectarian era is marked at it’s beginning by the Nectarin basin formation event.) 

But one other explanation *is* possible–the placement of Alphonsus in the Nectarian period is in error. This wouldn’t appear too likely– except for the existence of a virtually identical crater, Albufeda D, which also lies in direct radial relation to the Nectaris basin and is already recognized as a probably Nectaris basin secondary (Byrne, Wilhelms). The graphic below might help clarify things a bit:

So maybe Alphonsus is a bit older than we have thought. Or maybe these are just chance, unrelated impacts which coincidentally point back to the same basin?

Variable Rille?!

The last feature of particular interest in the area is a really neat one I think, which is why I saved it for last–its a variable rille in Arzachel!

Lying on the eastern floor of Arzachel is what appears to be a sinuous rille but which is in reality an arculate rille and a fracture of Arzachel’s floor, a portion of which, at least, formed in a similar way to the Straight wall. This created a feature that varies along its length from a depression to a cliff face:

The effect of this varying landscape in a telescopic view is similar to that seen of Arzachel near the bottom of the closeup graphic shown earlier; the southern portion being a ‘black line’ rille gradually fading from sight as it narrows, and the northern portion (from Arzachel T) being a wide whiteline (in morning sun) or a broad black cliff-shadow in the evening sun. Take your pick; Rima Arzachel offers “rilley” different views.

Written by CityAstronomy.com

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