Whitepeak Observatory, Tacoma, WA

Ptolemy's Neighborhood


Ptolemy's neighborhood is a popular one for moon gazers and that 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 interesting formations found on the Moon as well. It's making it's monthly appearance in the next couple of days-- so don't neglect to take a closer look at this fascinating and historical area!

The Old Astronomer's Club

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 not supplanted until Copernicus' contrary heliocentric ideas were eventually confirmed as a more accurate interpretation of reality when Galileo was finally able to observe 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 a Arab astronomer whose work was later developed by Copernicus. Al-Battani determined a period for the solar year that was most accurate; within three minutes of the modern value in fact and this 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 thyat 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 important ancient documents, last preserved in Arabic, back to the Latin language, in Toledo, Spain, an act which was responsible promulgating the ancient wisdom of the Greeks throughout a latinized but darkened medieval Europe.

So it seems 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 which illustrates the main features of this area as well as a geological map which illustrates it's history.

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 is Catena Davy (reference) and craterlets Ptolemaeus H and HA (Wilhelms). The Davy chain is a well-known feature with a variety of explanations offered as to it's cause, ranging from the foregoing to a comet impact ala Shoemaker-levy on Jupiter. But 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'll 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 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 something called megaregolith, in other words crushed and broken regolith but spread over moonwide scales instead of locally created by smaller impacts. It's 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, mostly in the form of older, Nectarian and Pre-nectarian crater rims.

Famous Crash sites!

Notice the *two* space probe landings upon the rather ordinary looking piece of mare called Sinus Medii to the north? What's so special here? Well, for one thing this area marks the closest point on the Moon to the Earth and was targeted repeatedly by the Surveyor project as a favored manned landing site. But it was an exceedingly unlucky destination; the first to target this area, Surveyor 2, was lost completely. The second, Surveyor 4, was lost 2 minutes before touchdown. Only the third attempt at notorious Sinus Medii (three times is a charm?) was successful with Surveyor 6 landing safely and proving tot he doubting Thomas' of the Apollo program once and for all that Thomas Gold's nightmarish 'bottomless oceans of dust' just didn't exist on the Moon, thus making it 'safe' for landings. The next Surveyor did some actual science instead.

Other points of interest shown in the larger view are a trio of Nectarian era craters; Albategnius and it's oddly darker floored sibling, Klein, which we'll look closer at later and the very odd crater Alpatragius with it's totally weird too big and too domey 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:

It's easy to assume (and most do) that this is a rendition of the whole Moon---but could it really be just a small 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 15minutes of arc in diameter at 30x!) What is that huge 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 now... Several experts have concluded that it is none other than Albategnius, but curiouusly 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 together denote a larger scale feature of interest in this area--the presence of a major 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 together 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 an rather anonymous crater called Klein, laying within the confines of the walled plain Abategnius. The oddity here is the albedo, or the color of this crater's floor--you'll notice it is much darker than the floor of the plain it lies within--obviously it's 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 it's floor (the rough looking area on the western side) as well as within a craterform immediately adjacent (d) which contains a plain & suspiciously volcanic looking feature (inset) indicate a strong possibility of a history of volcanic activity here.

Albategnius 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 has been written about the crater Alphonsus, famous for it's Lunar Transient Phenomena sightings as well as it's 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 interesting (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 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 missions that were scientific in nature rather then simply reconnaissance for potential Apollo landing sites. It was also the first Moon video broadcast live on television way back 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 it's 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 it's 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. 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?

A Variable Rille?!

The last feature of special interest in the area is a really neat one I think, which is why I saved it for last--it's 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 which varies along it's 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! ;)

Comments, corrections, additions, etc welcome as always. Happy lunar observing! back to main page