Study the Moon

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Article 3 of 6 on Studying the Moon

Observing Along the T: Features Both Similar and Strange

Michael Packer

Comments: m·DOT·packer·AT·Yahoo·DOT·com


There is an aspect to viewing volcanic geology on the Moon that is more meaningful than examining craters. Our Moon is part of that domain of celestial bodies we call other worlds. And observing geology out there that’s similar to the geology down here, gives an order to the universe that makes it a little more neighborly. Take a look at the images below:

Sheild Volcanoes on Earth (Galapagos), Jupiter's Moon Io, and the Moon (Mons Rümker is a field of shield volcanos)

Shield Volcanos are strikingly similar throughout our solar system. And they play a significant role in postulating the evolution of planetary systems around other stars as well as our own Sun. Here's a look at another feature:

San Andreas Fault California and Linear Faults Rima Hyginus and Rima Ariadaeus on the Moon

While the linear faults Rima Hyginus and Ariadaeus are not made from the orogenics that formed the San Andreas Fault in California, all the faults are tectonic in nature. And the sections dipicted above all happen to be similar in width (~5 km), similar in height (2000 ft) and are hundreds of kilometers long. If you ever have the chance to visit Point Reyes National Seashore, think of the moon as you drive through this huge rille. The experience can be "out of this world" as with walking the rim of Barringer Crater in Arizona.

Observing Similar and Strange Features Along the T: The features I jovially put under the heading of similar and strange include lunar domes, rilles and scarps, crater chains, Apollo landing sites and both clair-obscur and sun-ray lighting-effects. Similar and strange is how they might seem to the first time oberver. But in fact the examples that embody them, and which are presented below, are as much typical as they are natural wonders of the Moon. As in the last article on craters, I’ll give an overview and some background information on the objects. Two consecutive days of lunar observing will then introduce a few more features and finish up the article.

Background: Table 1 lists the range of geologic-based features you’ll want to familiarize yourself with. The seas, lakes, marshes, et cetera are all basaltic lava plains from low viscosity magma making its way to the surface and flooding low lying basins. Dorsum are tectonic features created when basaltic lava plains first cooled and contracted. Domes are shield volcanos and are discussed after rilles and scarps.


Table 1                                     Lunar Features


Large Impact Crater




Impact Crater




Crater Chain




Shield Volcano


Crater Ray


Mare Ridges






Scarp or Fault












Three Types of Rilles (really!): On the topic of the Moon I occasionally hear that rilles are "sunken lava tubes". But that's only a 3rd of the story. And it may be less than a 3rd when you consider that the word rille describes any long, narrow groove that resembles a channel. The word rille has been used to describe features on Venus, Mars, and the moons of Jupiter. So in other words, it's a useful general term to describe a terrestrial feature. Here are definitions for the 3 common rilles found on the moon:

Sinuous Rilles (Sunken lava tubes): Sinuous rilles meander in a curved path like a river, and are thought to be the remains of collapsed lava tubes or extinct lava flows. They usually begin at an volcanic dome, then meander and sometimes split as they are followed across the surface. Examples: Rima Conon and Rima Birt. Please note the key word sunken or collapsed. Sinuous valleys like Schröter and Alpes are not rilles but are carved from rivers of surface lava. But both Vallis Schröter and Vallis Alps have rilles running down the center of their floors! Although difficult to observe or image without decent seeing, today's noise cancelation imaging by amateurs allows these rilles to be captured. See Steve Keene's image of Valley Alps.

Concentric or Arcuate Rilles (Arcs of cracked mare): Concentric rilles have a smooth curve and are found on the edges of the dark lunar maria. Its believed they formed when the lava floods that created the mare cooled, contracted, and sank. Personally I think of the edge brownies coming out of the oven. See Rimae Pitatus below. Other excellent examples: Rimae Hypatia with Rimae Bradley and Fresnel likely candidates. Note 1: if concentric rilles are not strange enough, check the 15 km concentric crater (Hesiodus A) at the western edge of the image. Dozens of these small craters can be found on the moon near maria and are likely indicative of sub-surface geology (possibly volcanism) that allows the transient impact wave to propagate beyond the simple bowl shape. Note 2: If Hesiodus A is not odd enough, then check out the Hesiodus Sun Ray (see below for details).

Straight or Linear Rilles: Straight rilles follow long, linear paths and are believed to be grabens, sections of the crust that have sunk between two parallel faults forming a channel. These fault depressions are the result of themal heat expansion perpendicular to the fault line. These rilles can be readily identified if they pass through craters or mountain ranges. Two of the best examples (already shown above) are Rimae Archimedes and Rima Hyginus, although there is more geology going on with Hyginus. The crater chain that peppers Rima Hyginus are rimless eruptions craters which developed after the linear fault's formation. So the rille is both tectonic and volcanic in origin. 10 km Hyginus crater itself is likely the largest non-impact crater on the Moon.

Rupes, plural Rupis (Scarps, or Faults): Although they are not called rilles, rupis are faults so covering them after linear rilles seams like a good idea. The principle difference between rupes-faults and straight-rille-faults is the obvious offset in height between the faulted plans of rupes. This can result from a combination of volcanic, gravitational and seismic forces settling or rising adjacent planes. Localized surface cooling often plays a big role. As a section of surface cools it will contract (and shrink downward) relative to the surround. The loss of heat or thermal pressure can also allow gravitational forces to dominate causing the surface to collapse under its own weight. A rupes can result from an impact such as the one that formed the Nectarian Basin. One of the best known rupes is 134 km long and 490 m (1600 feet) high Rupes Recti (Straight Wall) shown below with sinuous rille Rima Birt. Often observed at sunrise one day after First Quarter, the showdow cast can make the wall seem towering and perfectly vertical although it actually slopes at an angle only a little greater than 20°. Another outstanding example is Rupes Cauchy which forms a pair of "hyperbolas" with Rima Cauchy.

Sinuous Rima Birt & Linear Rupes Recta
1d after First Quarter or Last Quarter

Arcuate Rimae Pitatus and Hesiodus A
1d after First Quarter or Last Quarter

10km Volcanic Hyginus & Linear Rima Hyginus
6d's after New Moon or 5d's after Full

Colongitude 12º or 186º

Colongitude 18º or 190º

Colongitude 357º or 167º



Lunar Domes, Lava Flows and Pyroclastic Deposits: The domes Mons Gruithuisen Gamma and Delta are located about 200km south of Sinus Iridum and are similar to volcanos on Earth. Both are from rocks rich in silica like sand on a beach such as Stinson Beach above. Unlike basaltic (mare) lava which flows in a more fluid manner, silica rich lava can form dome structures or shield volcanoes. At Colongitude 42º both Gamma and Delta will be visible at the T and should look positively Olympian. Gruithuisen Gamma is the more westerly of the two domes and spans 20 km in diameter, rising to 3900 ft. above the surface.  In the centre of the dome is a small 900m diameter crater. It is often used a test object for the resolution of scopes.  Gruithuisen Delta is slightly smaller at 13 km but stands 5000 ft. above the surface (source: esa). It is interesting to note that the basaltic mare that flooded the major impact basins was assayed to have low SiO2 content. Along with high temperature upon extrusion, the low viscosity of this erupted lava was found to be the same as motor oil at room temperature. This is about a factor of 10 more fluid than terrestrial lava resulting in fluid flows spreading out hundreds of miles. (source: Spudis article #21 and C. Meyer). Low viscosity lava also has the propensity to create lava tubes.

Vallis Schröteri, the best example of a lava river on the moon, is also nicely observed at Colongitude 42º (or 4 days after First Quarter/3 days after Last Quarter). 168 kilometers long and 1 km deep, the valley varies in width from 10 km to less than 1 km at its terminus and is worth following out to sea. The 6 km crater at the head of Schröter's Valley is believed to be the past eruptive vent where magma surfaced through 60 kilometers of lunar crust, created the valley, and flooded the NW region of Oceanus Procellarum. The valley's floor contains a long 0.2 km wide sinius rille which is not easy to image and near impossible to observe.

Pyroclastic Deposits are fragmented (clastic) rock material formed by a volcanic explosion or ejection from a volcanic vent. Sometimes containing useful volatiles such as ammonia, nitrogen, hydrogen and various oxides, the material cools rapidly forming mineral or glass like compounds ranging in size from boulders to ash. The debris lightens with space weathering so dark deposits represent recent activity on the geologic time scale. This dark material is best seen around fissures on the floor of craters such as Alphonus, Atlas and Franklin. On a larger scale, pyroclastic deposits reflect an explosive stage of basaltic volcanism that filled the nearside basins 3.75 billion years ago. Radar images show large debris fields around Valley Schröter (the Aristarchus Plateau) and other sections in proximity to the lunar seas. The VLA database lists some of these pyroclastic deposits.

Mons Gruithuisen Gamma & Delta
3d's after First Quarter or 2d's after Last

168 km long Vallis Schröteri
4d's after First Quarter or 3d's after Last

Pyroclastic Debris Inside Alphonsus
First Quarter or 6d's after Full

Colongitude 42º or 217º

Colongitude 18º or 190º

Also Full or Near Full Moon



Clair-obscur effects and Sun Rays: While observing along the T, one is occasionally hit with the interplay of light and shadow giving false but fun impressions of features such as giants letters like “X” and “V”, a giant staircase, and bridges. These impressions have been coined clair-obsur (French for light and shadow).


Both "Lunar X" or "Lunar V" can be seen at a Colongitude as early as 357º(a few hours after 1st quarter) on the terminator near craters Werner and Ukert respectively. I have seen both at the same time when the colongitude/solar latitude were 358º/-1.3º. The Zeno Steps can be seen around colongitude 110º. See for more info.


But my favorite effect is O’Neil’s Bridge at Promontorium Lavinium/Olivium which is a combination of Clair-obscur and Sun Ray. The effect got its name (at least the bridge part) after John O'Neill, former science editor of the old NY Herald Tribune, who saw a fictitious arch one night in 1953. I found it’s tricky to observe with the Sun Ray because the effect only lasts ~20 minutes. But if you see it under the right light there’s little question of it resembling a bridge at 125-175x. It’s rather astonishing because you see a sunbeam seemingly illuminate a narrows before going through the arch. The ray then apears to emerge and illuminate an arena some 15 km on the dark side of the terminator. I suggest using VMA and start observing ahead of time at a colongitude a little less than 130º. Click on the left table below for timings.

The right table above is for the Hesiodus Sunrise Ray or "Hesiodus Ray" which became popular after it was discussed in the July 1996 issue of "Sky & Telescope". The break in both Hesiodus's and Pitatus's crater wall plays a role in the rays formation and 44 kilometer span across the floor of Hesiodus. The ray is yet another interesting feature to look for around the Pitatus region. Here's an outstanding High Resolution Image of the area.  

Exceptional Night Along the T: Actually two consectutive nights are well worth the back-to-back view with colongitudes C-171º (~Day 21) and C-184º (~Day 22) respectively. In fact, there are so many outstanding features to study under this lighting I can only discuss a few.

Around the Apennines Mountains: As the Sun sets on the Moon on Lunar Day 21, C-171º, you can see the 3 basic classes of rilles on either side of the Apennines (Montes Apenninus). A small 80mm refractor will do. I particularly like the lighting at this time but it’s a fact that studying rilles under different lighting is best to appreciate their intricate meanderings and structure. In any case, look for Rima Fresnel (concentric rille), Rima Archimedes (linear fault), and Rima Conon (volcanic or sinuous rille). The later is on the back side of the Apennines. At the base of the Apennines you can see Rima Hadley and the Apollo 15 landing site but you’ll want a medium to large scope to see the rille. West and North of the Apennines, great views can be had of Archimedes, Autolycus and Aristillus. This is a really good time to study their ages. Going futher North, be sure to check out Cassini, Mons Piton and Vallis Alpes and other features up to the pole. The Copernicus Crater Chain: South of the Apennines, a supurb area to study on C-171º and C-184º (and even a 3rd night) is Copernicus and its secondary impacts to the North of Stadius. This is one of the best crater chains on the Moon but it is not called a cantena. In fact, it is not given any formal name that I am aware. Due to its location, Rükl refered to it in his atlas as "The Crater Chain Close to Stadius" - decriptive if nothing else.

Like Tycho rays, crater chains are secondary impacts that delineate the considerable blast radius and energy from a ground zero crater. But not all crater chains are secondary impacts. Comet Shoemaker-Levy 9 was proof that comets can break up and linearly strike a gravitational body. Also the craters that pepper Rima Hyginus are rimless eruptions craters rising from the linear fault. But the numerous and extensive crater chains between Eratosthenes and Copernicus are from Copernicus.

The Copernicus Crater Chain North of Stadius


From the dimensions and class of Copernicus, Eugene Shoemaker calculated that the asteroid that formed the crater was traveling at 17 km/s. And from the observed distribution of the secondary craters, Shoemaker further calculated that the craterlets could develop from 0.1-0.5 km rock fragments ejected from this 17 km/s impact.


Now wrapped around each craterlet are herringboned shaped dunes pointing back to Copernicus. To explain this feature, experiments from the NASA-Ames were conducted and showed that two projectiles that hit a target surface at same time, as with secondary craters, form waves of expanding ejecta. This expanding ejecta in turn interferes and falls back forming herringbone shaped piles – pointing back to the source. QED! The match between these experiments and Orbiter photos around Copernicus were dead on. See Chuck Wood’s book for more information and observe these craterlets and herringbone structures over several nights. It’s crackerjack.

Apollo 12 and 14 Bases: South of Copernicus the Apollo 12 mission landed on an area of the Ocean of Storms that had been visited by several earlier unmanned missions (Luna 5, Surveyor 3, and Ranger 7). The International Astronomical Union hence christened this region Mare Cognitum (Known Sea). The Fra Mauro Highlands is where the Apollo 13 mission was to land and where 14 finally did. Follow this link for a description of this well chosen area. Although the Apollo 11 site is in the Sea of Tranquillity (C-343º or C-153º are good colongitudes for observing), there was rhyme and reason for the location of this 1st lunar base as well. The landscape is complete with interesting features with Crater Sabine, Maskelyne G and the Rimae Hypatia network to the south. To the north the shield Volcanoes Arago Alpha/Beta are superb. To the east is 5 km ALC class crater Armstrong surrounded by faint dorsa. If you’re an early riser you can see this area under great illumination Four days after Full Moon. But the fortune in the cookie is taking in the scenery of the 1st manned mission to the moon. Turning Southeast of the Apollo 14 Base and back to the T on C-171º, the volcanic crater and fault Hyginus can be viewed close to the lunar sunset. A scope nudge South and Triesnecker crater with its network of rilles deserves study also (a 6" scope makes following the rilles easy). South of these rilles, look for the huge Tycho class craters Hipparchus, Ptolemaeus, Alphonsus, Arzachel. At these phases, one can easily study the best known area on the moon that exemplifies crater overlap and the relative age of craters. Further South along the T, we have two great nights to view the somewhat elusive sinuous Rima Birt (can you dertermine the direction and source of this 50 km long rille?) and the striking linear fault Straight Wall (6" or larger scope for Rima Birt recommended). The next area down the T is Pitatus and Hesiodus A. In addition to the rilles inside Pitatus look for the 309 kilometer Rima Hesiodus that runs West away from Hisiodus and see if you can observe the details of the triplet rille Rimae Hippalus. These rilles span 191 km are concentric to the Humorum Basin and were formed by crustal stress as the basin compressed under the weight of the Humorum lavas. There are several other finer rimae in this area to discover. Finally we make are way to Tycho and Clavius. Although these craters are not next to the T, the lighting is near perfect for observing the inner structures of both impacts and micro craters inside Clavius. Both craters benefit from favorable librations in latitude so be on the look for these times as well. And as you make your way to the South Pole look for Schiller and Moretus.

What’s next: The next article will be on sailing the lunar seas followed by favorable libration observations. And the last article covers the age of the Moon and its craters. Until then, try to get in a few consectutive nights of oberving along the T. If you observe all phases of the Moon in a month, you are a more dedicated observer than I.


Study the Moon

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Michael Packer