Although some stargazers may consider the Moon to be a nuisance that spoils their night-time viewing, the truth of the matter is that the Earth’s only natural satellite has enough features to offer a lifetime of observing, even if one never looks at anything else besides the Moon. However, since many of the estimated 300,000 lunar craters and dozens of mountains, rilles, lava domes, and other features are much the same, the trick to enjoying lunar observing is to look at objects at different times, when different illumination reveals contrasting features of the same object.
The ten objects on the list presented here are cases in point. Collectively, these objects represent the highest, deepest, biggest, and brightest objects of their class, but by viewing them at different times, each will reveal aspects of itself that may not have been visible during previous observations, which essentially, means that you will be looking at “new” objects almost every time you observe them.
Bear in mind though that the best lunar views are had just before and after the first quarter, but by using suitable filters, even large telescopes can be made to yield views of lunar features at almost any time during a lunation. Enjoy!
Object type: Mountain
Selenographic Coordinates: 19.92°N | 2.86°W
Named after: Christiaan Huygens- astronomer
Height: 5,500 m (18,046 ft)
Mons Huygens is the highest mountain on the Moon, and a relic of the violent impact that formed Mare Imbrium. In fact, Mons Huygens is only a small part of the Montes Apenninus (Apennine Mountain Range) that was formed along the eastern rim of the Imbrium impact basin.
However, although Mons Huygens is the highest mountain, it is not the highest point on the Moon. This point is known as the “Selenean summit” (Summit of the Moon), which is 10,786 metres (35,387 ft) above the centre of the Moon. Note though that this structure is not recognized as a mountain, since its slopes are on average only about 3 degrees steep. Being located on the far side of the Moon, the Selenean summit is thought by some planetary scientists to have been created by ejecta from the South Pole Aitken Impact Basin.
Object type: Linear Valley
Selenographic coordinates: 31.1°S | 56.0°E
Named after: Willebrord Snellius, Dutch astronomer and mathematician
Length: 592 km
Located in the rugged, heavily cratered southeastern parts of the near side lunar surface, the Vallis Snellius is 592km in length, making it the Moon’s longest named valley. It appears to have its origin in, or near the Mare Fecunditatis, and also appears to have been formed during the cataclysmic events that formed the Mare Fecunditatis. Although this valley is one of the major features of the Moon, it is heavily eroded by subsequent impacts, and much of its length can only be observed during conditions of favourable illumination.
Selenographic Coordinates: 18.4°N | 57.4°W
Named after: Ocean of Storms
Diameter: 2 568 km
At more than 1,600 miles (2,500 km) wide and covering an area of around 4 million square kms (1.5 million sq miles), Oceanus Procellarum is the biggest of the lunar mare (“seas”), and the only one to be called an Oceanus (“ocean”). The origin of this, the largest “ocean” of lava on the Moon remains uncertain. However, the square shape of the rift valleys (refer to the image) discovered by the GRAIL mission, during which gravity gradients on the Moon were investigated, seems to suggest that the basin is the result of internal processes driven by temperature changes, as opposed to being the result of an impact, which would have left a more circular scar.
Nonetheless, and regardless of the debate surrounding its origin, the Oceanus Procellarum contains among other features, the 32-km-wide Aristarchus ray crater, which is the brightest feature on the near side of the Moon, as well as the very prominent crater Copernicus, and the Carpathian Mountains between it and the Mare Imbrium.
Selenographic Coordinates: 20.6°S | 82.5°W
Named after: Lawrence Rooke, English astronomer
Diameter: 791 km
Montes Rook is 791 km in length, making it the longest mountain range on the lunar surface. Although much of the double-ring circular mountain range that encircles the Mare Orientale may be difficult to observe due to its location on the Moon’s western limb, it is sometimes possible to see individual peaks when the Sun is low on the lunar horizon during favourable librations. However, viewing this vast mountain range is best accomplished by projecting an image onto a white sphere to reduce the severe foreshortening that obscures detail of the circular structure in telescopic views.
Selenographic Coordinates: 6.4°N | 14.0°E
Named after: Nearby crater Ariadaeus
Length: 300 km
Over 300 kms long and 9 kms wide in places, Rima Ariadaeus is one of the most impressive straight rilles on the Moon. Unlike most other rilles, or rille systems on the Moon that were formed when hollow lava tubes collapsed, such as can be seen happening on Hawaii today, this particular feature seems likely to have formed when a section of the Moon’s crust subsided between two parallel fault lines. If this is indeed the origin of the feature, it should more properly be described as a “graben”, or fault trough, rather than as a “rille”.
Object type: Volcanic Rille
Selenographic Coordinates: 26.2°N | 50.8°W
Named after: Johann Hieronymus Schröter, German astronomer
Length: 140 km (86.9 miles)
Sinuous riles often emerge from a crater, and are defined by their long, meandering valleys, and steep walls. Located on the Aristarchus Plateau, Schroter’s Valley is the longest such example on the Moon, and at 160 kms in length, up to 10 kms wide, and 1,300 meters deep in places can easily be seen in Earth-based telescopes. This rille has its origin in a deep volcanic pit that resembles the shape of a snake’s head; hence, the feature’s other common name, the Cobra Head. This rille is also the largest of the three types (sinuous, arcuate, straight) of rille found on the Moon, and it follows a tortuous route for almost its entire length, before terminating at a 1,000-meter-high cliff on the edge of the Oceanus Procellarum.
Several transient lunar phenomena have been observed in and around the Cobra Head, most likely due to various gases escaping from the interior of the Moon. One other remarkable feature of Schroter’s Valley is that it contains a smaller rille on its floor, which can be observed and photographed through a medium-sized telescope when seeing conditions are good to excellent.
Selenographic Coordinates: 76.7°S | 16.9°W
Named after: Isaac Newton, mathematician
Diameter: 79 km
Depth: 6.1 km
Newton is recognized as the deepest impact crater on the near side of the Moon, and although foreshortening can obscure some of the most dramatic features of this crater, favourable librations, (and illumination), reveal that lightning can indeed strike twice in the same place. Newton D, a satellite crater seems to have struck the larger craters’ floor at a steep angle, judging by the huge difference in thickness between the north northeastern, and the southern rims of the main crater.
Selenographic Coordinates: 23.7°N | 47.4°W
Named after: Aristarchus of Samos, Greek astronomer
Depth: 3.7 km (2.3 miles)
Diameter: 40 km (24.9 miles)
Although the unaided human eye can generally not see objects on the Moon that are smaller than about 200 miles or so, the crater Aristarchus in the north-western part of the near side surface is an exception. This structure has an albedo of almost twice that of most lunar features, which makes it a readily visible naked-eye object, and the brightest reflectance object on the Moon.
Larger than the Grand Canyon, this crater is located in the Aristarchus Plateau, an area of the Moon that contains evidence of past volcanic activity, as well as many recorded instances of transient lunar phenomena, such as seemingly moving lights and other odd glows and/or luminescence. The Lunar Prospector spacecraft recently detected emissions of radon gas in this area, which no doubt explains at least some of the glows and/or strange luminescence in this area.
Object type: Megadome
Selenographic Coordinates: 16.61°N | 35.56°E
Diameter: 60.9 km (37.8 miles)
Height: 975 m (3 198 ft)
A megadome plateau is a flat, elevated dome-shaped plateau on the lunar crust, and the Gardner Megadome is largest that can be seen on the front side of the Moon. While the origin and evolution of most features on the Moon such as craters, collapse pits, crater rays, sinuous rilles, tectonic rilles, and volcanic domes are reasonably well understood today, the Gardner Megadome is the exception that proves the rule.
Most investigators interpret this structure as a vast volcanic dome; however, this interpretation is by no means certain, although the 90-degree section of what appears to be the remains of a volcanic caldera near the centre of the structure seems to argue in favour of the volcanic dome model. Nonetheless, until it is proven that this detail is indeed the remains of a caldera and not that of an impact crater rim, it is equally likely that the entire structure was raised during the formation of the Tranquillitatis and Serenitatis basins, or even that the area was lifted by a lava plume. The latter possibility is entirely feasible given that the entire surrounding area is a vast, volcanic area.
Selenographic Coordinates: 35°N | 17°W
Diameter: 1,146 km (712 miles)
Depth: 12 km (7.45 miles)
The Imbrium Basin formed around 3.85 billion years ago, and is the biggest impact basin on the nearside of the Moon, although the Pole-Aitken Basin is our natural satellite’s largest basin at 1,550 miles (2,500 km) wide and 8.1 miles (13 kms) deep, but is located on the far side (dark side) of the Moon, and therefore not visible to casual observers.
While there is some uncertainty about the size and mass of the object that created the Mare Imbrium, it was certainly massive enough to give the “Man in the Moon” his right eye. The impact was also severe enough to have raised an area of shattered surface material at a point directly opposite the Mare Imbrium, when the seismic waves set up by the impact broke through the other side of the Moon.
The Mare Imbrium has grooves that run through the mountains that form the northern rim of the vast lava lake, with the furrows that were cut through the mountains caused by massive pieces of ejecta after the proto-planet sized impactor hit the surface of the Moon. Careful observation during conditions of favourable illumination will clearly show the deep, almost parallel furrows and gouges left by what has been described as “…a shotgun blast across the surface of the Moon when the Mare Imbrium was created”.