Mare Orientale 19.0°S, 93.0°W

Mare Orientale (‘Eastern Sea’) is the lava-flooded, nearly circular portion of a very large impact basin. Whereas the diameter of Mare Orientale only amounts to 320 km, the basin measures about 930 km in diameter. The mare is surrounded by two ramparts, which enclose the lava surface with concentric rings, and which were created by seismic shock waves from the impact. These circular mountain ranges are known as Montes Rook (the Rook Mountains, named after Lawrence Rook, an English astronomer and observer of Jupiter's moons (1622–1666) and Montes Cordillera, where Montes Cordillera form the outer rampart.

Only the eastern outer regions of Mare Orientale are visible – at favourable libration angles – including:

the eastern edge of Montes Cordillera (20.0°S, 80.0°W),

the eastern edge of Montes Rook (20.0°S, 83.0°W),

Lacus Autumni (‘Lake of Autumn’, 14.0°S, 82.0°W), and Lacus Veris (‘Lake of Spring’, 13.0°S, 87.0°W).

Lacus Autumni is a narrow lava area, oriented north-south, about 250 km long at the inner edge of Montes Cordillera. Lacus Veris consists of two neighbouring, similarly narrow, lava areas with a total length of about 550 km at the inner edge of Montes Rook.

The ejecta from the Orientale impact may be traced out to distances of 1200 km, for example on the floor of the crater Schickard.

Serpentine Ridge

The eastern side of the mare surface is crossed by a series of low mare wrinkle ridges. Unofficially these are known as the Serpentine Ridge. The wrinkle ridges consist of two main segments, which are officially known as Dorsa Lister and Dorsa Smirnov. Both are about 10 km wide and extend, more-or-less following the trend of the edge of the mare, over considerable distances: Dorsa Lister for around 300 km and Dorsa Smirnov for about 130 km.

South of Dorsa Lister lies the crater Plinius with a diameter of 43 km. In between, oriented east—west is the Rimae Plinius rille system, with a length of 124 km. The small crater Very, with a diameter of just 5 km lies almost in the centre, directly on top of the ridge of Dorsa Smirnov.

Posidonius Gamma is the designation, no longer used, for a small peak on the Dorsa Smirnov wrinkle ridge. The mountain has a summit crater, which is surrounded by a small halo of bright ejecta. The rilles (Rimae Littrow, Rimae Chacornac) on the eastern edge of Mare Serenitatis are fracture zones, created by a subsidence of the Serenitatis Basin's central lava area.

In the western portion there are Dorsum Buckland, Dorsum Azara and a few others. All these mare wrinkle ridges in Mare Serenitatis branch and partially intersect one another. They are, in general, no more than about 200 m high, and are therefore only visible at very low solar elevations, when near the terminator.

Cassini 40.2°N, 4.6°E

Cassini is a very conspicuous crater, 56 km in diameter, lying at the southern end of the Alps. It is a prominent landmark in this area of the Moon. The outer crater wall appears rounded and smooth because of deposits of melted ejected material – as with Archimedes and Aristoteles.

On high-resolution images the ejecta, which lies in a ring around the outer crater rim, appears remarkably smooth and wavy. It gives that impression that the impact onto the lunar surface took place into semi-fluid material with a consistency like that of mud. Considered morphologically, the crater and its immediate surroundings resembles those craters on Mars that are classified as ‘rampart craters’. There, however, sub-surface ice plays a part, which is liquified by the impact and turns the soil surrounding the impact into a muddy consistency.

The floor of Cassini is slightly rounded and appears extremely smooth. A chain of hills stretches from the inner crater wall to the crater Cassini A (15 km). It is crossed by a narrow, valley-like structure. Cassini B (9 km) also lies inside the crater, and between them lie two mountain peaks. A craterlet of about 2 km in diameter lies directly on the northern crater wall.

Cassini M (8 km) lies immediately next to the outer rampart on the northwest. From there a valley-like trough runs more than 20 km northwards. An interrupted, very narrow, nameless rille lies between Cassini and the crater Calippus. Large telescopes using webcam techniques have revealed half-a-dozen additional crater pits on the floor of Cassini.

Sinus Asperitatis 6.0°S, 25.0°E

Sinus Asperitatis, ‘Bay of Asperity’, is a lava-covered surface about 180 km across. It lies north and east of the crater Theophilus and in the north passes into Mare Tranquillitatis. It has an appropriate name, because the surface of the lava is rough, furrowed, and uneven.

Torricelli 4.6°S, 28.5°E

Torricelli is a circular crater of about 22 km in diameter. The western crater wall is broken and overlapped by a smaller elliptical crater. Together both craters appear pear-shaped. Torricelli lies within a nameless, significantly larger ghost crater, the remnants of the crater wall of which may be seen only in the southeastern direction. The rest has been submerged by the Sinus Asperitatis lava. The whole region has been covered by ejecta from the Theophilus impact and is correspondingly craggy.

Censorinus 0.4°S, 32.7°E

A small, funnel-shaped crater with a diameter of just 3 km, lying on the southern border of Mare Tranquillitatis. It is surrounded by a halo of very bright ejecta. At high solar elevations it appears as one of the brightest objects on the Moon's surface.

Capella 7.5°N, 35.0°E

A crater, 49 km in diameter, lying on the northern edge of Mare Nectaris. Its central peak is extremely large and wide relative to the crater's diameter, and has a depression at its summit. The central peak was probably altered by post-volcanic events. Vallis Capella, a valley about 110 km long, crosses the crater, running in a southeasterly direction and has a slightly tapering appearance.

Cleomedes 27.7°N, 56.0°E

Cleomedes is a very conspicuous crater with a diameter of 126 km. There are small craters and a central peak visible on the floor. Across the northern portion runs Rima Cleomedes, a 30 km-long rille that is difficult to observe. (A large telescope, the correct lighting and a favourable libration are required.) The northern wall of the crater has eroded and has been breached by the craters Trailes (43 km, 28.4°N, 52.8°E), Cleomedes A (14 km) and Cleomedes E (20 km).

Geminus 34.5°N, 56.7°E

Messala 39.2°N, 60.5°E

Burckhardt 31.1°N, 56.5°E

North of Cleomedes lies Burckhardt, a 56 km-diameter crater. Burckhardt is superimposed on two smaller craters, Burckhardt E (39 km) and Burckhardt F (43 km). Normally, large craters are superimposed by smaller ones, but with Burckhardt the direct opposite is the case.

Even farther north lie the two craters Geminus and Messala. Geminus with a diameter of 85 km is another striking crater in this area of the Moon. The crater has a central peak and the southern wall is breached by a wide valley. Because of its size of 125 km, Messala would once have been classed as a ‘walled plain’. The floor is relatively smooth, but the view in large telescopes reveal some irregularities. Geminus C (16 km) and Messala G (29 km) are smaller craters that lie close to conspicuous rays of impact material, which are visible under high angles of illumination. The origin of the ray system cannot, however, be determined.

Gassendi 17.6°S, 40.1°W

The crater Gassendi is a prominent example of a crater that was altered after the impact, over a long period of time by post-impact volcanism and lava flows. It lies on the northern edge of Mare Humorum and has a diameter of 101 km and a depth of about 1.9 km. Dating the formation of the basin from measurements made by the European Moon probe Smart-1 gives a value of about 3.6 billion years. When the lava flows flooded the Humorum impact basin, large parts of the crater floor were also filled with lava. The crater wall and a few peaks of the central mountains are still visible. The whole floor of the crater is crossed by Rimae Gassendi, a complicated system of rilles, fissures and grabens. Located on the floor are the craterlets Gassendi P (2 km, in the northwest) and Gassendi M (3 km, in the southeast). The northern crater wall is breached by Gassendi A (33 km). The western wall shows a triangular landslip (similar to the one in the crater Plato).

The Helmet 16.7°S, 31.5°W

The Helmet is a low rise in the lunar crust with a diameter of about 60 km and a very low height of less than 200 metres. The structure is classified as a volcanic megadome. The surface is covered in rocky ridges and exhibits a few small craters. On colour-coded multispectral images of the region the structure stands out strinkingly like a ‘beacon’ from the surrounding mare lava through its brilliant colours.

Schickard 44.3°S, 55.3°W

Schickard is a very large, old crater, 206 km in diameter, with a relatively smooth and level floor. Under certain lighting conditions the crater's floor appears convex, bulging upwards slightly. Following its formation, the crater was re-structured by the Mare Orientale impact, almost 1200 km away. Bright ejecta from the Orientale impact may be observed on the crater's floor and the western wall exhibits radial, linear furrows and graben, oriented to the centre of Mare Orientale. Long after the Orientale impact, the northern and southern portion of the crater's floor were flooded with dark lava.

Wargentin 49.6°S, 60.2°W

Wargentin is one of the rare plateau craters, and is filled to the rim with dark lava. The crater's floor lies about 400 m above the surrounding terrain and is crossed by a mare wrinkle ridge. The network of the range of hills possibly marks the location of the former central mountains and ridges. A craterlet, surrounded by a dark halo, lies on the southern portion of the crater's floor. Because of its location near the limb, Wargentin is best observed at favourable libration angles.

Phocylides 52.7°S, 57.0°W

A large, lava-filled old crater, 120 km in diameter, and with 2.1 km high crater walls. The eastern wall is completely eroded. The crater Phocylides C, 46 km across, directly adjoins it on the northwest.

Grimaldi 5.5°S, 68.3°W

Grimaldi, because of its size, must be counted as an impact basin (similar to Mare Humboltianum), whose lava-flooded central portion is bounded by an inner wall that is about 200 km in diameter. The outer wall, which is only partially remaining, has a diameter of over 400 km. Grimaldi may thus be classed as a multi-ring basin.

Running parallel to the southeastern portion of the inner crater wall are Rimae Grimaldi, with an overall length of 230 km (with a maximum width of 2 km). They are narrow linear rilles, probably produced by the tension forces when the lava flows contracted as they cooled. In the northern portion of the crater's floor there is a large volcanic dome with a summit crater. A few additional domes may be observed with larger telescopes under favourable lighting and seeing conditions. Grimaldi is associated with a mascon, a gravitational anomaly. Because Grimaldi is near the western limb of the Moon, observation of the Grimaldi Basin and its rille system requires favourable libration angles.

Riccioli 3.3°S, 74.6°W

Riccioli is a large complex crater, 145 km in diameter. The Rimae Riccioli lie in the southern portion of the crater's floor, and stretch for a total length of over 400 km. They are linear rilles with a maximum width of 1.5 km. The northern portion of the crater floor is lava-flooded.

Picard 14.6°N, 54.7°E

Peirce 18.3°N, 53.5°E

Picard and Pierce, with diameters of 22 km and 18 km, respectively, are the only notable craters on the floor of Mare Crisium. Both exhibit sharply defined crater walls and are certainly relatively young when compared with the craters in the southern highlands. Picard is one of the few double-walled craters, that may be observed with small telescopes.

O'Neill's Bridge 15.2°N, 49.2°E

O'Neill's Bridge is not an official lunar designation, but a sort of nickname, first introduced by the amateur John O'Neill in 1953. The supposed bridge is visible shortly after Full Moon, when the terminator reaches the western edge of Mare Crisium. When seeing conditions are relatively poor (or the telescope's aperture is too small), the object resembles a bridge, joining the tips of the capes Promontorium Lavinium and Promontorium Olivium. (Neither designation is used officially nowadays.) If the seeing conditions are good and the instrument large enough, two small, eroded crater pits are revealed at the site. O'Neill's Bridge lies directly southeast of the crater Proclus.

Lick 12.4°N, 54.7°E

Yerkes 4.6°N, 51.7°E

Two, structurally almost identical, lava-flooded craters lie southeast and east of O'Neill's Bridge. Lick has a diameter of 31 km, and Yerkes is somewhat larger at 36 km. North of it lies Yerkes E (10 km), which is linked to Yerkes by a ridge.

Plato 51.6°N, 9.4°W

Plato is a large crater, 109 km in diameter, with an average depth of only 1 km. A few peaks on the eastern and western crater rim reach a height of 2 km. The crater is visible even with binoculars and is more-or-less divided in two at Half-Moon phases: half illuminated, and the other half lying in the darkness of the lunar night. Plato is sometimes referred to as ‘the big black lake’. In fact, the crater floor of Plato is one of the darkest areas on the nearside of the Moon. Plato is nearly circular, but because of limbforeshortening appears elliptical to an observer on Earth. Two large, triangular landslips are visible in the western edge of the crater. Similar wall or mountain landslides are also found in the craters Aristoteles and Gassendi.

Plato is extensively lava-flooded, and even the peaks of the central mountains – which must have been present in a crater of this size – lie below the lava surface. A normal crater of this size should be between 3 and 4 km deep. The upper surface of the lava appears completely flat, level, and without any features. Larger telescopes and good seeing conditions, however, enable five crater pits (about 2 km across) to be seen. Large telescopes with apertures of 300 mm and above and the use of webcam techniques show as many as 15 such pits.

Sirsalis 12.5°S, 60.4°W

Rimae Sirsalis 15.7°S, 61.7°W

Sirsalis is a crater 42 km across, with a central peak. It overlaps the crater Sirsalis A, 49 km across, which was formerly known as Bertaud. Sirsalis J, directly adjoining Sirsalis to the southeast, is a small crater, 12 km in diameter and the centre of a small ray system. The 3-km wide rille Rima Sirsalis run north-south on the eastern side of Sirsalis, and which is easily seen even in small telescopes. Rima Sirsalis is a fracture zone, possibly caused by the Orientale impact. To the south, Rima Sirsalis merges directly into the rille system Rimae Darwin. The widest of the Sirsalis rilles has an overall length of more than 300 km.

A narrow ridge begins at the northern wall of Sirsalis and forms the eastern wall of the crater Sirsalis Z (91 km), which casts a bizzare triangular shadow around the 12th day after New Moon. The floor of Sirsalis Z is smooth and level, and a distant portion of Rimae Grimaldi ends inside the northeastern crater wall.

Sirsalis E (8.0°S, 56.0°W) is a crater, 80 km across, that is almost completely flooded by the Oceanus-Procellarum lavas. Only the western wall and a small section of the eastern crater wall are visible.

Rima Hyginus 7.4°N, 7.8°E

Rima Hyginus is an extraordinary feature and, together with Rimae Triesnecker, is the best-known of all rilles. It consists of two linear sections, each of about 11 km in length, and is interrupted in the centre by the crater Hyginus (11 km, 7.8°N, 6.3°E). Hyginus is extended on the north by an unnamed small crater of about 4 km in diameter.

Both sections of the rille are about 400 m deep. The eastern section runs nearly parallel to Rima Ariadaeus, whereas the western section bends at the crater Hyginus by about 35° towards the north. This section is interrupted by nine craterlets, which lie directly in the rille. Rima Hyginus is probably an old lava channel and the craterlets may have been responsible for repeated eruptions of volcanic ash. This suspicion is strengthened by the fact that there are many nearby areas that are coated with dark volcanic ash (they are DMD regions). The crater Hyginus itself also appears to be of volcanic origin, because there is no form of crater wall and the floor is absolutely flat and level – it appears as if it were a cylinder punched out of the lunar surface.

About 35 km north of the crater Hyginus lies a small mountain, that appears spiral, rather like the letter ‘e’. The mountain has the former official name of ‘Schneckenberg’ (‘Snail Mountain’, at 9.0°N, 6.5°E).

Mare Nectaris

15.2°S, 35.5°E

Mare Nectaris (Sea of Nectar) is a relatively small and nearly circular lunar maria of about 350 km in diameter. It is the lava-flooded central portion of a large impact basin with an area of c. 100 000 km2. It arose in the Nectarian epoch (about 3.9 billion years ago), when the lunar surface was created, and is one of the oldest basins on the nearside of the Moon. Nectaris is a multi-ring basin identified by the ring-like arrangement of four concentric basin ramparts. Mare Nectaris is also the centre of a gravitational anomaly (a mascon).

Smaller telescopes show very few craters on the lava surface of Mare Nectaris (the largest of them is Rosse). Larger telescopes, however, show a wealth of craterlets and pits. One interesting surface feature, northeast of the crater Rosse, is a bright ray of ejecta from the Tycho impact. Large telescope reveal that, within the ray, there are many tiny secondary craters (in a so-called crater cluster).

Daguerre

11.9°S, 33.6°E

A ghost crater, 46 km in diameter, in the northern area of Mare Nectaris. A group of crater pits lies almost exactly in the centre of the crater's floor. Its southern half is crossed by the long ray of ejecta from Mädler. Directly adjoining it to the northwest lie a second, nameless ghost crater, that is very similar to Daguerre in structure.

Seleucus 21.0°N, 66.6°W

Seleucus is a prominent crater, east of Eddington, with a diameter of 43 km. It exhibits terraced inner walls and a small central peak. The eastern wall is covered in bright streaks, consisting of ejecta that originated in the distant crater Glushko (formerly known as Olbers A, 43 km).

Briggs 26.5°N, 69.1°W

A crater, 37 km in diameter, with a mountain ridge on its floor.

Eddington 21.3°N, 72.2°W

Struve 22.4°N, 77.1°W

Russell 26.5°N, 75.4°W

Eddington, Struve and Russell are impressive examples of large, lava-flooded craters with completely smooth floors. They lie close to the northwestern limb of the Moon, and are best observed under favourable libration angles. The crater diameters are 118, 164 and 103 km respectively. Struve and Russell merge into one another, and Eddington lies to the east of Struve. The 80° lunar meridian runs along the western wall of Struve.

Schiaparelli 23.4°N, 58.8°W

Schiaparelli is a conspicuous crater, 24 km in diameter, named after the Italian astronomer Schiaparelli, who initiated the long debate over martian canals. Schiaparelli lies at the end (or the beginning) of the 200-km long mare wrinkle ridges known as Dorsa Burnet. A bright streak of ejected material ends southwest of Schiaparelli. To the east lies the prominent Aristarchus plateau with Vallis Schröteri.

Posidonius 31.8°N, 29.9°E

Posidonius is a spectacular FFC crater, with a diameter of 95 km, lying on the northeastern edge of Mare Serenitatis. The crater walls are about 2.3 km above the crater floor. Long after the impact, the floor was uplifted by post-impact volcanism, which created the Rimae Posidonius rille system.

One of the rilles runs directly north at the foot of the western crater wall. At the end, the rille seems to run out into a series of crater pits. Along the eastern inner wall of the crater, following its curvature, there lies a prominent ridge (possibly landslides from the inner wall). Near the centre of the crater's floor is the crater Posidonius A (c. 15 km). Posidonius exhibits many remnants of central peaks. Northeast of the crater Posidonius A, separated by a broad rille, lies a central mountain with two sharp peaks. When observing with larger telescopes, they may be used to estimate seeing conditions and the resolution of the optics. The two peaks have a separation of 1.9 km which, at the Moon's average distance, corresponds fairly exactly to 1 arcsecond.

Chacornac

29.8°N, 31.7°E

An almost completely destroyed crater, 51 km in diameter, directly adjoining Posidonius to the south. The crater rim has a slightly hexagonal shape. Rimae Chacornac, some small linear rilles on the crater's floor and south of the crater, are visible at high magnifications.

Endymion 53.9°N, 57.0°E

With a favourable libration, Endymion is a very conspicuous complex crater on the Moon's northern limb. It has a diameter of 123 km and is 2.8 km deep. The crater's floor is smooth and has been flooded with unusually dark lava; there have been landslides in several places along the walls. The crater was formed during the Nectarian period. Large apertures reveal a few small and minute craters. Three small craters that are almost the same size lie directly to the west of the inner crater wall. Under high solar illumination, a system of bright rays of ejecta, of unknown origin, becomes visible crossing the crater.

Mare Humboldtianum 56.8°N, 81.5°E

Mare Humboldtianum, ‘Humboldt Sea’, is a difficult object for terrestrial observers, because it lies right on the Moon's northeastern limb. Part of it stretches onto the lunar farside, and as a result, favourable observational conditions are very closely linked to the libration. Mare Humboldtianum is the central, lava-flooded area of an impact bas in with several outer rings (i.e., a multi-ring basin), with an overall diameter of 640 km. The lava-covered surface has a diameter of about 270 km. With a favourable libration, larger telescopes clearly show individual structures within the Mare, and under high angles of illumination parts of the lava flow are crossed by bright rays.

de la Rue 59.1°N, 52.3°E

A large, heavily destroyed walled-plain with a diameter of 136 km, and both large and small craters on the floor. The crater is named after Warren de la Rue, a British amateur astronomer (1815–1889), who, amongst other things, was a pioneer of lunar photography.