Moongazing: Beginner’s guide to exploring the Moon. Royal Greenwich Observatory

Surface Features

      Almost every feature on the surface of the Moon visible from the Earth has been given a name. These names were originally assigned on a somewhat informal basis by early telescopic observers to honour great philosophers and artists of antiquity. As larger telescopes have revealed smaller features, names have been assigned more formally, and the process of cataloguing features is now governed by the International Astronomical Union (IAU.) Craters are named for notable deceased scientists, mathematicians, artists, scholars and explorers. The lunar maria (seas) have older Latin names, which reflect abstract states of mind and weather phenomena considered important to mariners. Lunar montes (mountain ranges) are typically named after terrestrial mountain ranges or nearby craters. Other features, with few exceptions, are named after nearby craters, maria or montes.

      Features on the Moon fall into one of several categories. Here each type of feature is described with a typical example shown as viewed through a large telescope.

      Craters – Fairly circular depressions usually formed from impacts. Occasionally, chains of craters are grouped together and collectively termed catena. Craters feature sloped walls and, on many occasions, central peaks, left over from the crater formation, at which point the lunar surface was locally molten by the energy of the impactor.

      Mare (plural: Maria) – Latin for ‘sea’. Large basins of solidified, ancient lava. The maria appear dark relative to the other terrain features. There is one large lunar ‘ocean’ in the Moon’s western hemisphere known as Oceanus Procellarum (Ocean of Storms).

      Mons – An individual mountain on the Moon. Lunar mountains were formed by a variety of processes, and vary greatly in size and height. The tallest are approaching 5 km in height, comparable to Vinson Massif, the highest peak in Antarctica.

      Montes – Large chains of mountain ranges formed by gigantic asteroid impacts billions of years ago. As with terrestrial ranges, prominent individual peaks often have their own names.

      Vallis (Plural: Valles) – A valley or system of valleys formed by lava flows and collapsed lava tubes. These features snake across the surface, often near craters connected to volcanism.

      Dorsum (Plural: Dorsa) – Derived from the Latin for the ‘back’, and connected to ridges or fins on the back of an animal, dorsa are subtle features resembling wrinkles in lunar maria. They are hard to see unless illuminated at a low angle, causing them to cast shadows.

      Rima (Plural: Rimae) – Latin for ‘fissure’. Rimae, sometimes called Rilles (German for ‘grooves’) are fissures or cracks in the lunar surface. Not to be confused with valleys, they are often jagged in appearance, with straight sections permeated by kinks. They are seismological features in the Moon’s crust, and are sometimes found in crater floors.

      Rupis (Plural: Rupes) – Latin for ‘rock’, rupes are escarpments in the lunar surface. They appear as large rifts, where a pronounced change in elevation can be seen. In fact, most rupes are very gentle slopes that are many kilometres wide.

      Lacus – Derived from the Latin for an opening, a lacus is a lake, which as its name suggests is a very small lunar mare. These features appear as dark, often patchy regions of dark, smooth plains.

      Sinus – Derived from the Latin for a gulf, a sinus on the Moon is a bay formed by a rugged ‘coastline’ of lunar highlands meeting a low elevation, smooth plain such as mare.

      Palus (Plural: Paludes) – Though its original Latin name is closer in meaning to a pool, paludes are generally translated as marches. They are low lying, but relatively rugged regions. Whilst they are not as dark as mare, they do have a relatively low albedo when compared with other types of rugged terrain.

       Images of the Apollo landing sites captured by NASA’s Lunar Reconnaissance Orbiter from low altitude. The remains of the Lunar Modules and various scientific instrument packages can be seen, as well as tracks left by the astronauts and rovers.

       North-East

      Due to atmospheric seeing, we are typically limited to observing features no smaller than one arcsecond in apparent size. On the Moon, this corresponds to just over a mile at best. This means that very small objects, such as those left on the Moon by the Apollo astronauts, cannot be resolved through the Earth’s atmosphere, but they have been imaged using the Lunar Reconnaissance Orbiter, which was only a few kilometres above the lunar surface.

      It’s useful to memorise prominent features on the Moon and consider its near side being divided into four quadrants as shown here. A good knowledge of the locations of the largest lunar maria will help you when choosing areas to explore during your moongazing sessions.

      Readers in the southern hemisphere should note that due to the convention of the coordinates on the Moon, it appears upside down in the sky compared with the charts in this guide. From Sydney, the Moon’s North Pole appears to be at the bottom. East and west are also reversed. From the northern hemisphere, the Moon’s eastern limb is on the right; from the southern hemisphere, it is on the left.

       South-East

       North-East

      The north-eastern quadrant of the Moon is dominated by the dark Mare Serenitatis (Sea of Serenity), landing site of Apollo 17, and Mare Tranquillitatis (Sea of Tranquility) where Neil Armstrong and Buzz Aldrin became the first humans to step foot on the lunar surface during the Apollo 11 mission. Mare Crisium (Sea of Crises) lies near the eastern limb, and south of it Mare Fecunditatis (Sea of Fertility).

       South-East

      The south-eastern quadrant of the Moon is remarkable for having no maria, save for the southern region of Mare Nectaris (Sea of Nectar). It’s mostly composed of rugged highlands, pockmarked with high energy impacts ranging right across the history of the Moon. The prominent Tycho Crater is found in this region, with its bright rays of ejecta visible over 1,600 km (1,000 miles) from the impact site. Astronauts on the Apollo 16 mission collected material from these rays to be analysed on Earth.

       South-West

       South-West