Selenography is the study of the surface and physical features of theMoon (also known asgeography of the Moon, orselenodesy).[1] Likegeography andareography, selenography is a subdiscipline within the field ofplanetary science. Historically, the principal concern of selenographists was the mapping and naming of thelunar terrane identifyingmaria,craters, mountain ranges, and other various features. This task was largely finished when high resolution images of thenear andfar sides of the Moon were obtained by orbiting spacecraft during the early space era. Nevertheless, some regions of the Moon remain poorly imaged (especially near the poles) and the exact locations of many features (likecrater depths) are uncertain by several kilometers. Today, selenography is considered to be a subdiscipline ofselenology, which itself is most often referred to as simply "Lunar science."
The word" selenography" is derived from theGreek wordΣελήνη (Selene, meaning Moon) andγράφω (graphō, meaning to write).
The idea that the Moon is not perfectly smooth originates to at leastc. 450 BC, whenDemocritus asserted that the Moon's "lofty mountains and hollow valleys" were the cause of its markings.[2] However, it was not until the end of the 15th century AD that serious selenography begin. Around AD 1603,William Gilbert made the first lunar drawing based on naked-eye observation. Others soon followed, and when thetelescope was invented, initial drawings of poor accuracy were made, but soon thereafter improved in tandem withoptics. In the early 18th century, thelibrations of the Moon were measured, which revealed that more than half of the lunar surface was visible to observers on Earth. In 1750, Johann Meyer produced the first reliable set of lunarcoordinates that permitted astronomers to locate lunar features.[citation needed]
Lunar mapping became systematic in 1779 whenJohann Schröter began meticulous observation and measurement of lunartopography. In 1834Johann Heinrich von Mädler published the first large cartograph (map) of the Moon, comprising 4 sheets, and he subsequently publishedThe Universal Selenography.[3] All lunar measurement was based on direct observation until March 1840, whenJ.W. Draper, using a 5-inch reflector, produced adaguerreotype of the Moon and thus introduced photography toastronomy. At first, the images were of very poor quality, but as with thetelescope 200 years earlier, their quality rapidly improved. By 1890 lunar photography had become a recognized subdiscipline of astronomy.
The 20th century witnessed more advances in selenography. In 1959, theSoviet spacecraftLuna 3 transmitted the first photographs of thefar side of the Moon, giving the first view of it in history. TheUnited States launched theRanger spacecraft between 1961 and 1965 to photograph the lunar surface until the instant they impacted it, theLunar Orbiters between 1966 and 1967 to photograph the Moon from orbit, and theSurveyors between 1966 and 1968 to photograph and softly land on the lunar surface. The SovietLunokhods1 (1970) and2 (1973) traversed almost 50 km of the lunar surface, making detailed photographs of the lunar surface. TheClementine spacecraft obtained the first nearly global cartograph (map) of the lunartopography, and alsomultispectral images. Successive missions transmitted photographs of increasing resolution.
The Moon has been measured by the methods oflaser altimetry andstereo image analysis, including data obtained during several missions. The most visibletopographical feature is the giant far-sideSouth Pole-Aitken basin, which possesses the lowestelevations of the Moon. The highest elevations are found just to the northeast of this basin, and it has been suggested that this area might represent thickejecta deposits that were emplaced during an oblique South Pole-Aitken basin impact event. Other large impact basins, such as themariaImbrium,Serenitatis,Crisium,Smythii, andOrientale, also possess regionally low elevations and elevatedrims.
Another distinguishing feature of the Moon's shape is that the elevations are on average about 1.9 km higher on the far side than the near side. If it is assumed that thecrust is inisostatic equilibrium, and that the density of the crust is everywhere the same, then the higher elevations would be associated with a thicker crust. Using gravity, topography andseismic data, the crust is thought to be on average about50 ± 15 km thick, with the far-side crust being on average thicker than the near side by about 15 km.[4][obsolete source]
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The oldest known illustration of the Moon was found in apassage grave inKnowth,County Meath,Ireland. The tomb wascarbon dated to 3330–2790 BC.[5]Leonardo da Vinci made and annotated some sketches of the Moon in c. 1500.William Gilbert made a drawing of the Moon in which he denominated a dozen surface features in the late 16th century; it was published posthumously inDe Mondo Nostro Sublunari Philosophia Nova. After the invention of thetelescope,Thomas Harriot (1609),Galileo Galilei (1609), andChristoph Scheiner (1614) made drawings also.[6]
Denominations of the surface features of the Moon, based on telescopic observation, were made byMichael van Langren in 1645. Many of his denominations were distinctlyCatholic, denominating craters in honor of Catholicroyalty and capes and promontories in honor of Catholicsaints. The lunarmaria were denominated inLatin for terrestrial seas and oceans. Minor craters were denominated in honor of astronomers, mathematicians, and other famous scholars.
In 1647,Johannes Hevelius produced the rival workSelenographia, which was the first lunar atlas. Hevelius ignored the nomenclature of Van Langren and instead denominated the lunartopography according to terrestrial features, such that the names of lunar features corresponded to the toponyms of their geographical terrestrial counterparts, especially as the latter were denominated by the ancientRoman andGreek civilizations. This work of Hevelius influenced his contemporary European astronomers, and theSelenographia was the standard reference on selenography for over a century.
Giambattista Riccioli,SJ, aCatholic priest and scholar who lived in northernItaly authored the present scheme ofLatin lunar nomenclature. HisAlmagestum novum was published in 1651 as summary of then current astronomical thinking and recent developments. In particular he outlined the arguments in favor of and against various cosmological models, both heliocentric and geocentric.Almagestum Novum contained scientific reference matter based on contemporary knowledge, and contemporary educators across Europe widely used it. Although this handbook of astronomy has long since been superseded, its system of lunar nomenclature is used even today.
The lunar illustrations in theAlmagestum novum were drawn by a fellowJesuit educator namedFrancesco Grimaldi, SJ. The nomenclature was based on a subdivision of the visible lunar surface into octants that were numbered in Roman style from I to VIII. Octant I referenced the northwest section and subsequent octants proceeded clockwise in alignment with compass directions. Thus Octant VI was to the south and includedClavius andTycho Craters.
TheLatin nomenclature had two components: the first denominated the broad features ofterrae (lands) andmaria (seas) and the second denominated the craters. Riccioli authored lunartoponyms derived from the names of various conditions, including climactic ones, whose causes were historically attributed to the Moon. Thus there were the seas of crises ("Mare Crisium"), serenity ("Mare Serenitatis"), and fertility ("Mare Fecunditatis"). There were also the seas of rain ("Mare Imbrium"), clouds ("Mare Nubium"), and cold ("Mare Frigoris"). The topographical features between themaria were comparably denominated, but were opposite the toponyms of themaria. Thus there were the lands of sterility ("Terra Sterilitatis"), heat ("Terra Caloris"), and life ("Terra Vitae"). However, these names for the highland regions were supplanted on later cartographs (maps). SeeList of features on the Moon for a complete list.
Many of the craters were denominated topically pursuant to the octant in which they were located. Craters in Octants I, II, and III were primarily denominated based on names fromancient Greece, such asPlato,Atlas, andArchimedes. Toward the middle in Octants IV, V, and VI craters were denominated based on names from the ancientRoman Empire, such asJulius Caesar,Tacitus, andTaruntius. Toward the southern half of the lunar cartograph (map) craters were denominated in honor of scholars, writers, andphilosophers ofmedieval Europe and Arabic regions. The outer extremes of Octants V, VI, and VII, and all of Octant VIII were denominated in honor of contemporaries ofGiambattista Riccioli. Features of Octant VIII were also denominated in honor ofCopernicus,Kepler, andGalileo. These persons were "banished" to it far from the "ancients," as a gesture to theCatholic Church.[citation needed] Many craters around theMare Nectaris were denominated in honor ofCatholicsaints pursuant to the nomenclature of Van Langren. All of them were, however, connected in some mode withastronomy. Later cartographs (maps) removed the "St." from theirtoponyms.
The lunar nomenclature ofGiambattista Riccioli was widely used after the publication of hisAlmagestum Novum, and many of its toponyms are presently used. The system was scientifically inclusive and was considered eloquent and poetic in style, and therefore it appealed widely to his contemporaries. It was also readily extensible with new toponyms for additional features. Thus it replaced the nomenclature of Van Langren and Hevelius.
Later astronomers and lunar cartographers augmented the nomenclature with additionaltoponyms. The most notable among these contributors wasJohann H. Schröter, who published a very detailed cartograph (map) of the Moon in 1791 titled theSelenotopografisches Fragmenten. Schröter's adoption ofRiccioli's nomenclature perpetuated it as the universally standard lunar nomenclature. A vote of theInternational Astronomical Union (IAU) in 1935 established the lunar nomenclature ofRiccioli, which included 600 lunar toponyms, as universally official and doctrinal.
The IAU later expanded and updated the lunar nomenclature in the 1960s, but new toponyms were limited to toponyms honoring deceased scientists. AfterSoviet spacecraft photographed thefar side of the Moon, many of the newly discovered features were denominated in honor of Soviet scientists and engineers. TheIAU assigned all subsequent new lunar toponyms. Some craters weredenominated in honor of space explorers.
Johann H. Mädler authored the nomenclature for satellite craters. The subsidiary craters surrounding a major crater were identified by a letter. These subsidiary craters were usually smaller than the crater with which they were associated, with some exceptions. The craters could be assigned letters "A" through "Z," with "I" omitted. Because the great majority of thetoponyms of craters were masculine, the major craters were generically denominated "patronymic" craters.
The assignment of the letters to satellite craters was originally somewhat haphazard. Letters were typically assigned to craters in order of significance rather than location. Precedence depended on the angle of illumination from theSun at the time of the telescopic observation, which could change during the lunar day. In many cases the assignments were seemingly random. In a number of cases the satellite crater was located closer to a major crater with which it was not associated. To identify the patronymic crater, Mädler placed the identifying letter to the side of the midpoint of the feature that was closest to the associated major crater. This also had the advantage of permitting omission of thetoponyms of the major craters from the cartographs (maps) when their subsidiary features were labelled.
Over time, lunar observers assigned many of the satellite craters aneponym. TheInternational Astronomical Union (IAU) assumed authority to denominate lunar features in 1919. The commission for denominating these features formally adopted the convention of using capital Roman letters to identify craters and valleys.
When suitable maps of the far side of the Moon became available by 1966,Ewen Whitaker denominated satellite features based on the angle of their location relative to the major crater with which they were associated. A satellite crater located due north of the major crater was identified as "Z". The full 360° circle around the major crater was then subdivided evenly into 24 parts, like a 24-hour clock. Each "hour" angle, running clockwise, was assigned a letter, beginning with "A" at 1 o'clock. The letters "I" and "O" were omitted, resulting in only 24 letters. Thus a crater due south of its major crater was identified as "M".
The Moon obviously lacks anymean sea level to be used asvertical datum. TheUSGS'sLunar Orbiter Laser Altimeter (LOLA), an instrument on NASA'sLunar Reconnaissance Orbiter (LRO), employs adigital elevation model (DEM) that uses the nominallunar radius of 1,737.4 km (1,079.6 mi).[7]Theselenoid (thegeoid for the Moon) has been measuredgravimetrically by theGRAIL twin satellites.[8]
The following historically notable lunar maps and atlases are arranged in chronological order by publication date.
democritus moon valleys and mountains.
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