Popular Astronomy January 1999
"Copernicus - Monarch of the Moon"
CCD Image: Mike Brown
900 million years ago, an asteroid the size of the Martian satellite Deimos careered into Earth's Moon on the southern shore of the Sea of Rains. The asteroid sliced many kilometres deep into the Moon's crust and the almighty explosion that ensued blasted out a giant crater, nearly a hundred kilometres wide, gouged out of solid rock within the space of a few minutes. And so the crater Copernicus was born.
Enormous streams of vaporised rock flew upwards in all directions away from the focus of the explosion, most of it arcing above the surrounding landscape in ballistic trajectories, to impact on the surface some distance from the new crater. Much of the material ejected from Copernicus was a fine spray of minute glassy particles that smothered the surrounding landscape and produced conspicuous long bright rays, but some large fragments of excavated crust formed their own small secondary impact craters in a mechanical (rather than explosive) fashion.
Having been so suddenly compressed by the impact, the lunar crust at the epicentre rapidly rebounded to thrust up great blocks of crust high above the new crater's floor, thus creating an impressive central mountain massif surrounded by superheated rock that formed about it vast pools of molten lava.
Copernicus was one of the last major impacts on the Moon. Its rayed rivals - Aristarchus in the Ocean of Storms and Tycho in the lunar southern uplands - were both formed in an identical manner only around 300 and 100 million years ago respectively, and both share the same characteristics as Copernicus. Though it was formed at such a relatively distant epoch, Copernicus arrived on the scene at least two billion years after large scale volcanic activity on the Moon had ceased, when the Moon had begun to mature after a troubled adolescence.
Copernicus is the Moon's most impressive impact crater. Its location can be identified with little difficulty with the naked eye when the Moon is near full, shining as a bright spot just below Mare Imbrium. On closer scrutiny with binoculars or a small telescope, the light rays of Copernicus can be seen to radiate across the Moon's surface to distances up to 800 kilometres. Magnified at higher powers through the eyepiece of an average size amateur telescope, Copernicus is breathtaking to behold when it has emerged from the morning terminator, or at the end of the lunar afternoon when near the evening terminator. According to Thomas Webb, a 19th century English astronomer who wrote the bestselling book "Celestial Objects for Common Telescopes":-
"Copernicus is one of the grandest craters...It has a central mountain, two of whose six heads are conspicuous; and a noble ring composed not only of terraces, but distinct heights separated by ravines; the summit [crater rim], a narrow ridge, not quite circular, rises [to] the height of Etna, after which Hevel [Johann Hewelcke] named it."
Indeed, Polish astronomer Johann Hewelcke had chosen to name the crater we know as Copernicus after fiery Mount Etna, designating the area around it Sicily, all of course surrounded by the calm expanse of a lunar Mediterranean Sea. He published three reasonably accurate maps of the Moon in his 1648 book "Selenographia" (Moon Charts), plus 40 smaller studies showing the features visible at various lunar phases. The names Hewelcke allocated for the lunar features were based largely on European geography, deliberately avoiding the naming of features after famous philosophers and scientists. Alas, Hewelcke's interesting nomenclature did not receive universal acceptance, but there remains on the Moon a few surviving Hevelian names, including the mountain ranges of the Alps, Apennines and the Caucasus Mountains.
So, too, was the scheme of lunar nomenclature proposed by Belgian Michel van Langren rejected. Van Langren, mathematician and cosmographer to the court of King Philip II of Spain, boldly proclaimed the names of the Spanish monarchy and aristocracy, with some biblical nomenclature thrown in for good measure. On van Langren's map the crater Copernicus is named Philip IV of Spain which resides snugly in "King Philip's Ocean". Van Langren was not a modest man - he chose for himself a prominent (132 km diameter) crater near the Moon's east limb - a feature which still bears his name.
It is the Italian Jesuit astronomer Giovanni Riccioli, a contemporary of Hewelcke and van Langren, to whom we are indebted for a great deal of the lunar nomenclature in use today. In his 1651 book "Almagestum Novum" (New Almagest) Riccioli presented two 28 cm diameter Moon maps that were redrawn after the work of the Italian astronomer Francesco Grimaldi. The lunar seas were named after moods (Seas of Tranquillity, Serenity) or terrestrial phenomena (Sea of Rains, Ocean or Storms) - a nomenclature influenced to a degree by ancient myths about the Moon's supposed supernatural effects on the Earth and its inhabitants. These names have survived, and so too Riccioli's naming of craters - more than 200 of them. Riccioli tended to name craters in the north after ancient scholars and classical philosophers, each surrounded by a clutch of that philosopher's pupils and advocates (Plato, Aristoteles). Southern craters were designated names after important people of the renaissance (Tycho, Maurolycus). The Jesuit Riccioli gives Copernicus a prominent place on the Moon - somewhat surprising, considering the effect the Copernican heliocentric theory had on Church dogma in the 16th century. However, Riccioli does not treat the heretic Galileo kindly, casting the great astronomer into an insignificant little crater in the middle of the Ocean of Storms!
Copernicus is surrounded by a reasonable amount of detail in the maps by Hewelcke and Grimaldi, and charts by Anton Schyrleus (de Rheita) published in 1645 and Francesco Fontana in 1646 also depict the extensive ray systems around Copernicus, in addition to ray systems around other craters. However, the more well known of Hewelcke's lunar maps tends to greatly exaggerate the lunar mountains and displays the ray system of Copernicus as a series or linear mountain ranges - something it quite obviously is not. One of the most realistic of the 17th century's Moon maps was drawn by Jean Cassini and engraved by Claude Mellan (who was 82 years old), and shows all the lunar features being illuminated from the west. The detail in and around Copernicus is truly stunning. As long before as 1636 Mellan had produced a particularly beautiful engraving of the last quarter Moon - so good it seems like a photograph.
By the 19th century Moon mapping had become an exacting pursuit, with the best maps having been compiled by the German astronomers Wilhelm Lohrmann (1824), Wilhelm Beer and Johann Mädler (Mappa Selenographica, 1834) and Julius Schmidt (1878). Schmidt's map shows all the detail of Copernicus as it can be discerned through a small telescope, with a good representation of the complicated terracing structure of the inner walls and the central peaks. The map, which drew heavily on Beer and Mädler's positional measurements, remains one of the best ever produced. Copernicus was closely scrutinised by the German selenographer Philip Fauth (1867-1942), who produced an intricate chart of the crater that holds up very well when compared with modern maps of the feature. In his book "The Moon in Modern Astronomy", Fauth shows his own chart of Copernicus and reproduces other drawings of the crater made by some of the well known astronomers of the era, including Edmund Neison (the one time BAA Lunar Section Director) with the probable intention of showing how inferior these other observers were at both observing and drawing! Interestingly, Fauth has been given a crater very close to Copernicus - a keyhole-shaped formation some 20 km in length that nestles in the Copernican ridges 50 km due south of Copernicus' rim.
In November 1966 the little US probe Lunar Orbiter 2 began orbiting the Moon. During its photographic survey the probe took a total of 184 photographs, including several showing Copernicus from a height of 20 km over the crater's southern ramparts. The close-up view of Copernicus, its blocky rim, terraces and central mountains was dramatic enough to lead some viewers into believing it was the "picture of the century". The Apollo 12 mission set down just 400 km south of Copernicus in November 1969, within sight of US robot softlander Surveyor 3 that had landed in April 1967. Samples of the material collected from around the landing site showed that the area was covered in piles of Copernican ejecta. NASA had hoped to include the smooth plains of the northern floor of Copernicus as a landing site for a later Apollo mission - a visually stunning landing that would have been, but alas, the Apollo Moon landing programme was cut short in December 1972.
What of the view of Copernicus to be obtained in amateur-sized telescopes? The crater's floor lies 3,760 metres beneath the crater's mountainous rim, and through a 150 mm reflector at high powers the floor's southern half can be seen to be hillier than the north. Through smaller telescopes this terrain cannot be adequately resolved, and instead the southern floor appears smooth and darker in tone than the north. The central floor is occupied by a complicated system of hills and mountains, the highest of which rises to 1,200 metres. Viewed from above, Copernicus' central peaks are somewhat comparable in size and configuration to the Virgin Islands east of Puerto Rico.
Complicated terracing structure adorns Copernicus' inner walls. When illuminated by an early morning lunar sun, so much detail on the inner west wall can be seen through the eyepiece that only a brave and confident observer would ever attempt to draw the whole lot during a single observing session. The crater is so complex that it has defeated my own attempts to adequately depict it on many occasions - now I have given up trying! When illuminated by an evening lunar sun, the inner east wall can be seen to be just as complicated. The 2.5 km craterlet Copernicus A sits snugly on the middle terrace due east of the central peaks; to its east, the rim displays a marked "kink" protruding towards the craterlet, probably the result of a landslide.
Copernicus' rim rises 900 metres above the surrounding terrain. The outer slopes are a network of concentric hills and furrows and radial ridges. The northwest and western slopes display a distinct boundary between the concentric features and the radial ridges, around 20 km from Copernicus' rim. This concentric topography was formed when the shock waves of the original impact were "frozen" into the rock and later modified by landslips, giving Copernicus the impression of having a substantial double wall. The radial ridges further out from the rim are composed of massive piles of ejected material.
This year, make a date to view the "Monarch of the Moon". Copernicus can be seen on or near the morning terminator on:- 26 Jan / 24 Feb / 26 Mar / 25 Apr / 24 May / 22 Jun / 22 Jul / 20 Aug / 19 Sep / 18 Oct / 17 Nov / 17 Dec. Copernicus can be followed as the Sun climbs higher during the course of the following two or three days, the shadows retreating to reveal detail on the craters walls and floor.
