In 1865, astronomical photography pioneer Lewis M. Rutherfurd captured some of the highest quality photos of the Moon of the time. Credit: Wikimedia Commons
In 1874, James Nasmyth and James Carpenter published an extraordinary book: The Moon: considered as a planet, a world and a satellite. Richly illustrated with spectacular images of lunar features based on their observations and exquisite plaster models, it served as a platform for the authors to advance their views on the nature and evolution of the lunar surface.
In a book review from the same year, British astronomer Sir Joseph Norman Lockyer wrote: “The illustrations in this book are so admirable, so far beyond those which one generally obtains of a celestial phenomenon, that one is tempted to refer to them first. No scholar of science has ever presented to his readers more truthful or surprising representations of natural objects than those presented here; and I may add that rarely, if ever, has the same care been taken to ensure such veracity.
New models
Carpenter was a British astronomer at the Royal Observatory in Greenwich, England. Nasmyth was a prominent Scottish engineer and inventor, as well as an amateur astronomer, philosopher and artist. Among other things, he invented the steam hammer and the Nasmyth-Cassegrain telescope design. Both men were later honored with the naming of lunar craters.
But what made their book so extraordinary? One of the major scientific questions of the time centered on the nature and origin of lunar craters. The authors were strong supporters of the widely held theory that the craters formed through volcanism, which drew parallels with the Campanian volcanic arc that includes Vesuvius in the Gulf of Naples. This theory was introduced in 1665 by Robert Hooke Micrograph as one of two possible ways lunar craters might have formed. The other was the impact theory of crater formation, which was not formalized until 1892 by the Grove geologist Karl Gilbert – and was not universally accepted until well into the 20th century. It finally gained real popularity in 1960, when Eugene Shoemaker demonstrated that, in stark contrast to the surrounding volcanic terrain in northern Arizona, the famous Meteor Crater (or Barringer Crater) was created by a large extraterrestrial impactor.
Working together, Nasmyth and Carpenter devised an ingenious mechanism for the supposed formation of craters through volcanism, called the fountain model. It explained how many of the most prominent and relatively young lunar craters, such as Tycho and Copernicus, had formed their protruding walls and central peaks. Theoretically it could include craters of almost any size, including those filled with lava.
They proposed that as the Moon gradually cooled and shrank after its formation, its outer shell ruptured and the molten lava underneath was ejected through the surface, much like volcanoes on Earth. Furthermore, since the Moon has no atmosphere and no oceans, it must have been a global phenomenon that created the extensively cratered surface we see today. In the authors’ words: “When the molten substrate broke its boundaries, expelled superfluous matter and produced the resulting volcanic features, after final solidification, it would resume the normal contraction process upon cooling.” Eventually “the skin, the outer layer of solid matter so to speak, wrinkles, forming alternating ridges and depressions or wrinkles.”
Recreating the Moon
In addition to promoting their theories of the formation of the lunar surface, Nasmyth and Carpenter were also eager to produce lunar charts that were more accurate than those drawn by hand up to that point, as well as to provide a level of three-dimensional realism that existing charts lacked. Photography, while useful, was still in its infancy and incapable of resolving the finer surface details easily visible through moderately sized telescopes.
As a result, Nasmyth embarked on an ambitious but captivating method of producing vastly superior representations of selected regions of the Moon. Drawing on his vast collection of exquisite telescope drawings, he created plaster models and lit them obliquely to match the sunrise and sunset shadow angles corresponding to his observations. These were then photographed and reproduced as woodcuts for the book. The results, in Lockyer’s words, “were perfect; much more perfect than any enlargement of photographs could have been.
Although they are exquisite works of art, Nasmyth’s images are not perfect in one fundamental respect: their vertical elevation is significantly overstated. In their book Epic moonWilliam Sheehan and Thomas Dobbins note that because Nasmyth used his models and not actual measurements of shadow angles to quantify elevations, they portray lunar features as taller and more jagged than they do.
But this error does not detract from the realistic appearance and location accuracy of its models. To demonstrate this, we directly compared some of them with telescopic digital images of the same regions obtained under comparable illumination angles. All digital images were taken with a Celestron 14-inch (C14) telescope (unless otherwise indicated) and various cameras, and processed by Leo Aerts.
Tell a story
One of the most impressive images in the book is the Theophilus, Cyrillus and Catharina group of craters. Their age differences are made evident by the progressive erosion of their respective walls and central peaks.
Theophilus, the northernmost crater, features terraced walls, a flat floor and a high central peak, making it the youngest of the trio. To the south-west is Cyril of similar appearance, although less well preserved, only slightly older than Theophilus. Nasmyth and Carpenter found in these two craters the perfect examples for their fountain model. In contrast, the largest and flattest crater of the trio, Catharina (southwest of Cyrillus) did not support their idea of crater formation. It is now clear that Catharina has been modified by several successive impacts, most notably its ancient rim destroyed, making it the oldest of the three.
Another trio of craters, from youngest to oldest, are Arzachel, Alphonsus and Ptolemaeus. Nasmyth and Carpenter saw this clustering as a nearly clear example of the progression of crater formation in their fountain model. The prominent circular wall and central peak of Arzachel indicated the initial phase of post-volcanic activity, while Alfonso was in the intermediate phase and Ptolemy was in the final phase, with the lower walls and a central peak completely engulfed by a smooth layer of lava. However, the latter represented a real challenge for them.
Because it lacked a central volcanic peak, or cone as they called it, Ptolemy, at 95 miles (154 kilometers) in size, did not easily fit into the theoretical size range of the fountain model. Like the Petavius crater, with a true size of 110 miles (177 km) in diameter (at the time of the book’s publication, the crater was measured at 78 miles [125 km] wide), Ptolemy had “no central cone” and was, “therefore, not manifestly volcanic like those possessing this characteristic”.
Nasmyth and Carpenter concluded that the only feasible method to explain why enigmatic basins like Mare Crisium had such extensive smooth floors was a spherical lifting force beneath the lunar surface: “[W]We see that an intense but extremely confined explosion… beneath the lunar crust must disturb a circular area of its surface, if the intervening material is homogeneous.
Curiously, despite being such strong advocates of large-scale volcanism on the lunar surface, the authors did not consider massive lava flooding of basins like Ptolemy’s as the most likely mechanism for its appearance. Ample evidence of past lava floods can be seen in their models of the Archimedes and Aristillus regions, along with the Plato-Montes Alpes complex. Both regions fall near Mare Imbrium, which formed from a collision with a large sim object about 3.9 billion years ago and was subsequently inundated by basaltic lava. This formed the smooth volcanic surface evident today, which still displays protruding mountain peaks and long wrinkled ridges.
Advanced for its time
Nasmyth and Carpenter were way off the mark with their volcanic theory of crater formation. Yet they showed remarkable foresight in proposing that the Moon’s surface contracted as it gradually cooled and shrank after its formation.
Based on earthquake activity recorded by surface seismometers left behind by the Apollo astronauts and more recent data from the Lunar Reconnaissance Orbiter, NASA scientists concluded that while the exterior of the Moon is now cold and solid, the interior it is still cooling and contracting. This internal contraction has the overall effect of forcing the total surface area to decrease and the crust to wither.
In retrospect, it is easy to understand why The Moon: Considered as a Planet, a World, and a Satellite attracted so much attention at the time. It was not universally accepted; Sheehan and Dobbins note in Epic Moon: “Although the volcanic theory has flourished in the years since Nasmyth and Carpenter published the fountain theory, it has not gone entirely unchallenged.” Scientists eventually hypothesized that the moons and planets formed through a process of accretion, which eventually exposed our airless Moon to bombardment and left it as we see it today, pockmarked with countless craters and basins of solidified lava.
However, although their theories were ultimately inaccurate, Nasmyth and Carpenter were among the first to propose credible mechanisms to explain the Moon’s chaotic-looking topography – and they did so with innovative and unique illustrations.
