Fun fact: 1990 is closer to the moon landing than it is to the present day. Now, as you gather your jaw from the floor, you might be even more surprised to learn that Neil Armstrong, Buzz Aldrin and Michael Collins knew exactly where they were going back in July of 1969. How? Because they had an atlas, of course!
How? David M. Harland, author of our popular guide Moon: From 4.5 billion years ago to the present, explains as he recounts how NASA scientists made a perfect map of the moon some 10 years before man’s small step using a technique called stratigraphic mapping…
In 1957 the National Academy of Sciences awarded Gerard Kuiper funding to start work on a new lunar atlas. Additional money was provided by the Air Force. The resulting Photographic Lunar Atlas was issued in 1960, with pictures printed on a scale at which the lunar disc would be 2.5m across.
Naturally the USGS wanted to map the Moon geologically. The key first step was to identify the various distinct geological units in terms of their textures, to delineate their outlines on a base map, and to use the principle of superposition to identify the sequence of their deposition. This was an example of stratigraphic analysis with the objective of deriving insight into the history of the surface.
The pioneering work on this was undertaken by Robert Hackman of the USGS Photogeology Branch in Washington DC, who demonstrated in 1960 that stratigraphic analysis could indeed be applied to the Moon.
When issued in 1961, Hackman’s map of what he identified as the ‘pre-maria’, ‘maria’, and ‘post-maria’ surface units marked a significant departure from the manner in which astronomers made their maps. The superposition relationships suggested to Hackman the maria were volcanic flows rather than splashes of impact melt.
Hackman drew particular attention to a patch of light-toned material between the crater Archimedes and the arcuate Apennine Mountains which partially enclose the Sea of Rains. The sequence was clear. The light-toned material was the floor of the cavity produced by the impact that formed the arc of mountains.
This had been struck to create Archimedes some time later, as revealed by the impact’s ejecta over the light-toned surface. The mare material which filled in the cavity had encroached, but because the light-toned patch was more elevated it had not been submerged by this lava.
Gene Shoemaker had independently made a superposition study of another section of the Moon to demonstrate the stratigraphy technique.
During a visit to a bookstore he had happened upon a photograph of the area around the crater Copernicus taken by Francis Pease at Mount Wilson in 1919. It was of sufficient clarity to distinguish craters as small as 1km across, so he had it enlarged and set about mapping.
Whereas Hackman had mapped only three units, Shoemaker used seven. In March 1960 he presented a paper showing that whereas much of the material excavated by Copernicus had been ‘hinged’ to create the rim and adjacent ejecta blanket, some material had been hurled farther to produce chains of small secondary craters.
These secondaries were less energetic because, for the ejecta to have fallen back it could not have exceeded the escape velocity of the Moon, which is an order of magnitude slower than the typical speed of a celestial impact.
Shoemaker’s stratigraphic mapping not only confirmed Copernicus to be an impact crater, it also refuted the assertion by those who favoured the volcanic origin of lunar craters that the chains of small craters close to large craters were associated with crustal fractures.
Shoemaker and Hackman presented a joint paper at the International Astronomical Union’s Symposium in December 1960. Meanwhile, a new round of ordinary surface mapping was also underway.
In 1959 the Air Force Aeronautical Chart and Information Center in St Louis, Missouri, began to employ airbrushing to represent topography in Lunar Astronautical Charts on a scale of 1:1,000,000.
In 1961 Kuiper convinced the Air Force that there was merit in using visual observations when compiling these charts, because the eye is able to resolve finer detail in brief moments of clarity than can be recorded during a photographic exposure.
The pictures would provide the positional basis for mapping and the visual observations would supply the detail. To discern the subtle topography these observations were to be made just after lunar sunrise or just before sunset.
Two large refracting telescopes were made routinely available for this activity – the 61cm of the Lowell Observatory in Flagstaff and the 92cm of the Lick Observatory in California.