1.8 The moon  (Page 3/5)

Mellan's three engravings are surely the finest artistic renderings of the Moon ever made, but they show an artist'sMoon, not an astronomer's Moon. Mellan wonderfully represented what he saw through the telescope: at first and last quarter thedetails at the edge of the Moon are washed out while the features near the terminator stand out starkly; conversely, atfull Moon the features in the center are washed out while those near the edge show prominent relief. Where the solar rays areperpendicular to the lunar surface they cast no shadows, but where they rake the surface they throw long shadows. Whatastronomers needed was a single map that showed all the features equally clearly--a composite view that pictured the Moon in away it never appeared in reality but was accurate in its placement of individual features.

The first such map was made by the Belgian cosmographer and astronomer Michael Florent van Langren in 1645. Two years latera much more influential effort was published by Johannes Hevelius. In 1647 Hevelius, a wealthy brewer in the Polish cityof Gdansk, published Selenographia, the first treatise entirely devoted to the Moon. Hevelius combined all the talents necessaryfor his task. He made his own lenses, constructed his own telescopes, observed the Moon on every clear night for severalyears, drew his observations, engraved them himself, and had the wealth to publish a sumptuous book at his own expense. InSelenographia he presented engravings of every conceivable phase of the Moon as well as three large plates of the full Moon : one of the ways the full Moon actually appeared through the telescope, one the way amaker of terrestrial maps might represent it (using the conventions of geographers), and one a composite map of alllunar features illuminated (impossibly) from the same side. It is this last map that was to be used by astronomers during lunareclipses. Hevelius also suggested a system of nomenclature based on earthly features.

Hevelius founded the science of selenography (after Selene, the goddess of the Moon) and showed astronomers how to representheavenly bodies. Selenographia was a model for all who came after him. All lunar maps since his time have used theconvention of single illumination (although while he used morning illumination modern maps use evening illumination aftervan Langren's model). He also instituted the practice of showing the entire lunar surface visible from the Earth, which, becauseof librations, is greater than a hemisphere. Hevelius's nomenclature, although used in Protestant countries until theeighteenth century, was replaced by the system published in 1651 by the Jesuit astronomer Giovanni Battista Riccioli, who gavethe large naked-eye spots the names of seas (Sea of Tranquillity, Sea of Storm, etc.) and the telescopic spots (nowcalled craters) the name of philosophers and astronomers (fig. 18). It should be pointed out that although Riccioli wrotehis Almagestum Novum ("New Almagest") in which this map appeared to combat the Copernican theory , he was evenhanded in assigning names: Copernicus and Kepler wereassigned prominent craters, and even Galileo received his due.

One last note. As the astronomical telescope with its inverted image came into use, astronomers quickly adopted the habit of representing the way they saw the Moon--upside down . This practice was followed until very recently. Lunar images are now constructed and stored digitallyand can be displayed in any orientation. Astronomers have therefore reverted to showing the Moon right side up.