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(sel-ĕ-nol -ŏ-jee) The scientific study of the Moon. See also moonquakes; Moon rocks; stratigraphy.



the scientific discipline that studies the structure and the chemical and mineralogical composition of the moon. The shape of the moon and the metrical characteristics of its relief are the subject of selenodesy. Selenology arose in the mid-20th century when it became possible to carry out direct investigations of the moon by means of artificial lunar satellites and instruments delivered to the surface of the moon by space vehicles.

In addition to astrophysical methods, selenology makes use of the techniques of geophysics and geochemistry. The difficulties associated with the investigation of the moon led to the development of methods of remote analysis of rocks, in particular, analysis based on the radioactivity of rocks; such analysis can be successfully carried out by means of instruments in artificial lunar satellites. Micromethods, which permit the study of specimens weighing milligrams or fractions of a milligram, have received extensive development in the investigation of specimens of lunar rock that have been brought to the earth. These methods include, in particular, scanning electron microscopy, the use of an electron microprobe, and neutron activation analysis. Investigations of the internal structure of the moon make use of active and passive seismic experiments and electromagnetic probing techniques. In the case of electromagnetic probing, studies are usually made of the electromagnetic fields excited within the moon by the electromagnetic field of the solar wind. Analysis of the motions of artificial lunar satellites has produced results of great importance for the study of the lunar gravitational field. These results have led to the discovery of mascons, which are large nonequilibrium concentrations of mass. Although the moon apparently lacks a general magnetic field at the present time, magnetic anomalies exist, which are evidence of the inhomogeneity of at least the outer layers of the moon.

Selenological studies have led to the concept of the moon as a cosmic body with a complex history of development. In the early stage of its existence the moon underwent complete, or nearly complete chemical differentiation. This situation apparently is characteristic to some extent of all terrestrial planets.

The methods of selenology are an extension and generalization of the methods of the earth sciences, but these methods cannot be simply transferred to lunar conditions. Because of the change in conditions, new factors are dominant, and the methods of investigation must consequently be modified. This concern with new factors has had an influence on the earth sciences. For example, in contrast to the earth, where water is the primary erosive agent, meteoritic impact is the principal erosive force on the moon, where there is no water. The elucidation of the role of meteorites in the formation of the lunar landscape has drawn the attention of investigators to the study of meteoritic craters on the earth.

The development of selenological research is of special interest because the moon is, as it were, a testing ground reproducing the conditions on bodies of the solar system that lack an atmosphere or have a rarefied atmosphere. Under these conditions, research using automatic equipment is particularly important, since the participation of man in the direct investigation of many bodies in the solar system entails substantial difficulties.



A branch of astronomy that treats of the moon, including such attributes as magnitude, motion and constitution. Also known as lunar geology.