Various changes, or geological events, and. their periodicity involved in the evolution of the earth can hardly be explained satisfactorily and completely only in terms of the earth itself. More recently, ever-increasing attention has been paid to the influence of the solar system and the galactic system, even that of the vast cosmic space. This paper intends to discuss the relationship between the earth's evolution and the galactic system, based on the revolution of the solar system and other space factors. It is well-known that one period in which the solar system moves around the gtlactic centre is defined as one cosmic year (ca. 220---250m.y.). A good many facts have shown that the periodicity of various changes involved in the evolution of the earth, such as climatic fluctuation, crustal movement, magmatic activity, biological evolution, eustatic movement, elemental synthesis, the reversal of polarity of the earth's magnetic field, the variation of the speed of earth's rotation, etc. is generally consistent with the concept of cosmic year, which reflects the internal connection between the evolution of the earth and the galactic system. The author has proposed that the revolution of the solar system orbiting around the galactic centre with an ellipticy of 0.07--0.1, the speed of earth's rotation, and the annual average distance between the sun and the earth, as well as some space physical fields, such as magnetic field,radiation field, etc. are all characterized by cosmic year periodic variatioons. Under the influence of these cosmic factors, various changes involved in the evolution of the earth are also characterized by cosmic year periodicity. The exploration of the regularities governing the earth's evolution and its origin from the concept of cosmic year periodicity, coupled with astronomical, cosmological and geological data, will give a great impetus to the rapid development of earth sciences. Research in this respect is of great significance both in theory and practice with respect to astronomy, geology and philosophy.
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