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Measurements are given in metric (usually SI) units, with an imperial conversion (to an appropriate accuracy) following in parentheses where appropriate. In historical contexts this convention is reversed so that, for example, the diameter of an early telescope is given first in inches. Densities, given in grams per cubic centimetre, are not converted, and neither are kilograms or tonnes. Large astronomical distances are usually given in light-years, but parsecs are sometimes used in a cosmological context. Particularly in tables, large numbers may be given in exponential form. Thus 103 is a thousand, 2 106 is two million, and so on. ‘Billion’ always means a thousand million, or 109. As is customary in astronomy, dates are expressed in the order year, month, day. Details of units of measurement, conversion factors and the principal abbreviations used in the book will be found in the tables on this page.  天文学百科.JPG |
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2016-05-28 21:17:05, 31.79 M
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