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[交流] 天文学百科（英文版）（原创）已有2人参与

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.

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