Using SI Units in Astronomy

A multitude of measurement units exist within astronomy, some of which are unique to the subject, causing discrepancies that are particularly apparent when astronomers collaborate with researchers from other disciplines in science and engineering. The International System of Units (SI) is based on seven fundamental units from which other units may be derived, but many astronomers are reluctant to drop their old and familiar systems. This handbook demonstrates the ease with which transformations from old units to SI units may be made. Using worked examples, the author argues that astronomers would benefit greatly if the reporting of astronomical research and the sharing of data were standardized to SI units. Each chapter reviews a different SI base unit, clarifying the connection between these units and those currently favoured by astronomers. This is an essential reference for all researchers in astronomy and astrophysics, and will also appeal to advanced students.

Richard Dodd has spent much of his astronomical career in New Zealand, including serving as Director of Carter Observatory, Wellington, and as an Honorary Lecturer in Physics at Victoria University of Wellington. Dr Dodd is Past President of the Royal Astronomical Society of New Zealand.

Preface	p. ix
Acknowledgements	p. xii
Introduction	p. 1
Using SI units in astronomy	p. 1
Layout and structure of the book	p. 2
Definitions of terms (lexicological, mathematical and statistical)	p. 3
A brief history of the standardization of units in general	p. 7
A brief history of the standardization of scientific units	p. 8
The future of SI units	p. 11
Summary and recommendations	p. 11
An introduction to SI units	p. 12
The set of SI base units	p. 12
The set of SI derived units	p. 12
Non-SI units currently accepted for use with SI units	p. 13
Other non-SI units	p. 14
Prefixes to SI units	p. 14
IAU recommendations regarding SI units	p. 20
Summary and recommendations	p. 23
Dimensional analysis	p. 24
Definition of dimensional analysis	p. 24
Dimensional equations	p. 25
Summary and recommendations	p. 29
Unit of angular measure (radian)	p. 30
SI definition of the radian	p. 30
Commonly used non-SI units of angular measure	p. 30
Spherical astronomy	p. 36
Angular distances and diameters	p. 46
Steradian	p. 48
Summary and recommendations	p. 53
Unit of time (second)	p. 54
SI definition of the second	p. 54
Definition of time	p. 54
Systems of time or time scales	p. 54
The hertz: unit of frequency	p. 60
Angular motion	p. 60
The determination of the ages of celestial bodies	p. 66
Summary and recommendations	p. 70
Unit of length (metre)	p. 72
SI definition of the metre	p. 72
Linear astronomical distances and diameters	p. 72
Linear motion	p. 83
Acceleration	p. 88
Area	p. 89
Volume	p. 89
Summary and recommendations	p. 91
Unit of mass (kilogram)	p. 92
SI definition of the kilogram	p. 92
The constant of gravitation	p. 94
Masses of astronomical bodies	p. 97
Density	p. 106
Force	p. 108
Moments of inertia and angular momentum	p. 109
Summary and recommendations	p. 111
Unit of luminous intensity (candela)	p. 113
SI definition of the candela	p. 113
Radiometry and photometry	p. 113
Magnitudes	p. 137
Summary and recommendations	p. 142
Unit of thermodynamic temperature (kelvin)	p. 146
SI definition of the kelvin	p. 146
Temperature scales	p. 147
Some examples of the temperatures of astronomical objects	p. 149
Blackbody radiation	p. 151
Spectral classification as a temperature sequence	p. 154
Model stellar atmospheres	p. 165
Summary and recommendations	p. 172
Unit of electric current (ampere)	p. 174
SI definition of the ampere	p. 174
SI and non-SI electrical and magnetic unit relationships	p. 175
Magnetic fields in astronomy	p. 183
Electric fields in astronomy	p. 194
Summary and recommendations	p. 195
Unit of amount of substance (mole)	p. 197
SI definition of the mole	p. 197
Avogadro's constant and atomic masses	p. 197
Astrochemistry and cosmochemistry	p. 202
Summary and recommendations	p. 204
Astronomical taxonomy	p. 206
Definition of taxonomy	p. 206
Classification in astronomy	p. 206
Classification of stellar objects	p. 207
Classification of Solar System objects	p. 215
Astronomical databases and virtual observatories	p. 216
Summary and recommendations	p. 218
References	p. 219
Index	p. 226
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Using SI Units in Astronomy

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