Book contents
- Frontmatter
- Contents
- Preface
- Acknowledgements
- 1 Introduction
- 2 An introduction to SI units
- 3 Dimensional analysis
- 4 Unit of angular measure (radian)
- 5 Unit of time (second)
- 6 Unit of length (metre)
- 7 Unit of mass (kilogram)
- 8 Unit of luminous intensity (candela)
- 9 Unit of thermodynamic temperature (kelvin)
- 10 Unit of electric current (ampere)
- 11 Unit of amount of substance (mole)
- 12 Astronomical taxonomy
- References
- Index
5 - Unit of time (second)
Published online by Cambridge University Press: 05 December 2011
Book contents
- Frontmatter
- Contents
- Preface
- Acknowledgements
- 1 Introduction
- 2 An introduction to SI units
- 3 Dimensional analysis
- 4 Unit of angular measure (radian)
- 5 Unit of time (second)
- 6 Unit of length (metre)
- 7 Unit of mass (kilogram)
- 8 Unit of luminous intensity (candela)
- 9 Unit of thermodynamic temperature (kelvin)
- 10 Unit of electric current (ampere)
- 11 Unit of amount of substance (mole)
- 12 Astronomical taxonomy
- References
- Index
Summary
SI definition of the second
The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.
This definition refers to a caesium atom in its ground state at a temperature of 0 K.
The dimension of time is [T], its unit is the second and its symbol is s.
Definition of time
Funk & Wagnalls New Standard Dictionary of the English Language (Funk et al., 1946) defines time, inter alia, as:
The general idea, relation, or fact of continuous or successive existence; or the abstract conception of duration as limitless, capable of division into measurable portions, and essentially comprising the relations of present, past and future.
A system of reckoning or measuring duration; as solar time; sidereal time; mean time.
Systems of time or time scales
There are two major systems of time: those based on the Earth's rotation and the orbital motions of the Earth, Moon and planets, known as dynamical time; and those based on atomic clocks and known as atomic time.
A time system or scale may be specified by two numbers, the origin from which the time intervals are to be measured and the number of predefined unit scale intervals measured since the time of origin (Leschiutta, 2001).
Dynamical time
Dynamical time may be thought of as the independent variable in the equations that describe the motions of the bodies in the Solar System.
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- Using SI Units in Astronomy , pp. 54 - 71Publisher: Cambridge University PressPrint publication year: 2011
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