Geometrized Unit
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In general relativity, it's quite useful to use geometrized unit, where everything has unit of kilometers. [Schutz]_
The principle of geometrized unit is to convert everything to length using :math:`c=G=1`. A precalculated value is
.. math::
1 = G/c^2 = 7.425\times 10^{-28}\mathrm{m} \mathrm{kg}^{-1}.
In the spirit of this conversion, we have the mass of the Sun :math:`M_\odot=2.0\times 10^{30}\mathrm{kg} = 1.5\times 10^{3} \mathrm{m}` and mass of earth :math:`M_\oplus = 6.0\times 10^{24}\mathrm{kg} = 4.4 \times 10^{-3} \mathrm{m}`.
References and Notes
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.. [Schutz] A First Course in General Relativity (Second Edition)