In astronomy the mean distance of the earth from the sun is sometimes taken as a unit of length.
In the present state of science the most universal standard of length which we could assume would be the wave length in vacuum of a particular kind of light, emitted by some widely diffused substance such as sodium, which has well-defined lines in its spectrum. Such a standard would be independent of any changes in the dimensions of the earth, and should be adopted by those who expect their writings to be more permanent than that body.
In treating of the dimensions of units we shall call the unit of length . If is the numerical value of a length, it is understood to be expressed in terms of the concrete unit , so that the actual length would be fully expressed by .
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![{\displaystyle l[L]}](https://wikimedia.org/api/rest_v1/media/math/render/svg/849ed978e3863e400f1b156a6c9d859d5e7416c7)
4.] (2) Time. The standard unit of time in all civilized countries is deduced from the time of rotation of the earth about its axis. The sidereal day, or the true period of rotation of the earth, can be ascertained with great exactness by the ordinary observations of astronomers; and the mean solar day can be deduced from this by our knowledge of the length of the year.
The unit of time adopted in all physical researches is one second of mean solar time.
In astronomy a year is sometimes used as a unit of time. A more universal unit of time might be found by taking the periodic time of vibration of the particular kind of light whose wave length is the unit of length.
We shall call the concrete unit of time , and the numerical measure of time .
![{\displaystyle [T]}](https://wikimedia.org/api/rest_v1/media/math/render/svg/9d2c8afd013f811776289b0d3f804a430888eccc)
5.] (3) Mass. The standard unit of mass is in this country the avoirdupois pound preserved in the Exchequer Chambers. The grain, which is often used as a unit, is defined to be the th part of this pound.
In the metrical system it is the gramme, which is theoretically the mass of a cubic centimètre of distilled water at standard temperature and pressure, but practically it is the thousandth part of a standard kilogramme preserved in Paris.
The accuracy with which the masses of bodies can be compared by weighing is far greater than that hitherto attained in the measurement of lengths, so that all masses ought, if possible, to be compared directly with the standard, and not deduced from experiments on water.
In descriptive astronomy the mass of the sun or that of the
