One of the main advantages of SI – the metric system – is that it can be used for any measurement task (from the kitchen to the science lab) – thus avoiding the need to learn a plethora of specific units for specific purposes. However, is there a case for making certain exceptions to this rule? (Warning: this article is for the technically-minded).
The Système International (SI) has been carefully designed as a coherent system of units intended for use in all applications independent of language and national origin. At its heart is the principle that one and only one unit is used for each type of measurement. The same unit is used regardless of scale and the former traditional practice of distinct units for different ranges is supplanted by the use of a common set of prefixes based on powers of ten as an optional alternative to exponential notation also based on powers of ten.
Some measurement applications however do have key units of measurement that, by virtue of their definition, are very convenient for the purpose and if were to be incorporated into the SI would create anomalies, contrary to its principles.
In this article the discussion will focus on astronomy which has a significant number of non-SI units in common use. Readers knowledgeable in other subjects will no doubt think of examples in their own sphere.
For a very recent example consider the tables of data from the NASA/Ames Kepler mission:
Note in particular the mass measurements: Solar (mass of Sun = 1), Jovian (mass of Jupiter = 1) and Earth masses which are in wide-spread use. The Astonomical Unit (AU where the mean distance from Sun to Earth = 1) is also very common. Other common units of distance are the parsec (distance of an object from Earth such that two position measurements made over a 6 month interval yield a discrepency of 1 arc-second due to parallax) and the light year (the distance travelled by an object in a year if moving at the speed of light).
The kilogram is probably used in astro-physics for calculation involving fundamental physical laws (e.g. the gravitational constant is a key parameter in the equations of stellar structure) but the results are presented and discussed in terms of the mass ratios referred to above. This is not because the SI is being rivalled by dogmatic alternatives but, probably, for valid scientific reasons. It is likely the comparisons are significant in helping to assimilate the data and to formulate or refine theories about the formation and evolution of stars and planets.
The AU has its uses in helping to visualize the scale and structure of the Solar system which is probably more memorable than distances in metres. 1 AU is about 150 Gm so conversion to metres isn’t too bad e.g. the mean distance of Pluto at roughly 40 AU is about 6000 Gm or 6 Tm. However, its use should really be confined to circumstances where a comparison with Earth’s orbit is significant (its use in the Kepler observations may well be important).
Incidentally, when interplanetary distances are expressed in metres suitable prefixes should be used e.g. Tm not billions of km.
The light year has some usefulness for larger distance scales. For example a galaxy 2 million light years away is effectively being seen as it was 2 million years ago. It is also a significant parameter for very distant objects because a quasar say 13 billion light years away is effectively a window on the very early universe. As it happens the light year is within 6% of 10 Pm (approx 9.46 x 1015 m).
In the opinion of this author the parsec, however, is overused. It is of course sensible for the intermediate results of parallax measurements (which is necessarily limited to relatively nearby distance determination) but a poor choice as a general purpose unit due to its awkward relationship to the light year (3.24 ly) and metre. Retaining it is almost (but not quite) as bad as retaining the mile alongside the kilometre.
As a general conclusion, standard SI units should be used where it is feasible to do so and non-SI units, where justifiable, should be rationalised to avoid unnecessary proliferation and confined to appropriate circumstances.
Readers may disagree with the tolerance expressed here to some non-SI units and are invited to comment or give other examples in areas of their own knowledge and interest.