[Physics] Why are cgs units the norm in astrophysics

astrophysicsunits

Other physics communities, e.g. the particle physics one, have their own set of units, custom-tailored to their own needs. Now, the astrophysics community is somewhat similar, in that a lot of measurements are often expressed in terms of corresponding numbers related to the Sun (solar mass, solar luminosity, solar radius, et cetera). However, when it comes to using down and dirty numbers, I've seen cgs units used way more often than SI units. Why is that? Is it just a matter of tradition or is there something more subtle at work?

Best Answer

It is convention and laziness (and I'm as guilty as anyone). In fact cgs units should not be used (according to the International Astronomical Union), in the same sense as Pluto should not be called a planet. Both were decisions made by the IAU. In the case of units, the IAU unambiguously endorsed the use of SI units, except for a short list of defined units common in Astronomy (like solar mass, parsec etc., although the electron-volt is also included).

So there you have it - the IAU has a policy, but most astronomers ignore it.

http://www.iau.org/publications/proceedings_rules/units/

Of the main journals ApJ, AJ, A&A don't specify, but MNRAS does suggest using SI units and points to the IAU policy.

Interestingly, the American Astronomical Society machine readable table standards document also says it requires SI units, but then points to a table which includes gram as an allowable unit of mass, however the electrical units are SI (Teslas, Webers etc,)?

https://aas.org/authors/aas-journal-unit-standards-machine-readable-tables

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