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For example, the relationship between the Earth and the sun can be described either in [[geocentric]] or [[heliocentric]] terms.  In other words, one can describe the sun as revolving around the Earth or the Earth revolving around the sun, and both are equally mathematically valid.  However, describing it in geocentric terms (with the sun revolving around the Earth) requires the introduction of fictitious forces to explain why the massive sun curves around the relatively tiny Earth.  The heliocentric description, however, requires no such fictitious forces, and is therefore the ''inertial'' frame of reference.  Thus, given the scientific principle of [[parsimony]], inertial reference frames are preferred.

For example, the relationship between the Earth and the sun can be described either in [[geocentric]] or [[heliocentric]] terms.  In other words, one can describe the sun as revolving around the Earth or the Earth revolving around the sun, and both are equally mathematically valid.  However, describing it in geocentric terms (with the sun revolving around the Earth) requires the introduction of fictitious forces to explain why the massive sun curves around the relatively tiny Earth.  The heliocentric description, however, requires no such fictitious forces, and is therefore the ''inertial'' frame of reference.  Thus, given the scientific principle of [[parsimony]], inertial reference frames are preferred.

It is important to note that [[Einstein]]'s theories of relativity apply only to ''inertial'' reference frames.

According to the theory of [[relativity]], fictitious forces are indistinguishable from gravity. Inertial frames only make sense as local approximations, if masses are present.