Linguofreak
Well-known member
Well, Mach numbers are used for calculating effects such as shock waves. Thus they are only really useful when discussing speed relative to the same gas whose speed of sound is being used to determine the Mach number, and when the object in question is traveling through that gas (or can be expected to interact with it in the near future), as opposed to some other.
I suppose you could measure the Sun's Mach number with respect to the interstellar medium, or the Earth's Mach number with respect to the solar wind, or the Mach number of the Solar wind with respect to the ISM, or the Mach number with respect to its own speed of sound that the solar wind is hitting the ISM at at the heliopause.
