sorry if this sounds a tad retarded but astronomical topics arent my forte...how do know if something is red-shifted without prior knowledge? say you've got light emitted from galaxy far far away at 500nm, and it ends up on earth at 510nm. how can you differentiate between 510nm actual radiation and 500nm radiation red shifted? it sounds like circular reasoning to me: by the amount of red shift, you can figure out the source velocity, except you don't know the original wavelength, so we can figure it out from the shift, except to know the original wavelength you need to know the shift already and hence the velocity.....aaand now i'm confused. i thought you could do it by recognising familiar groupings of line spectra...but surely the spectra will vary according to what the source is composed of etc.