Hubbles Discovery

Question

Why do you think Hubble's discovery that the universe is expanding would have been impossible without instruments that could precisely measure emission spectra?

Answer 1

In order to conclude the universe is expanding, you need to show there is a measurable red shift in the emission spectra of the galaxies.  If the galaxies are moving apart at a speed relatively small compared to the speed of light, the red shifts will be small.  Therefore, measurement of the red shift would require very accurate and precise optical instrumentation mounted on the telescopes.  Otherwise, the data could be inconclusive.