What provides the necessary fixed reference point for measuring the fractional spectral shift?

Answer

Known, unique sets of spectral lines in a laboratory on Earth

To accurately calculate the degree of spectral shift ($ rac{ u ext{observed}}{ u_{ ext{rest}}}$), astronomers must know what the spectrum looks like when the source is stationary. This reference point is established by studying the precise wavelengths of absorption lines (like the Balmer series for hydrogen) in a laboratory setting on Earth. Since every element produces a unique, known 'fingerprint' of spectral lines at rest, observing the entire pattern shift on the star allows astronomers to calculate the exact fractional displacement ($ rac{ ext{A}_{ ext{observed}} - ext{A}_{ ext{rest}}}{ ext{A}_{ ext{rest}}}$) which is critical for determining speed.

What provides the necessary fixed reference point for measuring the fractional spectral shift?

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