What every astronomer measures in the spectrum of a galaxy is the percentage by which a line is shifted from its laboratory wavelength. Astronomers habitually say they measure a velocity. That is incorrect. What they measure is a redshift, what they infer is a velocity. The only astronomer I ever knew who was meticulously accurate about this was Fritz Zwicky, who always used the term “indicative” recession velocity. For consistency with general astronomical usage we have expressed large redshifts as fractional shifts (Δλ/λ), but for smaller redshifts multiplied them by the velocity of light in km s−1 as if they were Doppler velocity shifts. (The speed of light is approximately 300,000 km s−1).

Corrected Values of the Solar Motion

Even though we have consistently used the correct term—redshift or blueshift—for the measured quantity (whatever may cause the shift) we still have to remove from this measure the effect of any bona fide motions that we do know about, such as the orbital velocities of the earth around the sun and the sun around the galactic center. Redshifts of galaxies are initially measured with respect to the telescope that observes them. Then they are normaly given a small correction for the earth's motion around the sun (less than 30 km s−1) and called heliocentric redshifts. The motion of the sun must then be removed.

The motion of the sun with respect to the coordinate frame of the nearby galaxies consists mainly of a rotation of the solar neighborhood around the center of our own Galaxy.