Complex sentence: In 1912, Vesto Slipher used spectroscopy to measure the radial velocity of M31 with respect to our solar system--the largest velocity yet measured, at.
Split into simpler sentences: In 1912, Vesto Slipher used spectroscopy to measure the radial velocity of M31 with respect to our solar system. This was, at the time, the largest velocity yet measured, at.