Original: In 1996 astronomers Baliunas, Sokoloff, and Soon reported measurements of a sample of stars' Calcium II H and K spectral lines and thereby measured a rotational period of 37 days for 51 Pegasi.
Simplified: In 1996 astronomers Baliunas, Sokoloff, and Soon calculated that it takes 51 Pegasi b 37 days to orbit 51 Pegasi.