Simplify the following text for a Grade 9 native English-speaking person.
Original: In 1996 astronomers Baliunas , Sokoloff , and Soon 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 .