Simplify the following text for a Grade 8 native English-speaking person.
Original: From 1979 to 1987 he was principal conductor of the London Symphony Orchestra , and from 1982 to 1986 he was principal guest conductor of the Chicago Symphony Orchestra .
Simplified: He was principal conductor of the London Symphony Orchestra from 1979 to 1987 . In the US , he was principal guest conductor of the Chicago Symphony Orchestra from 1982 to 1986 .