Simplify the following text for a Grade 9 native English-speaking person.
Original: Pickering estimated the diameter at 38 miles ( 61 km ) , but since he also gave 42 miles ( 68 km ) as the diameter of Phoebe , he was clearly overestimating the albedo ; using the modern figure for Phoebe gives Themis a diameter of 200 km .
Simplified: Pickering thought that the diameter of Themis was about 38 miles ( 61 km ) . Pickering had also discovered the moon Phoebe , but modern astronomers know that Pickering made a mistake when he gave 42 miles ( 68 km ) as the diameter of Phoebe .