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. Because astronomers know how Pickering made his mistake, they can say that if Themis existed, it would have a diameter of 200 km.