Complex sentence: Her uncle Frank Arthur Bellamy was the senior of the two assistants at the Radcliffe Observatory of the University of Oxford; in 1899, at 17 years old, Ethel started to work for him, on a part-time basis from home, as an assistant.
Split into simpler sentences: Her uncle Frank Arthur Bellamy was the senior of the two assistants at the Radcliffe Observatory of the University of Oxford. In 1899, at 17 years old, Ethel started working for her uncle Frank Arthur Bellamy on a part-time basis from home, as an assistant.