Sentence:  The favoured explanation, adopted by Maraldi, was based on the assumption that the star was rotating and composed of two different parts, a bright and a dark region; if so, it might only be visible when the bright region turned towards Earth. Whatever the explanation, <WSD>Mira</WSD> had puzzled the astronomers because of its random character and the irregular periods between its observations. 
Meaning of the word marked with <WSD> tags:  A binary star in the constellation Cetus, Omicron (ο) Ceti. The system contains a variable red giant and a white dwarf. Its brightness varies from a magnitude 2 at its brightest to a magnitude 10 at its dimmest.