Sentence:  [<WSD>Markab</WSD> (the south-westernmost star in the square) is one of the stars from which the moon's distance is calculated in the Nautical Almanac, for the purpose of finding the longitude by lunar observations.] 
Meaning of the word marked with <WSD> tags:  A (subgiant), visible as a second-(magnitude) bluish star in the northern (constellation) of (Pegasus), one of four stars in the (asterism) of the Great Square of Pegasus, one of the (lunar stars) observed in navigation.