We use the semi colon for 2 reasons:
1. To manage a large and unwieldy list which already contains commas.
Division 2: Thatto Heath Railway Hotel B 228, BICC 213; Greenalls 213, Carr Mill 194; British Plate B 238, ICI 204; Fibreglass 198, St Marys 251. (Liverpool Daily Post and Echo]. u.p., n.d., Leisure material)
Ham, cheese and eggs; ham, pickles and crisps; ham, chutney and cucumber.
2. To separate closely related independent clauses.
A constable brought Wycliffe’s coffee; he sipped it and made a wry face. (Wycliffe and the scapegoat. Burley, W J. London: Corgi Books, 1987, pp. 27-158.)
Cepheids are absolutely regular, so that their magnitude at any moment can always be predicted, and they are very useful to astronomers, because their real luminosities are linked with their periods; the longer the period, the more powerful the star. (Exploring the night sky with binoculars. Moore, Patrick. Cambridge: Cambridge University Press, 1990, pp. 13-155. 2001).