Answer: Reference for an Essay Published in a Book with Many Authors
A proper format could be:
Author’s name, “Title of the essay“, in Name of one of the book’s authors et al., Title of the book, city where the book was published: publisher’s name, year of publication, page number (where the reference can be found; “p.” if the information is on a single page and “pp.” if the information continues on more than a page).
Robert Radford, “The Moon and the Sun”, in Tim Thomas et al., Planets and Stars, Montreal: Horizon Publishing, 2009, pp. 43-44.