Harvey, Anthony; Barry Cork; Maurice Allward; Teresa Ballús; Roser Oromi (1978). "Cite-Que". Qué Sabes del Universo 1 (orihinal na pamagat: New World of Knowledge: Our Earth and the Universe). Ediciones Nauta, S.A. (nilimbag sa Espanya) / William Collins Sons and Company Limited, ISBN 84-278-0441-5, ISBN 84-278-0453-9.{{cite ensiklopedya}}: CS1 maint: date auto-translated (link)