Anderson, Earl R. Folk-Taxonomies in Early English. Madison, N.J.:Fairleigh Dickinson University Press,2003,p.219. ISBN 978-0-8386-3916-0.«"On St. Martin's day (11 November) winter begins, summer takes its end, harvest is completed. ...This text is one of many that preserves vestiges of the ancient Indo-European system of two seasons, winter and summer."»
National Space Science Data Center (Centro de Datos de la NASA, Estados Unidos).«EarthFactSheet, Orbital parameters»(en inglés).[Consulta: 9 setembre 2011].