"Glossary"Archiviato il 19 agosto 2017 in Internet Archive. in Astronomical Almanac Online. (2018). Washington DC: United States Naval Observatory. s.v. obliquity.
Earth Is tilted, su timeanddate.com. URL consultato il 25 agosto 2017.
web.archive.org
"Glossary"Archiviato il 19 agosto 2017 in Internet Archive. in Astronomical Almanac Online. (2018). Washington DC: United States Naval Observatory. s.v. obliquity.