Calendar

===Lunar===
{{Main|Lunar calendar}}
Not all calendars use the solar year as a unit. A lunar calendar is one in which days are numbered within each [[lunar phase]] cycle.  Because the length of the [[lunar month]] is not an even fraction of the length of the [[tropical year]], a purely lunar calendar quickly drifts against the seasons, which do not vary much near the equator.  It does, however, stay constant with respect to other phenomena, notably [[tide]]s. An example is the [[Islamic calendar]].
Alexander Marshack, in a controversial reading,<ref>James Elkins, ''[https://books.google.com/books?id=5Ku6YdWurMgC&pg=PA63&lpg=PA63 Our beautiful, dry, and distant texts]'' (1998) 63ff.</ref> believed that marks on a bone baton ({{Circa|25,000 BC}}) represented a lunar calendar. Other marked bones may also represent lunar calendars. Similarly, Michael Rappenglueck believes that marks on a 15,000-year-old cave painting represent a lunar calendar.<ref>{{cite news |url=http://news.bbc.co.uk/2/hi/science/nature/975360.stm |title=Oldest lunar calendar identified |work=BBC News |date=16 October 2000 |access-date=14 March 2013 |archive-date=11 February 2021 |archive-url=https://web.archive.org/web/20210211233146/http://news.bbc.co.uk/2/hi/science/nature/975360.stm |url-status=live }}</ref>