Calendar

===Solar===
{{main|Solar calendar}}

Solar calendars assign a ''date'' to each [[solar time|solar day]].<ref>{{Cite web |title=Introduction to Calendars |url=https://aa.usno.navy.mil/faq/calendars |access-date=2024-03-12 |website=aa.usno.navy.mil}}</ref> A day may consist of the period between [[sunrise]] and [[sunset]], with a following period of [[night]], or it may be a period between successive events such as two sunsets.<ref>{{Cite book |url=https://books.google.com/books?id=drMzAQAAMAAJ&newbks=0&printsec=frontcover&pg=PA983&dq=A+day+may+consist+of+the+period+between+sunrise+and+sunset,+with+a+following+period+of+night&hl=vi |title=The Jurist |date=1861 |publisher=S. Sweet |pages=983 |language=en}}</ref><ref>{{Cite book |last=Oxossi |first=Diego de |url=https://books.google.com/books?id=FN11EAAAQBAJ&newbks=0&printsec=frontcover&pg=PT37&dq=A+day+may+consist+of+the+period+between+sunrise+and+sunset,+with+a+following+period+of+night&hl=vi |title=Sacred Leaves: A Magical Guide to Orisha Herbal Witchcraft |date=2022-07-08 |publisher=Llewellyn Worldwide |isbn=978-0-7387-6721-5 |language=en}}</ref> The length of the interval between two such successive events may be allowed to vary slightly during the year, or it may be averaged into a [[solar time|mean solar day]]. Other types of calendar may also use a solar day.

The Egyptians appear to have been the first to develop a solar calendar,<ref>{{Cite web |title=Solar calendar {{!}} Ancient Egypt, Mayan, Aztec {{!}} Britannica |url=https://www.britannica.com/science/solar-calendar |access-date=2024-03-12 |website=www.britannica.com |language=en}}</ref> using as a fixed point the annual sunrise reappearance of the Dog Star—[[Sirius]], or Sothis—in the eastern sky, which coincided with the annual flooding of the Nile River.<ref>{{Cite book |url=https://books.google.com/books?id=mXsoAQAAIAAJ&newbks=0&printsec=frontcover&dq=The+Egyptians+appear+to+have+been+the+first+to+develop+a+solar+calendar,&q=The+Egyptians+appear+to+have+been+the+first+to+develop+a+solar+calendar,&hl=vi |title=The New Encyclopædia Britannica: Micropædia |date=1991 |publisher=Encyclopædia Britannica |isbn=978-0-85229-529-8 |pages=941 |language=en}}</ref><ref>{{Cite book |last=Lawson |first=Russell M. |url=https://books.google.com/books?id=kwTHEAAAQBAJ&newbks=0&printsec=frontcover&pg=PT179&dq=The+Egyptians+appear+to+have+been+the+first+to+develop+a+solar+calendar,&hl=vi |title=Science in the Ancient World: From Antiquity through the Middle Ages |date=2021-09-23 |publisher=Bloomsbury Publishing USA |isbn=979-8-216-14241-6 |language=en}}</ref><ref>{{Cite book |last=Muntz |first=Charles |url=https://books.google.com/books?id=VGHODQAAQBAJ&newbks=0&printsec=frontcover&pg=PT16&dq=The+Egyptians+appear+to+have+been+the+first+to+develop+a+solar+calendar,&hl=vi |title=Diodorus Siculus and the World of the Late Roman Republic |date=2017-01-02 |publisher=Oxford University Press |isbn=978-0-19-064901-2 |language=en}}</ref> They built a calendar with 365 days, divided into 12 months of 30 days each, with 5 extra days at the end of the year. However, they didn't include the extra bit of time in each year, and this caused their calendar to slowly become inaccurate.<ref>{{Cite web |title=Solar calendar &#124; chronology &#124; Britannica |url=https://www.britannica.com/science/solar-calendar}}</ref>