Rapa Nui calendar
The Rapa Nui calendar was the indigenous lunisolar calendar
Lunisolar calendar
A lunisolar calendar is a calendar in many cultures whose date indicates both the moon phase and the time of the solar year. If the solar year is defined as a tropical year then a lunisolar calendar will give an indication of the season; if it is taken as a sidereal year then the calendar will...

 of Easter Island
Easter Island
Easter Island is a Polynesian island in the southeastern Pacific Ocean, at the southeasternmost point of the Polynesian triangle. A special territory of Chile that was annexed in 1888, Easter Island is famous for its 887 extant monumental statues, called moai, created by the early Rapanui people...

. It is now obsolete.


William J. Thomson, paymaster on the USS Mohican
USS Mohican (1883)
The second USS Mohican was a steam sloop of war in the United States Navy. She was named for the Mohican tribe.-Construction:Mohican was laid down by Mare Island Navy Yard, California, 4 September 1872, funded with the repair money allocated for the first ; launched 27 December 1883; sponsored by...

spent twelve days on Easter Island from December 19 to 30, 1886. Among the data Thomson collected were the names of the nights of the lunar month
Lunar month
In lunar calendars, a lunar month is the time between two identical syzygies . There are many variations. In Middle-Eastern and European traditions, the month starts when the young crescent moon becomes first visible at evening after conjunction with the Sun one or two days before that evening...

 and of the months of the year:
The natives reckoned their time, and in fact do so still by moons or months, commencing the year with August, which was, according to the traditions,the time when Hotu-Matua and his followers landed upon the island.


Thomson recorded the months as follows:
Rapanui name Meaning Western equivalent, 1886–1887
Anekena August
Hora-iti little summer September
Hora-nui big summer October
Tangarouri part of November
Kotuti November and December
Ruti December and January
Koro January
Tuaharo February
Tetuupu March
Tarahao April
Vaitu-nui big winter May
Vaitu-poto short winter June
Maro or Temaro July


The month was divided in two, beginning with the new and full moon. Thomson recorded the calendar at the time of his visit to the island as follows. The new moon occurred on November 25 and again on the night of December 24; Thompson records the crescent was first visible on November 26.
Rapanui name Meaning Western
equivalent, 1886
Kokore tahi first kokore November 27
Kokore rua second kokore November 28
Kokore toru third kokore November 29
Kokore hâ fourth kokore November 30
Kokore rima fifth kokore December 1
Kokore ono sixth kokore December 2
Maharu first quarter December 3
Ohua December 4
Otua December 5
Ohotu December 6
Maure December 7
Ina-ira December 8
Rakau December 9
Omotohi full moon December 10
Kokore tahi first kokore December 11
Kokore rua second kokore December 12
Kokore toru third kokore December 13
Kokore hâ fourth kokore December 14
Kokore rima fifth kokore December 15
Tapume December 16
Matua December 17
Orongo first quarter [sic] December 18
Orongo taane December 19
Mauri nui December 20
Marui [sic] kero December 21
Omutu December 22
Tueo December 23
Oata new moon December 24
Oari December 25
Kokore tahi first kokore December 26

The three sources we have correspond with each other except for two intercalary
Intercalation is the insertion of a leap day, week or month into some calendar years to make the calendar follow the seasons or moon phases. Lunisolar calendars may require intercalations of both days and months.- Solar calendars :...

 days (in bold), and the night of the new moon in Englert, which seems to have been confused with one of these. Beginning with (o)ata, the night of the new moon, they are:
day Englert Thomson Métraux day Englert Thomson Métraux
*1 oata oata ata *15 omotohi omotohi motohi
2 ohiro oari ari 16 kokore tahi kokore tahi kokore tahi
3 kokore tahi kokore tahi kokore tahi 17 kokore rua kokore rua kokore rua
4 kokore rua kokore rua kokore rua 18 kokore toru kokore toru kokore toru
5 kokore toru kokore toru kokore toru 19 kokore hâ kokore ha kokore ha
6 kokore hâ kokore ha kokore ha 20 kokore rima kokore rima kokore rima
7 kokore rima kokore rima kokore rima 21 tapume tapume tapume
8 kokore ono kokore ono kokore ono 22 matua matua matua
*9 maharu maharu maharu *23 orongo orongo rongo
10 ohua ohua hua 24 orongo taane orongo tane rongo tane
11 otua otua atua 25 mauri nui mauri nui mauri nui
x ohotu hotu 26 mauri karo mauri kero mauri kero
12 maure maure maure 27 omutu omutu mutu
13 ina-ira ina-ira ina-ira 28 tireo tireo tireo
14 rakau rakau rakau x hiro

*New moon, full moon, and first and last quarters.

The kokore are unnamed (though numbered) nights; tahi, rua, toru, haa, rima, ono are the numerals 1–6. The word kokore is cognate with Hawaiian ‘a‘ole "no" and Tahitian ‘aore "there is/are not"; here it may mean "without [a name], nameless".http://www.netaxs.com/~trance/mamari.html


The calendar collected by Thomson is notable in that it contains thirteen months. All other authors mention only twelve, and Métraux and Barthel find fault with Thomson:
Thomson translates Anakena as August and suggests that the year began at that time because Hotu-Matua landed at Anakena in that month, but my informants and Roussel (1869) give Anakena as July.

We are basing the substitution on the lists by Metraux and Englert (ME:51; HM:310), which are in agreement. Thomson's list is off by one month.

However, Guy calculated the dates of the new moon for years 1885 to 1887 and showed that Thomson's list fit the phases of the moon for 1886. He concluded that the ancient Rapanui used a lunisolar calendar with kotuti its embolismic month (AKA "leap month"), and that Thomson chanced to land on Easter Island in a year with a leap month.

The days hotu and hiro appear to be intercalary. A 28-day calendar month needs one to two intercalary days to keep in phase with the 29½-day lunar month. One of the rongorongo
Rongorongo is a system of glyphs discovered in the 19th century on Easter Island that appears to be writing or proto-writing. It cannot be read despite numerous attempts at decipherment. Although some calendrical and what might prove to be genealogical information has been identified, not even...

tablets may describe a rule for when to add these days.
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