Rapa Nui calendar


The Rapa Nui calendar was the indigenous lunisolar calendar of Easter Island. It is now obsolete.

Attestation

William J. Thomson, paymaster on the USS Mohican, 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 and of the months of the year:

Months

Thomson recorded the months as follows:

Days

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.
The three sources we have correspond with each other except for two intercalary days, and the night of the new moon in Englert, which seems to have been confused with one of these. Beginning with ata, the night of the new moon, they are:
dayEnglertThomsonMétrauxdayEnglertThomsonMétraux
*1oataoataata *15omotohiomotohimotohi
2ohirooariari 16kokore tahikokore tahikokore tahi
3kokore tahikokore tahikokore tahi 17kokore ruakokore ruakokore rua
4kokore ruakokore ruakokore rua 18kokore torukokore torukokore toru
5kokore torukokore torukokore toru 19kokore hâkokore hakokore ha
6kokore hâkokore hakokore ha20kokore rimakokore rimakokore rima
7kokore rimakokore rimakokore rima21tapumetapumetapume
8kokore onokokore onokokore ono22matuamatuamatua
*9maharumaharumaharu*23orongoorongorongo
10ohuaohuahua24orongo taaneorongo tanerongo tane
11otuaotuaatua25mauri nuimauri nuimauri nui
x—ohotuhotu26mauri karomauri keromauri kero
12mauremauremaure27omutuomutumutu
13ina-iraina-iraina-ira28tireotireotireo
14rakaurakaurakau x——hiro

The kokore are unnamed 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 , nameless".

Analysis

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