Arqueoastronomía en Rapa Nui

Knowledge of the movements of the stars and planets and the seasonal changes, the definition of a lunar calendar and a systematic observation of celestial phenomena is a constituent part of the Rapanui heritage that was originally used for navigation and later, once settled on land, for agriculture. Unlike almost all of the other Polynesian islands,  Rapa Nui is sub-tropical, which causes changes in the weather over the cycle of the year.

The change of the seasons and the phases of the moon, especially new moon (Ohiro) and full moon (Omotohi), were once accompanied by rituals and ceremonies which would determine the propitious times for fishing, planting and harvesting. For the Rapanui, the year began with the appearance of the star cluster of the Pleiades (Matariki) following the winter solstice.

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The Lunar Calendar and Archaeoastronomy in Rapa Nui

Archaeeoastronomical tradition in Rapa Nui

Tradition tells us that there used to be centers for celestial education, such as the Ana U´i Hetu´u (nwhere the stars are observed”) cave near Tahai and solar observatories from where the constellations and astronomical phenomena could be watched with greater clarity due to natural or man-made markings which would allow very precise alignments.  The astronomer-priests would announce the beginning of a season and warn about signs observed in the skies, some say from stone towers called Tupa, 2 to 3 meters (6 to 10 feet) high and the same in diameter with a shape not unlike an igloo made of stones.

One of the largest Tupa is found behind Ahu Ra´ai from where it is possible to see the sun rise on the morning of the summer solstice directly over a point on Poike Hill called Pu´a Katiki and set behind Pu´i Hill.

Ana U´i He Tu´u

Astronomers, Archaeologists and Anthropologists tell us

The astronomer William Liller studied and remeasured in the field the notes made by the archaeologist and anthropologist with the Norwegian expedition of 1955, William Mulloy, which were the first to seriously consider possible astronomical orientations of the monuments.

He focused on 23 Ahu located along the shore, where the outreaching extremes were set almost perpendicular to the coast line in a north-south direction. He supposed that these placements could be intentional. Inland, at more than a kilometer (3000 feet) from the ocean, he noticed some platforms which seemed to face the rising sun of the shortest day of the year, the winter solstice.

His explanation was that the farmers and country folk lived inland. They needed to pay attention to the solstices of winter and summer, since the dates of the four seasons would determine the success or failure of their crops. On the seaside, the fishermen were more interested in the four cardinal points to establish their fishing grounds and orient themselves at sea, for which the equinoxes were more useful.

The most notable inland astronomical site is Ahu Huri A Urenga, which faces directly to the sunrise on the day of the winter solstice, June 21st. This platform and statue is located between Puna Pau and Orito Hill. It is now considered a pre-historical solar observatory, built with an incredible level of highly sophisticated precision. A peculiarity of this Ahu is that it has a Moai (statue) with 4 hands, the only one of its kind. In the opposite direction from the sunrise, not far from there, is another platform called Ahu Akava, located within the property of the current Provincial Governor, Pedro Edmunds-Paoa. That Moai faces straight to the point in the sea where the sun sets on the same June 21st winter solstice. According to Edmunds, along with this alineation is the Ahu Ko Te Pei, 3,500 meters (2.3 miles) away, which triangulates with the other two to offer lines of vision between the sunrise and sunset of the equinox. 

Moai & Petero Edmunds

Map of the Ahu Huri A Urenga

Solstices and Equinoxes

Outside the edge of the rock paved plaza of Ahu Huri A Urenga, Liller found 5 circular cavities and four pairs of cupules (holes like cups) which point with precision toward the principal points of solar elevation and which could have been used as artefacts for solar alignment. Unfortunately, they are not visible today as they are overgrown by tall grass.  All these details, including that it was built on a rise which favored visibilitiy, reaffirm Mulloy’s conclusion that Ahu Huri A Urenga was carefully designed and built to be a Stone Age observatory. Other examples may be Ahu Tahiri which indicates the equinox, Ahu Vinapu 2 which marks the summer solstice, Ahu Heki´i which alligns with the winter solstice and Ahu Ra´ai y Ahu Tongariki which are perpendicular to the summer solstice.


Domes with stars in Ahu Huri A Urenga

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Up until now only the sun and its rising and setting had been considered important by the archaeologists. But isn’t it possible that there were also observatories for the moon, the stars and the planets? There is no firm evidence thus far, but some Ahu and Moai names would indicate that they were dedicated to the stars, such as Ahu Ura Uranga Te Mahina (“the ruddy moon”) at Akahanga Bay or Ahu Moai A Mata Mea (“statue of the planet Mars”) near Tahai. In addition, there are many petroglyphs which refer to celestial phenomena on Papa U´i Hetu´u at Poike peninsula and on Papa Mahina near Hanga Ho´onu (La Pérouse) Bay.  All these suggest an astronomical purpose. The Spanish astronomer, Juan Antonio Belmonte, together with the anthropologist, Edmundo Edwards, after studying 30 sites, reached a different conclusion. According to them, many of the orientations that Liller took as equinoxial were, in reality, focused on the Belt of Orión (Tautoru) asterism and the majority of those that might be solsticial are realted to the location of the Pleiades (Matariki), the most important star cluster of Rapanui culture.

For this researchers the Moai Huri A Urenga looks out towards the dawning of the Pleiades right before sunrise, on the winter solstice. These orientations were represented in their lunar calendar. As we can see, the mystery of Rapa Nui does not want to be revealed. Archaeoastronomy certainly deserves more study.

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