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Chankillo and the Lunar Standstills
Amelia Carolina Sparavigna
Politecnico di Torino
Here we investigate the possibility that the site of Chankillo in Peru could had been used to observe the moon and the lunar standstill period. We use for simulations the Photographer's Ephemeris software.
Chankillo is an archaeological site located in the coastal desert of Peru. The site is composed by a hilltop fort, the nearby Thirteen Towers solar observatory, and other buildings in the area (see Figure 1). The site is dated to about 300 BCE, and appears to have been used until the Spanish Conquest . As described by Wikipedia, the "regularly-spaced thirteen towers of Chankillo were constructed atop the ridge of a low hill running near north to south, forming a "toothed" horizon with narrow gaps at regular intervals". This toothed horizon was considered by Clive Ruggles and Ivan Ghezzi as a frame of reference for astronomical observations concerning the sun . Using a software giving sunrise and sunset directions on satellite maps (Sollumis.com), we simulated the proposal by Ghezzi and Ruggles of Chankillo as a solar observatory .
In an arxiv preprint , the author (Noah Brosch) tells us that, thanks to a discussion with Yvon Georgelin at the Observatoire Astronomique de Marseille-Provence, he had learned of the existence of Chankillo and that this site seems had served as a lunar observatory too. "Ghezzi & Ruggles identified two structures, … as possible observation points for the sunrise and sunset positions at certain dates such as the solstices and the equinox, … Alternatively, Georgelin proposed that the site was used to observe the rising of the Moon from the "fort", and the 13 towers served to account for the changing declination of the Moon" . Actually, the "primary function of the site was, according to Georgelin, to enable the prediction of times when a Solar or Lunar eclipse would be possible .
For what concerns the declination of the Moon, let us remember that our satellite has an apparent motion in the sky, which is more complex than that of the sun. We have that the sunrise direction oscillates between the two solstice positions during a year, whereas the moon does the same during a nodal period (about 27 days). Moreover, the moon has a period – the lunar standstill period (18.613 years) – on which the values of the extreme directions (standstills) are changing. In this manner there are major and minor standstills, of which we can calculate moonrise/moonset directions. These directions are depending on latitude. For a latitude of about 45°, like that of Torino for instance, we have that the minor and major northern moonrise azimuths (directions) are 47.40° and 65.65° (angles are given from true north). The minor and major southern moonrise azimuths are 116.35° and 132.58°. The azimuths of sunrise on summer and winter solstices are between these lunar azimuths.
To show how the moonrise azimuths are changing during the lunar standstill period we have at our disposal a software. It is the Photographer's Ephemeris. It gives the directions of moonrise/moonset and sunrise/sunset for any day of any year on satellite maps and we have used it in several occasions (see for instance [4-8].
Let us investigate the Georgelin's proposal of a lunar observatory at Chankillo by using this software. We can see the simulations in the Figures 2 and 3. For the southern moonrise on a major lunar standstill, the moon, seen from the fort, is rising behind the southern tower. For the southern moonrise direction on a minor lunar standstill, the moon is rising behind the northern tower. Therefore, from the simulation, we have that the Georgelin's proposal is well-posed. Combining the observation of the sun and the moon, people at Chankillo had the possibility to know when a Solar or Lunar eclipse would be possible.
 Brosch, N. (2011). Thinking about Archeoastronomy. arXiv preprint arXiv:1103.5600.
 Ghezzi, I., & Ruggles, C. (2007). Chankillo: A 2300-Year-Old Solar Observatory in Coastal Peru". Science. 315 (5816): 1239–1243. ISSN 0036-8075. PMID 17332405. doi:10.1126/science.1136415.
 Sparavigna, A. C. (2012). The solar towers of Chankillo. arXiv preprint arXiv:1208.3580.
 Sparavigna, A. C. (2016). The Temple Complex of Ggantija and the Major Lunar Standstill as Given by the Photographer's Ephemeris (August 24, 2016). Available at SSRN: https://ssrn.com/abstract=2828614 or http://dx.doi.org/10.2139/ssrn.2828614
 Sparavigna, A. C. (2016). Augusta Emerita and the Major Lunar Standstill of 24 BC (July 10, 2016). PHILICA Article Number 635. Available at SSRN: https://ssrn.com/abstract=2807544
 Sparavigna, A. (2016). Astronomical Alignments of Ales Stenar along Sunset and Moonset Directions. PHILICA.COM Article number 663.
 Sparavigna, A. C. (2016). Megalithic Quadrangles and the Ancient Astronomy (November 1, 2016). Available at SSRN: https://ssrn.com/abstract=2862330 or http://dx.doi.org/10.2139/ssrn.2862330
 Sparavigna, A. C. (2016). An Astronomical Analysis of Some of the Diagonal Avenues of Washington (June 3, 2016). PHILICA, Article 613. Available at SSRN: https://ssrn.com/abstract=2789700
Figure 1: The archaeological site of Chankillo in Google Earth. In the lower part of the image we are also showing an elevation profile.
Figure 2: Thanks to the Photographer's Ephemeris, we can see the southern moonrise direction (blue line) on a major lunar standstill. Seen from the fort, the moon is rising behind the southern tower.
Figure 3: Southern moonrise direction (blue line) on a minor lunar standstill. Seen from the fort, the moon is rising behind the northern tower. (Courtesy: Photographer's Ephemeris).
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Sparavigna, A. (2017). Chankillo and the Lunar Standstills. PHILICA.COM Article number 1131.