Building J (Observatory)

17.0439°N, 96.7672°W.

Building J is the most architecturally peculiar structure on Monte Albán's Main Plaza in the Valley of Oaxaca, Mexico — an arrowhead-shaped platform oriented 45 degrees off the cardinal grid that governs the rest of the site. Built during the Late Formative around 100 BCE under the early Zapotec, Building J's interior corridors are aligned with stellar phenomena, and its façade is studded with more than 40 carved "conquest slabs" depicting place-glyphs of subjugated towns shown upside-down beneath the head of the conquering ruler. Anthony Aveni demonstrated in the 1970s that the structure's principal axis points to the rising of Capella, alpha Aurigae, on the day of the first solar zenith passage at this latitude. Excavated by Alfonso Caso beginning in 1931, the building remains the clearest pre-Columbian observatory in Mesoamerica.

Building J is a building shaped like an arrow, pointed at a star that rises on the day the sun stands directly overhead.