Observatories Built to Read the Sky by Eye, from Chichen Itza to Jaipur

El Caracol was built around AD 906 at Chichen Itza, and its elevated sightlines let Maya astronomers follow Venus, whose full cycle of appearances they recorded as about 584 days, with five cycles equal to eight solar years.

Founded around 1259 under the Ilkhanid ruler Hulagu Khan and directed by Nasir al-Din al-Tusi, Maragheh gathered astronomers from across the Islamic world and China around a major library, and its instruments influenced later observatory design.

Built in 1276 in the early Yuan dynasty, Gaocheng used a tall gnomon and a 31.19 meter horizontal shadow scale to fix the tropical year at 365 days, 5 hours, 49 minutes and 20 seconds, a value in accord with the Gregorian calendar.

At Samarkand, Ulugh Beg built a fifteenth-century observatory around a huge underground sextant that was used to measure the duration of the year, planetary periods and eclipses; its star catalog, the Zij-i Sultani, stayed among the most accurate for over a century.

Built in 1442 during the Ming dynasty and described as a pretelescopic observatory, it still holds large bronze instruments, including an armillary sphere and a celestial globe, and is among the oldest surviving purpose-built observatories in the world.

Constructed from 1576 to 1580 on the island of Ven for Tycho Brahe, Uraniborg was the first custom-built observatory in modern Europe and the last built without a telescope; its naked-eye measurements later underpinned Kepler's laws of planetary motion.

Completed in 1577 on a hill above Constantinople, this observatory had a giant armillary sphere and an accurate mechanical astronomical clock for timing the planets, yet it was demolished by order of the Sultan only a few years later.

Begun in 1734 by Sawai Jai Singh, the Jaipur Jantar Mantar is a collection of 19 masonry instruments for measuring time, predicting eclipses and locating stars by naked-eye observation, and it is a UNESCO World Heritage Site.