September 11, 2010

Giant Sundial

The largest sundial I have ever seen is in the Jantar Mantar in Jaipur, India. It looms over a remarkable collection of naked-eye astronomical instruments, where large scale and geometrical ingenuity make up for the absence of optical magnification. The term "jantar" means "instrument" and "mantar" may be interpreted as "formula" or "calculation."

Constructed for Maharaja Jai Singh II at his new capital and fabricated out of masonry, marble, and bronze between 1727 and 1734, the dozen or so instruments that constitute the collection were used for making remarkably precise determinations of astronomical position without the aid of telescopes.

Together, the devices could be used to measure time, predict eclipses, track star locations, ascertain declinations of planets, and determine celestial altitudes.

The largest instrument (samrat yantra) casts a shadow that tells the time of day. Its triangular gnomon, 90 feet high, is angled at 27°, Jaipur's latitude. The triangle's hypotenuse rests parallel to Earth's axis. A quadrant of a circle lies on either side of the gnomon, parallel to the plane of the equator.

The small cupola at the top of the samrat yantra was used as a platform for announcing eclipses and the arrival of the monsoon season.

Another astronomical instrument (jai prakash yantra) consists of a pair of large hemispherical bowls, which serve as a reflection of the sky above. Crossing wires stretched across each bowl hold a centered metal ring so that every point in the sky can be reflected to a corresponding point on the bowl through the ring.

The two bowls complement each other. The open spaces in one correspond to surfaces in the other. The cutouts allow observers to view positions without inadvertently blocking the light. When a shadow falls within a cutout in one bowl, an observer simply moves to the other bowl.

Bronze devices related to astrolabes allowed the measurement of time and the positions of celestial objects. 

Telling time from the smaller of the two giant sundials in the Jantar Mantar.

In July, the Jantar Mantar in Jaipur was added to the UNESCO World Heritage List as an "expression of the astronomical skills and cosmological concepts of the court of a scholarly prince at the end of the Mughal period" in Indian history.

Photos by I. Peterson

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