Nadivalaya Yantra

An equinoctial sundial of ancient design Jai Singh built built Nadivalaya Yantras at Jaipur, Ujjain, and Varanasi

Introduction

The Nadivalaya incorporates two circular surfaces, parallel to the equatorial plane, with metal rods pointing toward the poles mounted at their centers. Devices similar to the Nadivalaya were known to Hindu astronomers before Jai Singh’s time and references appear in Hindu literature as early as the 8th century. The Nadivalaya is essentially an equinoctial sundial built in two parts—one facing south and the other north.
How it works

The dial faces are built along an east-west line, and are parallel to the plane of the equator. With this arrangement, the sun illuminates the northern face during the summer months and the southern face during the winter months. When the sun crosses the celestial equator, its rays will be parallel to the planes of the Nadivalaya and will illuminate both faces. On all other days only one or the other face will be illuminated. This feature makes the Nadivalaya an excellent indicator of the equinoxes. The surfaces of the instrument are graduated along the outer edge in units of time, with the smallest division being one minute. Zero is indicated at top and bottom, so that time can be measured from noon or midnight. The Nadivalaya may also be used for determining the declination of objects at night. A string is attached to the central pin and extends to the scale at the rim of the instrument. The string is held taut and a metal strip or slit is aligned with the object being observed, the central pin, and the string. The point on the graduated scale where the string crosses will indicate the hour angle of the object