About

Maharajah Sawai Jai Singh II of Jaipur constructed five astronomical observatories in northern India between 1724 and 1730 – in Jaipur, Delhi, Varanasi, Ujjain, and Mathura, with the one in Delhi being the oldest, and the one in Jaipur being the largest and most elaborate. Each of these observatories is different in layout and size, although the instruments are similar. The Mathura observatory, and the fort in which it was housed, were demolished just before 1857.
The Jaipur observatory is by far the most elaborate of the five and houses sixteen masonry instruments and six made of metal.

The objective behind the construction of these observatories was to create astronomical instruments that would be more accurate and permanent than the brass instruments in use at the time. Jai Singh’s solution was to make the instruments on a massive scale, and to make them of stone and masonry.

The Samrat Yantra, is an equinoctial sundial which can measure time to a precision of two seconds.
The essential parts of the Samrat Yantra are the gnomon (a triangular wall with its hypotenuse parallel to earth’s axis), and a pair of quadrants on either side, lying parallel to the plane of the equator. On a clear day, as the sun passes from east to west, the shadow of the gnomon falls on the scale of the quadrant, indicating local time.
The gnomon is about 90 feet high, with a base of 147 feet. It is placed in a North-South direction with an angle equal to the latitude of its location (i.e., 27°). A small chhatri is built on top of the gnomon.
The face of the two quadrants is made of stone, and is marked with fine notches, each of which represent about two seconds. The western quadrant measures six hours of the morning, while the eastern quadrant measures six hours of the afternoon. The radius of each arc is 49 feet and 10 inches. Jaipur is on 75.787° East latitude, which is about 7.5 degrees west of 82.5° East latitude, which is the base for Indian Standard time. Thus, any measures of time at the Jaipur observatory will be about 26 minutes behind Indian Standard Time.

The Rama Yantra consists of a pair of cylindrical structures, open to the sky, each with a pole at the center. The pole and walls are of equal height, which is also equal to the radius of the structure. The floor and interior surface of the walls are inscribed with scales indicating angles of altitude and azimuth.
The Ram Yantra is used to observe the position of any celestial object by aligning the object in the sky with both the top of the central pillar, and the point on the floor or wall that completes the alignment. In the daytime, the sun’s position is directly observed at the point where the shadow of the top of the pillar falls on the floor or wall. At night, an observer uses a sighting guide to align the star or planet with the top of the pillar and interpolates the point on floor or wall where the imaginary line intersects.
The instrument is most accurate near the intersection of floor and wall. Here, the markings are at their widest spacing, and give an accuracy of +/- 1° of arc. The accuracy diminishes as one approaches the centre of the instrument. The Ram Yantra is constructed with twelve sectors, and has a similar instrument also with twelve sectors. The gap between the sectors allows the observer to move freely and make observations. The two structures complement each other in the sense that where one has a void, the other has the marked scale, and vice versa. The observer can move from one instrument to the other in order to take the readings.
A pair of Rama Yantra towers of much smaller scale stand to the west of the larger Rama Yantra. It is understood that the smaller versions were built first and served as models for the larger version.

The Jai Prakash is a complex instrument based on concept of a hemispherical sundial. It is a bowl-shaped instrument, built partly above and partly below ground level. The diameter at the rim of the bowl is 17.5 feet. The interior surface is divided into segments, and recessed steps between the segments provide access for the observers. A taut cross-wire, suspended at the level of the rim, holds a metal plate with circular opening directly over the center of the bowl. This plate serves as a sighting device for night observations, and casts an easily identifiable shadow on the interior surface of the bowl for solar observation.
The surfaces of the Jai Prakash are engraved with markings corresponding to an inverted view of both the azimuth-altitude, or horizon, and equatorial coordinate systems used to describe the position of celestial objects.
The Jai Prakash yantra is also comprised of two complementary structures which allow observers to freely access the inscribed surfaces to take measurements, as in the case of the Rama Yantra. In the Jai Prakash, the hemisphere is divided into sectors representing one hour of observation (15 degrees).

The Digamsa Yantra consists of two concentric cylindrical walls surrounding a central pillar. The top surfaces of the inner and outer walls are marked in angular divisions as fine as 1/10th of a degree. The instrument is used to determine the azimuth of a celestial object by sighting the object from the outer edge of the inner wall through the center of a crosswire suspended from the outer wall. The inner wall and pillar are about 1m in height, and the outer wall about 2m in height. Two cross-wires intersect at the center of the pillar. The observer aligns the point of intersection with the celestial object being observed, while a weighted string connected to the center of the pillar is cast over the outer wall in such a manner that it aligns perfectly with the line of sight. The azimuth markings on the top of the outer wall are accurate up to six minutes of an arc.

The Nadivalaya is an equinoctial sundial built in two parts—one facing south and the other north. It has two circular surfaces, parallel to the equatorial plane, with metal rods pointing toward the poles mounted at their centers.
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. 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.
Each dial plate is divided into three circular scales – two of which have markings for hours and minutes, while the third is marked for determining the zenith distance.

The Rasivalaya Yantra is also an equinoctial dial with gnomon aligned to the earth’s axis and quadrants parallel to the equator. It comprises a set of twelve instruments, each instrument being aligned with one of the twelve constellations of the zodiac.
The gnomon and quadrants of each instrument are aligned with the pole and plane of the ecliptic at the moment the first point in the constellation crosses the meridian. The gnomon points to the ecliptic pole when the zodiacal constellation transits the meridian. The celestial longitude of the object can be measured at that instant using the markings on the quadrants, while the latitude can be measured using the markings along the length of the gnomon. The angles of the gnomons vary from 3.5° (nearly flat) to 50.5° (quite steep).
Each of the gnomons of the Rasivalaya cluster has a tile with a pictorial depiction of the zodiac sign it is associated with, under the gnomon where it intersects with the quadrants.

The Unnatamsa Yantra measures the altitude (angular height) of a celestial object in the sky.
The large brass circle hung from the supporting beam is the measuring instrument of the Unnatamsa. The brass circle is pivoted to rotate freely around its vertical axis. The ring has two cross-beams in the vertical and horizontal directions. A sighting tube us pivoted at the center of the circle. It can be moved in the vertical direction to align it towards any celestial object. The vertical movement of the sighting tube, and the horizontal rotation of the brass ring can be adjusted to sight the celestial object of interest and read the altitude against the graduations marked on the rim of the circle.
The rim of the brass circle is divided to an accuracy of one tenth of a degree. Larger divisions of one degree and six degrees are also marked on the circle.

The Daksinottara Bhitti is a meridian dial. It incorporates a quadrant or half-circle inscribed on a north-south wall, with a metal rod at its center. It is an instrument built into a wall placed exactly in the north-south axis.
The instrument measures the altitude or angular height of a celestial object when it crosses the meridian. The meridian is the arc defined by extending the radial latitudes infinitely into space. The instrument uses either the semi-circular arc built on the western wall, or the intersecting arcs on the eastern wall, to measure the meridian altitude of a celestial object
The circular arc of the western wall and the intersecting quadrants of the eastern wall have ninety degrees of altitude marked ad the bottom of the sale, and zero degrees marked at its top. The altitude is measured with the help of the shadow of the rods (gnomon) fixed on either side.
The markings on the scales are in units of degrees and are further sub-divided providing an accuracy of two minutes of the arc for measurement.

The Krantivritta is an instrument that is meant to measure the celestial latitude and celestial longitude of an object in the sky. It is also used for measuring solar sign of the Sun in the day time. Tehe instrument consists of two brass circles of equal diameter pivoted together at one point on their circumference, maintaining an angle of 23.5° between them. The base circle is made of stone and is called the Nadivritta. It always remains parallel to the equator of the earth, while the outer brass circles can be rotated to be parallel to the ecliptic at any given moment. The Nadivritta has sixty degrees marked on its circular scale, whereas the twelve zodiac signs are marked on the outer brass circle. A sighting bar is pivoted to the outer ecliptic circle and can be rotated about the ecliptic pole when the outer circle is parallel to the ecliptic.
The sighting bar is rotated and brought into alignment with the celestial object. The readings on the quadrant if the sighting bar and the brass circle at this point, give the latitude and longitude of the object, respectively.

The Yantra Raj is an adaptation of an astrolabe. It is used for measuring ascendants, altitude, time, and the position of the Sun and that of some other celestial objects in the sky. It can also be used in the computations of celestial positions and their changes. The instrument also measures time from the positions of stars in the night, and that of the Sun during the day.
This circular instrument is marked with 360 degrees on it, with divisions in ghatis. 90 altitude circles, prominent stars, constellations, and celestial circles are also marked on the instrument, at their respective places. It has a separate attachment for a sighting tube to be fixed in the center of the instrument.

The Laghu Samrat Yantra is a scaled down version of the giant Samrat Yantra, and can give time to an accuracy of 20 seconds.

The Chakra Yantra is a ring instrument which measures the global co-ordinates of declination and the hour angle of a celestial object. Declination is the angular distance north or south from the celestial equator. The hour circle of a celestial object isa great circle that passes through the object and the celestial poles. The angle between an observer’s meridian, and the hour circle of the celestial body is the hour angle.
The ring is pivoted about a polar axis at the southern end of the instrument. A sighting tube is mounted to pass through the center of the ring. Using the movement of the whole ring about the polar axis, and the movement of the sighting tube about its perpendicular axis, a celestial object can be sighted.
The rim of the circle has markings for the 360 degrees, with each degree being further sub-divided into 10. The plate around the polar axis pivot has a scale of 60 ghatis. Once the celestial object is sighted, the position of the sighting tube on the two scales can be used to read the declination and the hour angle.

Trivia

Major restorations of the four extant observatories occurred between 1901 and 1923 under the direction of Gokul Chand Bhavan.

The Western system of hours, minutes, and seconds replaced the Hindu system of time measurement on some of the instrument scales during these restorations.

The beautiful marble surfaces at Jaipur were added in 1945 to replace the deteriorating plaster scales of the 1901 restoration.