One of the main attractions in the so called ‘Pink’ city of Jaipur is the World Heritage Site of Jantar Mantar astronomical observatory. This impressive collection of astronomical instruments were built by Sawai Jai Singh, a Mughal commander, dated 1728. Sawai Jai Singh built five observatories, one in Jaipur, Delhi, Ujjain, Banaras (Varanasi) and Mathura. By far the Jantar Mantar in Jaipur is the biggest and the best preserved. This was the place where Sawai Jai Singh himself used to sit for astronomical observations.
A walk among these mostly massive structures is a must in Jaipur, even if you are not familiar with astronomical concepts. It really is an 18th century architectural exhibit of acumen and brilliance. Let’s move on and ‘analyze’ the single instruments, but first a brief introduction of Jantar Mantar.
Brief introduction of Jantar Mantar
In 1718 the ruler Jai Singh thought of constructing an observatory and for it he studied several books on the subject of astronomy. He wasn’t alone in this arduous adventure, he was helped by Pt. Jagannath Samrat (a true polyglot) and also by Pt. Keval Ramji, a numerical Ephimerista person who studies the daily motions and positions of the planets. The two scholars helped the Maharaja in fulfilling his great ambition by translating in Sanskrit several authoritative works on mathematics and astronomy, written in various languages. Sawai Jai Singh sent the scholars to several foreign countries like Greece, Britain, Arabia Peninsula, and Portugal to study the science of Astronomy. Those journeys had a huge impact on the realization of Jai Singh’s dream.
(Unnat: elevated, Amsa: division, degree of arc)
Unnatamsa is an instrument for measuring altitude – the angular height of an object in the sky.
The large graduated brass circle hung from the supporting beam is the measuring instrument of the Unnatamsa. The brass circle is pivoted to rotate freely around a vertical axis. The ring has two cross beams in the vertical and horizontal directions. A sighting tube is pivoted at the centre of the circle. It can be moved in the vertical direction to align it towards any celestial object. This pivoting of the Unnatamsa is analogous to the AltAz mounting of a modern telescope.
The vertical movement of the sighting tube and the horizontal rotation of the brass ring can be adjusted to sight the celestial object and read the altitude against the graduations on the rim of the circle.
The rim of the brass circle has graduations marked in such a way that the smallest division is one tenth of a degree. Larger divisions of 1 degree and 6 degree are also marked on the circle.
(Dakshinottara Bhitti: meridian wall)
Dakshinottara Bhitti is an instrument built into a wall placed exactly in the north-south direction.
The instrument measures the altitude or the angular height of a celestial object when it crosses the meridian. The meridian is the arc defined by the norht, south, and the zenith. When an object crosses this arc in the sky, halfway between its rising and setting times, it is said to transit the meridian.
The instrument uses either the semi-circular arc built on the west facing wall, or the intersecting arcs of the east facing wall, to measure the meridian altitude of a celestial object.
The circular arc of the west facing wall and the intersecting quadrants of the east facing wall have 90 degrees of altitude marked at the bottom of the scale and 0 degree altitude marked at its top. Altitude is measured with the help of the shadow of the rods (gnomon) fixed on either sides.
The marking on the scales are in units of degrees that have been further subdivided into 10 main divisions each with further subdivisions of 3 small units which yield a least count of 2 minutes of arc for the instrument.
(Vrihat: large/great, Samrat Yantra: supreme instrument)
The Vrihat Samrat Yantra is a sundial that can give the time to an accuracy of 2 seconds.
The shadow of the triangular wall, which is placed in the north-south direction with an angle equal to the latitude of this location (27 degrees north), moves equal distances in equal time intervals, on the eastern and western side quadrants. This movement is calibrated to read the tocal time. The western and eastern quadrants are divided into 6 hours each, for the morning and the afternoon segments respectively. Each hour is divided in 15 minutes and later in 1 minute parts. The minute part has ten subdivisions each of 6 seconds each of which again has three small divisions of 2 seconds each.
The corrections factor which is to be added for the day is desplayed in the Observatory. It is used to convert the time obtained from this instrument to the clock time.
The triangular wall of the Yantra has scale marked 0 degrees to 23 30’degrees north and south for measuring the declination, which is the angular position of a celestial object with respect to the equator.
The Vrihat Samart Yantra is the largest of all of Jai Singh’s instruments, with a height of about 90 feet and a base length of 147 feet. The radius of each of the quadrants is 49 feet and 10 inches.
(Sasthamsa : sextant)
The Sasthamsa Yantra, or sextant, are two huge semi circular concave arcs lying in the meridian. There are two identical units of the Sasthamsa, one each inside the eastern and the western supporting structures of the Samrat Yantra.
Each unit of the instrument has two semi circulars scales for measuring the declination and zenith distance respectively. It also has two pinholes placed high above. A circular image of the Sun is projected onto the scales for some time (approx 2 minutes), through the pinholes, when the Sun transits the meridian. The altitude and zenith distance are measured on the scales as per the image location. When this image is captured on a white paper sheet, the black spots of the Sun and the vibrations in its image as it moves, are clearly visible.
The circular scales have markings to note the declination or the angular distance from the equator, as well as the zenith distance or the angle away from the zenith.
The declination scale of the Sasthamsa reads values between 23° 30′ degrees north and south. The zenith distance scale is marked from 0 to 60 degrees. The scales of the Sasthamsa are divided into degrees and minutes with the least count of 1 minute.
The Digamsa is a cylindrical instrument that has a simple method of determining the azimuth of a celestial object. Digamsa or the azimuth of a celestial object is the relative angular position of the object measured eastwards, starting from the direction north.
The instrument consists of a small knob (gnomon) placed in the centre of three co-axial cylinders. To determine the azimuth of a celestial object at night, an ordinary string is needed to be attached to the central knob. The other end of string is suspended over one of the outer cylinders, using conventional weights. The circular rim of the cylinders is marked into 360 degrees and further subdivisions to indicate the azimuth.
The string is moved over the rim of either of the outer cylinders and aligned to sight the celestial object. By this process, a vertical plane is defined that contains the object and a point on the horizon. The azimuth can be read from the marking scale on the rim of the cylinder where the string is resting.
Likewise, to measure the azimuth of the sun, cross wires are stretched on the outer cylinder in east-west and north-south directions. Shadow of the center of cross wires on the circular scale tells the azimuth.
(Nadivalaya : equatorial instrument/ circular dial)
The Nadivalaya has two circular plates, facing north and south which are its dials. The wall of the plates is inclined towards the south at such an angle that the instrument remains parallel to the plane of Earth’s equator. The rods(gnomom) emerging perpendicularly from the plates are parallel to the axis of rotation of the Earth. The shadows of the rods move along the scales on the dial plates, indicating the local time.
Each dial plate is divided into three circular scales, two of which have markings for the hours and minutes. The third scale is marked for the determination of the ghatis and palas (zenith distance). To the time indicated by the circular scale, a correction factor as displayed for the day at the observatory, needs to be added to obtain the clock time.
The dial plate facing south is sunlit from the autumn equinox to the spring equinox and is to be used for telling the time. The dial plate facing north is sunlit from the spring equinox to the autumn equinox and is to be used for the purpose.
(Laghu : small, Samrat Yantra : supreme instrument)
The Laghu Samrat Yantra is a sundial that can give the time to an accuracy of 20 seconds.
The shadow of the triangular Wall of the Yantra falling on the eastern and western side quadrants, tells the local time. The triangular wall, with the angle inside the wall equal to the latitude of this location is placed exactly in the north-south direction.
The shadow of the triangular wall moves equal distances in equal time intervals on the quadrants. This movement is calibrated to read the local time.
The western and eastern quadrants are divided into subdivisions, each of 6 hours, for the morning and the afternoon segments respectively. Each hour is further divided into four 15 minutes divisions which are subdivided into 5 minutes and 1 minute divisions; and each 1 minute division is subdivided into 3 divisions each of 20 seconds.
The correction factor, to be added to convert the sundial time to the clock time for the day is displayed near the instrument.
The triangular wall of the Yantra has a scale marked 0 degree to 23 1/2 degrees north and south for measuring the declination or the angular position of a celestial body with respect to the equator.
(Dhruva : Pole Star, Darshak Pattika : viewing plate)
The Dhruvadarshak Pattika is perhaps the simplest of all the instruments found in the observatory. It is in the form of a small trapezoidal structure whose upper surface points towards the Pole Star on a clear dark night.
The angle in the triangular wall is supposed to be equal to the latitude of the observatory’s location (27 degrees north). The instrument is erected on a 3.07 m long and 54 cm wide stone masonry base. Its lower end is about 76 cm above the ground and the upper end 2.32 m.
This instrument was the compass of an earlier age.
(Chakra: circle, Yantra: instrument)
The Chakra Yantra is a ring instrument which measures the global coordinates 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 the celestial object is a 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 passing through the centre of the ring is mounted. Using the movement of the whole ring about the polar axis and the movement of the sighting tube about the perpendicular axis, a celestial object can be sighted.
The rim of the circle has scale of 360 degrees with each degree division divided into 10 subdivisions. The plate around the polar axis pivot has 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.
(Ram: a name, Yantra: instrument)
The Ram Yantra can measure the local coordinates of altitude and azimuth of a celestial object.
The angular height of an object, from the horizon is the altitude. The azimuth is the relative angular position of the object measured eastwards, staring from the direction north. The complementary units are so designed that the shadow of the gnomon falls on a Sector of one of the instruments if it falls in the gap for the other instrument.
When the shadow falls at the top of the wall of the instrument, the altitude of the sun is zero. When the shadow is at the junction between the wall and the floor, the altitude of the sun is 45 degrees. Altitudes between 45 to 90 degrees can be read in a radial direction on the floor of the instrument.
The circular ring (horizontal circle) near the roof of the instrument has 360 degrees scale for the azimuth, along the circumference. Each degree division is further divided into minute divisions and the smallest division is one fifth of a degree.
(Rasivalaya: zodiacic circle)
Rasivalaya are instruments for measuring the celestial latitude and longitude of the celestial bodies. There are twelve instruments which represent the twelve signs of the zodiac, one for each measurement to be done when the corresponding sign of the zodiac transits the meridian.
Each unit consists of a triangular gnomon and a quadrant perpendicular to it, analogous to the Samrat Yantra. However, these differ with each other regarding the shape, size and the angle of the gnomon.
Celestial longitude of an object is measured along the ecliptic, while the angular distance of an object to the north or south of the ecliptic is its celestial latitude. Ecliptic is the annual apparent path of the sun in the sky.
The gnomon of a Rasivalaya Instrument points to the ecliptic pole, when the zodiacal constellation corresponding to that Rasivalaya transits the meridian. At that instant, the celestial longitude of the object can be measured using the quadrant of the Rasivalaya while the markings along the length of the gnomon can be used to determine the celestial latitude.
The Rasivalaya is found only in the Jaipur Observatory.