JANTAR MANTAR (ASTRONOMICAL OBSERVATORY)

Jai Singh II designed this strange complex of seventeen buildings between 1728 and 1734. Passionate about astronomy, he designed most of these measuring instruments. The Jantar Mantar is a UNESCO World Heritage Site since 2010. The purpose of all these instruments was to calculate the position of the planets in order to establish astral themes and to define the dates of major events (weddings, travel ...). We present here the main instruments. Their usefulness and instructions for use are sometimes complex and we strongly recommend that you seek the services of a guide.

Laghu Samrat Yantra: the ramp of this sundial points directly towards the North Pole. The shadow cast by the sundial allows one to know the exact time on the inlaid marks of the curved scale.

Dhruvdarshak Pattika: this small trapezoidal instrument served as a compass. The ramp points to the North Star and the angle of the triangle is equivalent to the latitude of the observatory (27° N).

Nadi Valaya Yantra: this is a rather complex sundial made of two discs, one facing south and the other north. They are tilted to be parallel to the plane of the equator. The needles placed in the plates are parallel to the axis of rotation of the earth. Their shadow projected on the graduated dials allow to know the minutes and the time. On the other hand, a correction factor must be applied to know the exact time. The south-facing plate gives the time from autumn to spring, while the north-facing plate gives the time from spring to autumn.

Unnantansha Yantra: it allows to measure the altitude of an object in the sky. It is a large graduated bronze ring, suspended from a low wall. It is crossed by a horizontal and a vertical axis. In the center of the ring, a rotating telescope can be placed and moved along the vertical axis. The rotation of the ring and the placement of the bezel allow to position oneself on any celestial body and to read its altitude on the graduated dial.

Dakshinottar Bhatti Yatra: this instrument measures the altitude or angular height of a celestial body when it passes the meridian (at its zenith). It is a high wall built in the north-south axis. To calculate the height of the angle, the observer can use either the semicircular arc at the bottom of the western wall or the intersection of the arcs on the eastern wall. The height is measured by the shadow cast by the needle planted on each side of the wall.

Vrihat Samrat Yantra: this impressive windowed wall can tell the time with an accuracy of two seconds. It is placed in a north-south axis with the angle of the top equal to the latitude of the position of the Jantar Matar. The shadow of the triangle moves at a constant speed on the dials placed in the east and west. These dials are subdivided into ranges of 6 hours, themselves subdivided into equal parts to arrive at a marking of 2 seconds.

Shashtansha Yantra: there are two examples placed on either side of the Laghu Samrat Yantra. This sextant is used to measure the latitude and the distance of the sun's zenith point. When the sun transits the meridian, its rays pass through two holes placed at the top of the instrument. A round shadow is projected on the semi-circular scale.

Rashivalaya Yantra: this set of twelve instruments allows to measure the latitude and longitude of a celestial body. Each instrument represents a sign of the zodiac and allows measurements to be taken when the sign transits the meridian. Consisting of a gnomon (which will project the shadow) and a dial perpendicular to it, on the same principle as the Laghu Samrat Yantra.

Jai Prakash Yantra: this bowl planted in the ground represents a map of the sky bordered by the zodiacal zones. A small ring is stretched over the bowl and the shadow cast indicates the position of the sun in the sky. The cutouts in the sky map allow someone to stand inside the bowl and take measurements.

Chakra Yantra: this ring-shaped object allows to measure the coordinates of the declination and the angle of a celestial object. The declination is the angular distance of a celestial body from the equator.

Kapali Yantra: the sun eclipses all other planets and stars during the day. What would the sky look like if we could observe everything at the same time? This is the idea of the Kapali Yantra which works on the same principle as the Jai Prakash. Two concave hemispheres are buried in the ground. Each of them presents a map of the sky, with planets and stars, their positions and movements.

Ram Yantra: it allows to measure the coordinates of the altitude and azimuth of a celestial body. When the shadow falls on top of the wall, the position of the sun is 0°; at the intersection of the wall and the ground, its position is 45°. The altitudes between 45° and 90° are measured by the radials on the ground.

Digansha Yantra: the hemisphere planted in the ground is used to calculate the azimuth of a celestial object. The azimuth is the angle between the direction of an object and the reference direction. Here, the reference direction is north and the direction of the object is east.