JANTAR MANTAR (ASTRONOMICAL OBSERVATORY)

Jai Singh II designed this strange complex of seventeen buildings between 1728 and 1734. A keen astronomer, he designed most of the measuring instruments. The Jantar Mantar has been 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 define the auspicious dates for major events (weddings, travel, etc.). Here we present the main instruments. Their use and instructions are sometimes complex, and we strongly recommend that you enlist the services of a guide.

Laghu Samrat Yantra: the ramp of this sundial points directly towards the North Pole. The inlaid markings on the curved scale show the exact time.

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 observatory's latitude (27° N).

Nadi Valaya Yantra: this is a rather complex sundial made of two discs, one facing south and the other north. They are inclined so as to be parallel to the plane of the equator. The hands on the plates are parallel to the earth's axis of rotation. Their shadows are projected onto the graduated dials, enabling us to tell the minutes and hours. However, a correction factor must be applied to determine 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: used to measure the altitude of an object in the sky. It consists of a large graduated bronze ring suspended from a low wall. A horizontal and a vertical axis run through it. At the center of the ring, a pivoting telescope can be placed and moved along the vertical axis. Rotation of the ring and placement of the telescope allow you to position yourself on any celestial body and read its altitude on the graduated dial.

Dakshinottar Bhatti Yatra: this instrument measures the altitude or angular height of a celestial body as it passes the meridian (at its zenith). It is a high wall built on a north-south axis. To calculate the height of the angle, the observer can use either the semi-circular arc at the bottom of the west wall or the intersection of the arcs on the east wall. Altitude is measured using the shadow cast by the needle planted on either side of the wall.

Vrihat Samrat Yantra: this impressive windowed wall can tell the time to within two seconds. It is set on a north-south axis, with the angle of the apex equal to the latitude of the Jantar Mantar's position. The shadow of the triangle moves at a constant speed over the dials to 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 on either side of the Laghu Samrat Yantra. This sextant measures the latitude and distance from the sun's zenith. When the sun transits the meridian, its rays pass through two holes placed at the top of the instrument. A round shadow is cast on the semicircular scale.

Rashivalaya Yantra: this set of twelve instruments measures the latitude and longitude of a celestial body. Each instrument represents a sign of the zodiac and enables measurements to be taken when the sign transits the meridian. Consisting of a gnomon (which casts 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 cut-outs in the sky map allow someone to stand inside the bowl and take measurements.

Chakra Yantra: this ring-shaped object measures the declination and angle coordinates of a celestial object. 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? That's the idea behind the Kapali Yantra, which works on the same principle as the Jai Prakash. Two concave hemispheres are buried in the ground. Each presents a map of the sky, with planets and stars, their positions and movements.

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

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