Chakra Yantra Jantar Mantar Observatory

The Chakra Yantra is the pair of upright metal circles in the photo. They stand between a pair of hemispheres (bowl shapes) lowered into the ground in front of and behind the metal circles. The pair of hemispheres is the Kapali Yantra, discussed on the next page.

As for the Chakra Yantra, it is an instrument for finding the right ascension and declination of a planet or other celestial body observed at night.

To understand these coordinates, it may help if you visualize the earth's lines of longitude and latitude projected up onto the sky, such that the north star corresponds to earth's north pole. Earthly latitude is the angular distance north or south of the equator, and earthly longitude is the angular distance around the polar axis as measured from any arbitrary starting point - in this case, the meridian of Jaipur.

The projection into the heavens of earth's latitude lines is called the "declination", and the projection of earth's longitude lines is called the "right ascension". (See Basic Celestial Phenomena for more information about this.)

Now say you want to locate a planet in the sky. You do that the same way you would locate a spot on earth, by moving down from the north pole to the correct latitude and then moving around that circle of latitude until you reach the correct longitude. When you do this on the celestial sphere instead of the earthly one, you get the celestial coordinates of declination and right ascension.

The Chakra Yantra lets you measure these coordinates. The axis of the metal circle (I will discuss only one of the pair; the other is, apparently, the same) is perpendicular to the crossbar (notice where the axis is attached to the upright posts). The axis points towards the north pole, and the circle pivots around this axis. In the middle of the circle is a hole for attaching a sighting tube, which pivots around the circumference of the circle. (You don't see a sighting tube in the photo, because one is not currently attached to the instrument, so just pretend that it's there.)

To find a planet, imagine that the whole contraption, circle and sighting tube, is aimed at the north star (perpendicular to the crossbar), and that the planet is (let's say) 10 degrees below and 20 degrees to the right of the north star. Imagine how you have to move the sighting tube to see the planet. First of all, you pivot the sighting tube 10 degrees down the metal circle. This is the declination, 10 degrees below north, marked as such on the circle. Then you have to pivot the metal circle 20 degrees counterclockwise around its axis. This gives you the right ascension, which is marked on a disk at the base of the axis.