The Ram Yantra can measure the local co-ordinates 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, starting from the direction North.
Each of the cylindrical Ram Yantra consists of a circular wall, floor sectors and a gnomon at the center. The gaps inside the instrument are for facilitating the movement of observers to read the markings and hence, the complimentary instrument is designed in such a way that, the shadow falls on a sector of one of the instruments, if it falls in the gap for the other instrument. Spliced together, the two instruments make a whole cylinder whose inner wall and the floor is a complete reflection of the sky above.
The complete circumference of the Ram Yantra Cylinder encloses 360 degrees of Azimuth. This is then divided into different segments of Azimuth, in the wall and floor sectors of the instrument. The 12 wall and floor sectors inside each of the two cylindrical instruments, are of unequal width. The East Ram Yantra has 12 degree wide sectors separated by 18 degree wide gaps, while the West Ram Yantra has 18 degree wide sectors separated by 12 degree wide gaps.
Azimuth angles are marked on the floor and walls of the Ram Yantra as sectors. The circular ring near the (non-existent) roof of the building has markings for the Azimuth. The smallest division is one fifth of a degree. Azimuth is zero towards the North and increases eastwards.
Below, is a view from inside the west Ram Yantra. Counting the boxes in the horizontal direction, on the walls shows that there are 18 divisions here corresponding to 18 degrees of Azimuth.
The height of the walls and the gnomon has been designed to be exactly equal to the inside radius of the cylinder measured from the outer circumference of the gnomon.
It is the Altitude markings in tangents of scale, that form the beautiful simplicity of usage of this instrument. When the shadow falls at the top of the wall of the instrument - the Altitude of Sun is zero.
When the shadow is at the junction between the wall and the floor, the Altitude of the Sun is 45 degrees. Another 45 degrees are marked inward from the wall towards the circumference of the Gnomon - so that, altitudes between 45 to 90 degrees can be read off on the floor of the instrument. The scale is not linear any more (unlike for the Azimuth) - the length of the shadow and the height of the gnomon (equal to the length of the floor sector) give the required Altitude.
Tan (Altitude) = Gnomon Height / Shadow Length