Jantar Mantar Observatory
The Rashivalaya instruments were mentioned earlier as examples of sundials. However their orientation is unusual, since they do not point due north. This is a clue to their purpose, which is to calculate sidereal, rather than solar, time. The advantage of using sidereal coordinates is that they depend only on the annual orbit of the earth around the sun, not on the earth's daily rotation.
Sidereal time is measured relative to the ecliptic, the path of earth's orbit across the heavens. The ecliptic is divided into 12 parts for convenience, each part named after a constellation that is located there. The 12 constellations are called the "Zodiac". (See Basic Celestial Phenomena for more information about this.)
In the Rashivalaya Yantras, each of the 12 instruments is associated with one of the 12 signs of the zodiac.
For any given instrument, say "Pisces", the curved scale of that instrument lines up with the (1/12) part of the ecliptic that contains the constellation Pisces. The twelve instruments, acting together, are therefore actually a single instrument that covers the entire ecliptic.
To make an observation, first choose the instrument that covers the constellation currently occupied by the sun (the Jai Prakash yantra will tell you which constellation this is). Then, the position of the gnomon's shadow on the scale gives the sidereal time - that is, the celestial longitude of the sun in sidereal coordinates.