16th Century Astronomy & Astrology

Jantar Mantar means “Instrument for Calculation”. Jai Singh, founder of Jaipur built five sites, each with multiple instruments for astronomical observation. We visited the one in Jaipur which is quite well perversed including the world’s largest sundial, shown here. The edge of the inclined ramp casts a shadow either to the east or the west depending on the time of day. The slope is carefully constructed to be parallel to the earth’s axis of rotation. Jaipur is at 27 degrees north latitude so the angle to the ground is also 27 degrees.


The basic principle of operation is more easily seen in the second picture of the “small” (prototype sundial which was built first and) which is 1/10 the size of the big one. The shadow of the long ramp falls on the inside of the cylindrical dial (being examined by the tourists in the second picture). The cylinder is split in two and spread apart so that each edge of the ramp is exactly at the center of the corresponding quarter cylinder. The cylinder of the large sundial has marks representing 2 second time intervals. The graduations are carved in white marble and so are still quite sharp. The smaller sundial is only marked with 20 second time intervals. It was overcast and drizzling when we visited so we could not see the operation of the sundial. The sun subtends an angle of about half a degree which corresponds to about 2 minutes of time at the earth rate of rotation. So it seems that it would be difficult to read the sundial to even 20 seconds of resolution, let alone 2 seconds.


There is a second instrument near the "small" sundial which consists of two flat, circular surfaces perpendicular to the earth’s axis, one facing north, the other south. By observing which face is illuminated by the sun you can tell whether the sun is north or south of the equator. There is a post in the center of each face and marks by which you can also tell the time, albeit much less precisely than with the large sundials.

This instrument is designed to measure the position in the sky of the sun at anytime of the day. It consists of small hole in a disk suspended by wires over a hemispherical dish. This picture shows only half of the instrument. The other half, which is located nearby, completes the hemisphere because sections which are missing in this part are present in that one and vice-aversa. The reason strips are missing is to allow an observer to get close enough to the inside surface of the hemisphere to read the location of the spot at the center of the disk. The sundials also have marks on the edges of the ramps so an observer could in principle make the measurement of the sun’s declination using those marks. But this instrument is marked with two sets of spherical coordinates, one centered about the vertical and another centered a line parallel to the earth’s polar axis, eliminating tedious calculations. When discussing this instrument, the guide explained that Indian astrology is much more precise than western astronomy. This is because it accounts for not only your zodiac sign but position in the sky of the sun at the time of your birth.

There is a second two part instrument which appears to allow the same measurements. Is consists of two large, vertical cylinders with poles in the center. The walls and floor of the cylinders is marked with radial divisions and vertical divisions so the altitude and azimuth of the sun can be read from it’s shadow.



There is also an instrument dedicated to observations related each zodiacal constellation. They appear to be similar to the large sundials and each has a ramp with a different slope and is pointed in a different azimuthal direction. The guide explained that each points at a different elevation because of the different positions of the zodiacal constellations relative to the equator. However she did not know why they are aligned in different azimuthal directions or exactly how they were used.