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 Pir - Ghaib Observatory

A 14th century hunting lodge and Observatory built by Feroz Shah Tughlak.

The Oldest surving Astronomical Observatory in India?

(After writing the following and also giving out information in the press regarding the possibility of the Pir Ghaib being the oldest surviving astronomy observatory in India, a reading of the very popular book "The Great Arc" by John Keay has shown that, perhaps, the interpretation of the cylindrical structure and the hole in the roof of the monument as a 14th century Zenith tube is incorrect. The monument underwent some transformations when it was used as a survey tower by Everest, while making his baseline measurements for the Great Trigonometrical Survey. The history of this monument, even if it does not contain the oldest surviving astronomical instruments in the country, is yet very interesting. Work is still under progress to put together as much of the history of the monument as possible.)


There is so much of India’s astronomical heritage that lies hidden in monuments, sculptures and forgotten manuscripts. One such instance is in the heart of the northern ridge of Delhi in a monument known as Pir Ghaib. A place known for a “mysterious” feat of a saint who had disappeared from the monument.


This monument was built as a kushak (hunting lodge) and observatory by Feroz Shah Tuglak. Later legend turned it into Pir Ghaib. This is also a place that was the centre of action during the siege of Delhi in 1857. From the 11th of May to early June the Ridge was under the control of the citizens of Delhi and the sepoys. However, following the Badli ka Serai battle, the ridge was abandoned and later occupied as a strategic position by the British, who then lay siege to the walled city of Delhi. The Pir Ghaib was one of the places occupied by them and used as an outpost. The heavy battery used by the British, during the siege was stationed very near the Observatory. The lane behind the observatory was within easy access of firing from the walled city, and was termed the Valley of Death by the British soldiers (H.C. Fanshawe (1902), Delhi- Past and Present).


The British clearly identified the Pir Ghaib monument as an observatory, although there are also many disparaging British references to the monument as a “so called observatory”. The book Delhi: Past and Present, by H.C. Fanshawe mentions a report by an earlier British traveler Finch, who talks of a cylindrical structure in the Observatory and the presence of a globe and a half Moon on its top. There are two water colour drawings in the British Library, India Office at London, of the Observatory, made in 1857, by Francis Latter Tandy. There is also a sketch by J.R. Turnbull, of a view from the Flagstaff Tower, of the Observatory, the nearby Chauburji Mosque and Bara Hindu Rao’s house. (Copies of these sketches, if obtained from the India Office of the British Library and preserved in the archives of the Archeological Survey of India, would be useful for further historical researches in this connection).


Was it known as an observatory in the few centuries prior to 1857? There do not seem to be indications that Sawai Jai Singh, early in the 18th century, was aware of the presence of the astronomical structures of this monument. A search through source material on Indian Astronomical studies, that were used as reference by Sawai Jai Singh, would confirm or negate this inference. In the extensive studies made by Virendra Nath Sharma, of the source materials used by Jai Singh, there is no mention of the Pir Ghaib.


What exactly are the surviving astronomical structures of this monument that clearly identify this as an intended observatory (in addition to being a hunting lodge) as built by Firoz Shah Tughlak? One very identifiable structure is a cylindrical pillar coming vertically out of the roof of the monument. The upper surface of the cylindrical pillar could have been used as a dial for measuring the azimuth of the Sun or the Moon, by inserting a vertical pole into the small hole at the center of the circular disk covering the topmost surface of the pillar. Used in this way, this structure would have been a primitive analogue of the Ram Yantra built and perfected by Jai Singh. This inference, however, is speculative. Although there are historian’s accounts (Mohammad Habib and Khalik Ahmed Nizami) that talk of sundial usage from this observatory, it is not yet clear what exactly were the sundial structures in this momument.


The cylindrical structure on the roof of the Observatory - a Zenith Tube? The openings near the floor of the roof could have served as air vents or could have also been used for communication between observers placed in the room below and on the roof.
What is more interesting in connection with the cylindrical structure on the roof, is that, it is hollow and the hole in its centre looks down onto the floor in an apartment inside the monument. From this room, looking up into the hole in the cylindrical tube on the roof, one could observe the passing of stars that are exactly overhead at any given moment.


Not many celestial bodies would do that, from any given location. The Sun would never pass exactly overhead, as seen from Delhi, as it is to the North of the Tropic of Cancer. As seen from Delhi and all latitudes of 28.5 degrees to the North or South of the Equator, the Moon does appear exactly overhead, but, only once in about 18 years. These years are called the Major Lunar Standstill years. In addition to this, a few stars would have been seen to move exactly overhead, once in the night. Some faint stars of the Cygnus and Hercules constellations would have been good candidates for observers using a Zenith Tube from Delhi, during the period of Feroz Shah Tughlak. The passage of these stars overhead, observed through the Zenith tube (perhaps with cross wires placed across it) would have been used to correct the accumulated errors in the water clocks.


In the book on the Delhi Sultanate, by Mohammad Habib and Khalik Ahmed Nizami, there are references to the fact that the tower of this monument was used to keep a water clock that would mark time for an hour. The water container would be refilled after every hour and the time announced from the top of the tower. Using a sundial, the water clock would be corrected for errors once in the day. The Zenith tube or the hollow cylinder could also have been used to correct the water clock once in the night, when certain stars passed overhead. This usage would imply that a sophisticated understanding of the diurnal and annual movement of the stars and sundial equations, existed with the astronomers who must have been employed by Feroz Shah Tuglak. It is well known that the Astronomer Mahendra Suri, who wrote a treatise on the Astrolabe, worked in the court of Feroz Shah Tughlak. However, references to the usage of Zenith tubes for time measurement, by Mahendra Suri or other astronomers of that time, are yet to be found.


A very intriguing aspect of the Zenith Tube is the presence of two ventilator openings to the North and South of the cylindrical Zenith Tube, on the roof floor of the monument, as can be seen in Figure 2. These openings allow communication between the room below and the roof and could have been used in this way when observers were present in both of these locations. A more interesting possibility is whether, the astronomers and the architects involved in the design and construction of the Observatory had been aware of the need for ventilation in a Zenith Tube to ensure that refraction effects due to heating of air in the tube did not wash out the visibility of many fainter stars. There is a 1980 study of the effect of building design on the visibility of stars through a modern day photographic Zenith Tube by T.J. Rafferty of the US Naval Observatory, where he concludes that the absence of ventilation near a Photographic Zenith Tube would result in the decrease of visibility of a large number of faint stars. This study was related to modern day equipment that would see many faint stars, but, it is interesting to speculate whether the medieval observers working for Feroz Shah Tughlak were aware of such refraction effects for observations of naked eye stars and designed the opening in the Zenith Tube of the Observatory to minimize such effects!


What is the most interesting aspect of this monument as an observatory, is the fact that, this could perhaps be the only Zenith Tube in the world that was used to study the maximum northward swing of the Moon, during Major Lunar Standstill years. Every month the Moon makes North-South swings as it moves around the Earth and goes through its phases. The orbit of the Moon around the Earth is tilted to the orbit of the Earth around the Sun, by about 5 degrees. The Sun moves a maximum of 23.5 degrees North and South of the equator, in what we call the Uttarayana and Dakshinayana movements of the Sun. These maximum northward and southward movements of the Sun are relatively simple and denote the Summer and Winter Solstices. The north-south swings of the Moon are more complex. There are the Minor Lunar standstill years when the maximum swing of the Moon to the south or north will be 5 degrees subtracted from 23.5 degrees, or 18.5 degrees. There are the Major Lunar Standstill years when the maximum north-south swings of the Moon are 5 degrees added to 23.5 degrees or 28.5 degrees North or South of the Equator. These Major Lunar Standstills happen once in about 18 years. What this means is that from latitudes of 28.5 North or South of the Equator, it is once in 18 years that there is a possibility of the Moon appearing exactly overhead.

Well, Delhi is one location that is at such latitudes, and witnesses the Moon moving overhead only during the Major Lunar Standstill years. It is possible that the Zenith Tube of the Pir Ghaib was also used to observe the small differences in the position of the Moon from one Major Lunar Standstill to another, which could be used to refine the orbital elements of the Moon. This seems speculative, perhaps. The one additional aspect which points in this direction is that the Zenith Tube when seen from the floor below subtends an angle of about 0.5 degrees in the sky, which is also the angular diameter of the Moon. All in all, there is a lot of study needed to pinpoint the exact nature and usage of this observatory.

The Pir Ghaib is graded as Archaeological Value A in "Delhi The Built Heritage: A Listing'' by the Indian National Trust for Art and Cultural Heritage (INTACH). There are references here and there, to the fact that this monument is an observatory, in the book “Delhi Bole” by Vishnu Khanna, for instance. However, it is not generally recognized as an observatory, with some instruments surviving, and it also lacks recognition as possibly the oldest surviving observatory in the country. It is very necessary to recognize that this perhaps, is the oldest surviving astronomical observatory in the country and its upkeep looked into, keeping this in mind. At present, there are several cracks in the roof of the apartment that looks up into the Zenith Tube. The presence of “diyas” in the apartments which seem to be lighted regularly, needs to be studied for possible damage to the interiors of the monument. The southern apartment in the monument is overtaken by bats and other animals, which makes it difficult to study possible astronomical alignments through the various windows of the monument.


Nehru Planetarium is investigating the nature of the possible astronomical structures in the Pir Ghaib Observatory in further detail. There is a need to investigate and document the vast astronomical heritage hidden in many of our monuments and bring this heritage alive.

Written by Dr. N. Rathnasree, Director, Nehru Planetarium and Anurag Garg, Educator at the Nehru Planetarium, who are investigating the Astronomical usage of the Pir Ghaib Observatory.