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Site: Jantar Mantar Observatory instruments for Time Measurement

N. Rathnasree, Nehru Planetarium, Nehru Memorial Museum and Library, New Delhi

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In the main, the Samrat Yantra constructions of the Jantar Mantar observatories have been designed as equal hour sundials for time measurement. The Samrat Yantra have built in capabilities to make certain equatorial co-ordinate measurements as well. The Samrat yantra finds a representation in all the extant Jantar Mantar observatories - those at Delhi, Jaipur, Ujjain and Varanasi. At Jaipur, there are two versions of the same construction - the Laghu Samrat Yantra (with a least count of 20 seconds in time measurement) and the Brihat Samrat Yantra (with a least count of 2 seconds, in time measurement).

Brihat Samrat Yantra of the Jaipur Jantar Mantar Observatory

At Delhi, the Samrat Yantra is very similar in structure to the Jaipur and other observatory designs, however, in its present state, the central half of the instrument has been covered over with cement during a 20th century maintainence and thus, a part of the quadrant remains missing.

The Samrat Yantra principle is also incorporated on the eastern and western edges of the Misra Yantra, at the Delhi observatory.

The Misra Yantra of the Delhi Jantar Mantar Observatory. The eastern and western triangular walls and the attached quadrants function analogous to the Samrat Yantra

So, how do these Samrat Yantra work towards a measurement of time? They work as an equinoctial or equal hour sundial. The term equinoctial refers to the fact that the quadrant arcs of the sundial are in the plane of the equator. The triangular central wall of the instrument is oriented such that it points towards the North celestial pole. The angle in the right angled triangle is made to be equal to the latitude of the location. With such a configuration, on the quadrant arc, the shadow of the triangular wall moves equal distances in equal intervals of time. It is this movement that is used to tell time.

When the shadow of the triangular gnomon falls on the upper end of western quadrant, it is 6:00 AM local solar time. At solar noon the shadow disappears from the western quadrant and reappears on the eastern quadrant. At 6:00 PM Solar time, the shadow hits the topmost point of the scale marked on the eastern quadrant.

Solar time, however, is not the same as our clock time. There is a correction factor that has to be added to the solar time to convert it to the clock time. This correction factor is different for different dates of the year. The Jaipur observatory shows the correction factor to be applied for each day of the year, near the Laghu Samrat Yantra.

The correction factors come about, from two causes. One is a constant correction which needs to be applied on account of the fact that the whole country uses one longitude line as a reference for time, while a sundial tells time according to the local meridian and thus according to the exact longitude of the location of its construction.

There is yet another correction factor which comes into play, from a combination of the fact that the orbit of the Earth around the Sun is an ellipse (this is negligible) and the fact that the spin axis of Earth's rotation is tilted with respect to its orbit plane. This results in a different correction to be applied every day of the year to synchronise sundial time with civil time.

There are many online ephemeris calculators which would give the correction factor for any date and location, which could be used to obtain this correction factor by anyone wishing to make time measurements with any of the extant Samrat Yantra instruments of the Jantar Mantar observatories at Delhi, Jaipur, Varanasi and Ujjain or from a constructed model at any other location. This site of the NOAA, for instance, gives the Solar Noon time for any given location and date. The difference of the Solar Noon time from 12:00 hours is the correction factor to be applied for any time measurement from that location, for that date.

Using any of the time measuring instruments of the observatories, the sundial time can be measured and then converted to clock time using the correction factor obtained from such ephemeris tables.

There are other instruments of the observatories with variations of constructions, which are also meant for meaurements.

One such instrument is the Nadivalaya, present in the Jaipur and Ujjain observatories.

The South facing dial of the Nadivalaya at the Jaipur Jantar Mantar observatory

This is a version of an equinoctial sundial, which is different from the design of the Samrat Yantra. The two circular plates that are facing the North and the South are inclined towards the South at such an angle that they are parallel to the plane of the equator of the Earth. At the centre of the circular dials are rods that are projecting perpendicularly outwards. These rods are then parallel to the axis of rotation of the Earth. The movement of the shadow of these rods, on the circular dial, indicates the time.

From the Autumn Equinox to the Spring Equinox, the dial plate facing South will be sunlit and is to be used for telling the time. From the Spring Equinox to the Autumn Equinox, the dial plate facing North is sunlit and is to be used for telling the time. Around the time of the Equinoxes, both the dial plates are seen to be sunlit.

Time measurements with this instrument are usually simple, except that, sometimes the shadow of the rod may fall woefully short of the arc with the scales drawn on it, and a mental extrapolation of the shadowline to the scales on the arc becomes necessary.

There have been a number of activities and observing programs conducted by the Nehru Planetarium, New Delhi, to engage students and interested visitors towards taking time (and other) measurements with the instruments of the Delhi, Jaipur and Ujjain observatories.

Here are some obserations of time, with the Nadivalaya at the Jaipur observatory, which are a part of the periodic reports on the usage of the Jaipur Jantar Mantar observatory instruments, being submitted to the UNESCO, by the Nehru Planetarium, New Delhi.

Date of ObservationsNadivalaya TimeClock Time
 (After applying correction) 
May 9th 201114:20:4014:24:17
(Correction : 23m10s)  
May 10th 201107:21:4807:18:59
(Correction : 23m8s)  

(To be completed)

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Page last modified on January 05, 2015, at 10:30 AM EST