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(This narration was also submitted to Mrs. Dharmendra Kanwar, to be considered for inclusion, in the Rupa Publishers book on the Jantar Mantar, Jaipur)

The English humorist Hilaire Belloc said,

I am a sundial, and I make a botch Of what is done far better by a watch

Well, he said it because he had not measured time with Jaipur’s Brihat Samrat Yantra, the largest sundial in the world!

The Brihat Samrat Yantra is a gigantic sundial built into an excavation in the ground. The movement of the shadow of the triangular wall, on the arched segments, has been calibrated to read the time to an accuracy of 2 seconds, on this instrument!

The triangular wall of the instrument is the gnomon. In any sundial, the gnomon is that part of the instrument, whose shadow can indicate the position of the Sun in the sky.

The triangular wall has been placed exactly in the North-South direction. The angle inside the sloping wall of this triangle, is supposed to be equal to the latitude of the observatory. This ensures that the gnomon points towards the North Celestial Pole. This is the imaginary point reached, when the axis of rotation of the Earth, is projected on to the sky.

The “Staircase to Nowhere”, in fact, points to the Pole Star, or the Dhruv Tara!

The arched segments (known as quadrants) on either side of the triangular wall are perpendicular to the inclined wall of the Gnomon, and are therefore, parallel to the plane of the Equator of the Earth.

What is achieved by this configuration of the sundial? The shadow of a gnomon pointing towards the North celestial pole, falling on the arched quadrants parallel to the equator, moves equal distances in equal intervals of time. This movement can therefore be calibrated in a very simple manner, to read the time. This is one version of an equinoctial or an equal hour sundial. When you place a gnomon or a pole in an arbitrary direction, its shadow does not move equal distances in equal intervals of time. You can sense that by studying the shadow movement around a cricket wicket or any vertical pole.

Let us now, look at the way that the Brihat Samrat Yantra gives us the 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.

The quadrant arcs, therefore, are divided into 6 hours each, for the morning and the afternoon segments. The hours are sub divided into 1 half hour and further into 1 minute divisions. The 1 minute divisions are divided into 10 divisions, each of which are further subdivided into 3 divisions. The instrument can therefore read solar time to an accuracy of 2 seconds.

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. The correction factor arises from the fact that the axis of rotation of Earth, is tilted with respect to the plane in which it revolves around the Sun, and from the fact that the orbit of the Earth around the Sun is an ellipse and not a circle. The sundial therefore runs sometimes slower and sometimes faster than a civil clock, which keeps the same interval of time for every day of the year. This correction factor, therefore, is different for different dates of the year.

It is also to be kept in mind, that the instruments of the Jaipur Jantar Mantar observatory, give the Local Solar Time, with respect to the longitude of Jaipur. The Indian Standard Time actually uses a longitude of 82.5 degrees as the standard longitude. There is therefore, an additional constant correction factor that has to be added to the sundial time, as seen from the Jaipur instruments. The black board placed under the Gnomon of the instrument, by the Observatory staff, gives the correction factor for the day, which takes into account both of these corrections.

There are some factors related to the possible accuracy that can be achieved, while observing with this gigantic instrument. Shadows tend to be not so sharply defined when cast from such a great height and spread over such large dimensions. Any sundial shadow has the darker umbra and the lighter penumbra, arising from the fact that the Sun is not a point source of light, but, is an extended object. If one is standing very close to the quadrant, in order to mark the exact position of the shadow, the distinction between the umbra and the penumbra seems to disappear. This makes it difficult to pinpoint the exact location of the edge of the shadow.

The difficulty may be eliminated by using a short of piece of string. This is done by superimposing on the penumbra, the shadow of a thin object such as a needle or a string. By holding a one to two cm long taut string parallel to the shadow edge, about one cm or so above the instrument's surface, and reading the scale where the string's shadow merges with the shadow of the gnomon edge, it would be possible to determine time with the intended accuracy of the instrument.

So, go ahead, observe the shadow on the quadrant carefully, note the solar time, add the correction factor, and then check with the correct clock time. You will be surprised by the accuracy of this 300 year old instrument!

The Brihat Samrat Yantra has yet another functionality, built into its triangular gnomon and the arched quadrants. It can measure the angular position of the Sun in the sky, with respect to the Equator.

Through the year, the Sun goes through the Uttarayana (the Northward) and the Dakshinayana, (the Southward), movements, as we know. On the days of the equinoxes, the Sun is perpendicular over the Equator and its angular distance from the Equator would then be zero. It moves Northward in summer, reaching a maximum angle of 23.5 degrees North of the Equator, on the day of the Summer Solstice. In winter, it moves southward, reaching a maximum angular distance from the equator, of 23.5 degrees South.

These Uttarayana and Dakshinayana movements of the Sun can be easily measured, using the Brihat Samrat Yantra.

It is not just the Sun, the instrument can determine the Declination of any object in the sky. The Declination is the angular distance of an object in the sky, from the Celestial Equator. (Celestial Equator is the projection of the equator of the Earth, on to the sky).

Look at the markings on the surface of the triangular wall of the Brihat Samrat Yantra. These markings, for measuring the Declination, are in degrees, which have been divided into 6 parts that are further subdivided into 5 parts. The Declination can, therefore be read to an accuracy of 2 minutes of arc.

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Page last modified on June 17, 2008, at 04:49 AM EST