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Signs and Shadows: The Jantar Mantar


Moved to India in recent weeks I have not updated the blog. Now that the first adjustment is over, I hope to resume a bit 'rhythm ...

One of the things I've done in the meantime, however, was to take the train (in Sleeper Class: those with bars on the windows) to go from Delhi to Jaipur , the "pink city" - as it seems to me painted in orange rather than pink. There, the first thing I saw was the Jantar Mantar - built by Jai Singh II, the early eighteenth century, to make astronomical observations, and been rebuilt and restored in the early twentieth century. From

Aesthetically, the Jantar Mantar is an amazing thing. Area quiet and well cared for in the chaos of Jaipur, has a fine bunch of marble and masonry astronomical instruments. I like a good skeptic, was interested to go and see for yourself something that I had read in a magazine extension, uh, I suspect around 1983 or so. That is, that many of the expensive tools of the Jantar Mantar are, for simple reasons of optics, much less useful than it could imagine.

Sundials huge as Samrat Yantra (the largest in the world, with a gnomon 23 meters high) were designed by Jai Singh to project a large shadow on the dial and then allow to better measure the position del sole nel cielo. Però, il sole non è una sorgente luminosa puntiforme: ha un diametro ben apprezzabile. E di conseguenza, mentre l'ombra di un oggetto è molto nitida a pochi centimetri di distanza, la distinzione tra luce e buio diventa molto meno chiara quando la distanza tra l'oggetto stesso e il piano di proiezione aumenta. Qui, l'ombra che cade sui segni tracciati per terra per le meridiane più grandi è troppo indistinta per misurazioni precise.

Vanno un po' meglio le cose con strumenti più piccoli e più raffinati, come la Laghu Samrat Yantra (fotografata qui accanto), che proietta l'ombra dello gnomone su lastre di marmo suddivise con estrema precisione. Anche in questo caso, However, the shadow is too indistinct to allow measurements with an accuracy greater than 20 seconds. It is a problem without solutions: shadows do not allow too short too precise measures, but increasing the size of the early sundial slams against the optical boundary.

I am inclined to draw a general lesson: do well and scrupulously signs is important, but it can help to a certain point. If you want to go further, in many situations to a certain point you stop doing things the old way and you must find an alternative solution ...