The Astronomical Observatories of Jai Singh c16.5

c16.5

The Samrāṭ Yantra might be considered as a section in the plane of the equator of the hemisphere of Berosus, and with the bead extended into a line parallel to the axis; but so could any dial be referred to the same origin. In the British Museum are many dials of the seventeenth and early eighteenth centuries constructed exactly on the same principle as the Samrāṭ.¹ The Samrāṭ instrument is an equal hour instrument, or equinoctial, as such instruments are often called. The evolution of the equal hour instrument is of considerable interest. In early instruments the time from sunrise to sunset is divided into 12 equal portions. These portions of time vary in length from one-twelfth of the longest day to one-twelfth of the shortest day. They were therefore called unequal hours, also temporal hours and planetary hours (see page 22). Of these varying hours, naturally, the equinoctial hours were the mean, and, on the introduction of mechanical clocks, the equinoctial hour became the standard for sun-time measurements. According to Delambre, Abdul Ḥasan (al-Ḥasan b. ‘Alī b. ‘Omar al-Marrakōshī, Abū ‘Alī) was the first to introduce the constant hour notion among the Muslims; but he seems to have employed, in the usual manner, the horizontal plane for the shadow traces, while Jai Singh’s instrument receives the shadow of the inclined gnomon on a circular arc lying in the plane of the equator, and, thereby, secures, in the simplest manner, equal hours throughout the year. The direct origin of the tangential scales on the gnomon, for measuring the declination of the sun, is not known; although Ibn Yūnus, and other Muslim writers on astronomy, had worked out the theory.² Hindu astronomers did not employ tangent scales, and refer to no other dial than the vertical gnomon, and to no other dial measurements than the length of the shadow. They made no direct angular measurements, and an angular dial would have been almost contrary to the spirit of their teaching.³ The only other instruments that can be attributed to Jai Singh’s genius are the Digamśa Yantra and Rām Yantra, but these are simply enlargements, in masonry work, of the azimuth and combined azimuth and altitude instruments of the Muslims.