Astronomical Instruments

Early instruments included the gnomon (sanku) to determine the length of the noon solar shadow and to keep track of the year and a water clock in the form of a pot (ghata) with a small hole at bottom for water flow and thus time measurement; both were known to have been used in Mesopotamia (Pingree 1963, p. 232). Pingree notes that Hip-parchos (as reported by the Roman Strabo) and Eratosthenes mention that Seleucid ambassadors at the court of the Mauryan king made observations with the gnomon in India. Even in Vedic times, the gnomon was used to establish east-west lines using equal solar altitude shadow directions. These were established by noting the directions when the solar shadow reached a preset length marked on a radius established with a cord equal to the height of the gnomon (so that the altitude was 45°) on either side of the celestial meridian. A line joining the termini of the shadow lengths then established the east-west line (see Parpola 1994, p. 206).

In the 18th century, the Maharajah Jai Singh of Jaipur constructed elaborate observatories using classical instruments both at Jaipur and at Delhi. Both observatories are well preserved and constitute popular tourist attractions. The Delhi Observatory is known as the Jantar Mantar. The instruments, described more fully in §3.3, include the Samrat Yantra, used to measure hour angles and declinations (Figure 9.10a); the

Rama Yantra, cylindrical towers used to measure altitudes and azimuths (Figures 9.10b,c); the Jayaprakasa Yantra, hemispherical bowls, also used to measure altitudes and azimuths (Figure 9.10d), but practical only for the shadow-casting Sun and Moon; a combination instrument called the Misra Yantra (Figure 9.10e). The latter consisted of four instruments: the Samrat Yantra, the Niyat Chakra Yantra, which indicates the declination of the Sun at four specified times of day, when apparent noon (solar transit) occurs at four other observatories around the world (Notke, Japan; Saritchen; Zurich; and Greenwich); the Dakshinobhitti, to calculate meridian altitude or zenith distance; and the Kark Rashivala, which gave the zodiacal sign of the Sun's location—a classical Indian (and Greek) way of referring to the position of an object in the sky.

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