Which one of the following longitudes determines the Indian Standard Time ?

Indian Standard Time (IST) is determined by the Standard Meridian of India, which is located at 82°30' E longitude [5]. This longitude is mathematically equivalent to 82.5° E because 30 minutes (30') represent half of a degree (0.5°) [t1]. The meridian passes through Mirzapur in Uttar Pradesh, near Prayagraj [c1][c5]. India adopted this specific longitude to ensure a uniform time across the country, which spans approximately 30 degrees of longitude from west to east [t1][t5]. This choice follows the international convention of selecting standard meridians in multiples of 7°30' [c2]. Consequently, IST is exactly 5 hours and 30 minutes ahead of Greenwich Mean Time (GMT+5:30) [c1][c3]. The meridian also traverses the states of Madhya Pradesh, Chhattisgarh, Odisha, and Andhra Pradesh [t7].

Sources

1. [2] INDIA PHYSICAL ENVIRONMENT, Geography Class XI (NCERT 2025 ed.) > Chapter 1: India — Location > INDIA – LOCATION > p. 2
2. [5] https://www.britannica.com/science/Indian-time-zone