Which ancient Indian city, an important centre of astronomy, was regarded as the 'Greenwich of India'?

Ujjain, in Madhya Pradesh, is often called the "Greenwich of India" because, in ancient Indian astronomy, the prime meridian (zero longitude) was taken to pass through Ujjain, just as the modern Prime Meridian passes through Greenwich, England.

Ujjain's importance in astronomy:

• Ancient Indian astronomical texts, such as the Surya Siddhanta, used the meridian of Ujjain as the reference line for calculating time and the positions of planets.
• It was one of the main centres of astronomical study in ancient India.
• The great astronomer and mathematician Varahamihira (6th century CE) is associated with Ujjain, where he wrote the "Pancha Siddhantika" and "Brihat Samhita".
• Brahmagupta (7th century) is also linked with the Ujjain school of astronomy.

Location:

• The Tropic of Cancer passes close to Ujjain, which added to its importance for astronomical observations.
• Ujjain is situated on the banks of the Kshipra river.

The Jantar Mantar (Vedh Shala) of Ujjain:

• Maharaja Sawai Jai Singh II of Jaipur built five observatories in the early 18th century, at Delhi, Jaipur, Ujjain, Varanasi and Mathura.
• The Ujjain observatory (about 1725) has instruments such as the Samrat Yantra (a large sundial), the Nadivalaya Yantra and the Digamsha Yantra.
• The Mathura observatory no longer exists.

Ujjain in history and culture:

• It was the ancient city of Avantika, capital of the Avanti mahajanapada.
• It is linked with the legendary King Vikramaditya, after whom the Vikram Samvat calendar is named.
• The poet Kalidasa is traditionally associated with Ujjain.
• The Mahakaleshwar Temple, one of the twelve Jyotirlingas, is in Ujjain.
• It is one of the four places where the Kumbh Mela (Simhastha) is held.

Modern recognition:

The Vikramaditya Vedic Clock, unveiled in Ujjain in 2024, displays time according to the traditional Indian time-keeping system.

Why not the other options?

• ✗Varanasi is a great centre of learning and has a Jantar Mantar, but it was not the reference meridian.
• ✗Pataliputra was the capital of the Mauryas and Guptas; Aryabhata worked near it.
• ✗Kanchipuram was an important centre of learning in the south.