The Ujjain Observatory, located in the city of Ujjain in Madhya Pradesh, India, is believed to be the oldest astronomical observatory in the world. It was established around 500 BCE and was used for centuries to observe celestial events and make astronomical calculations.

Jai Singh II, the Maharaja of Jaipur, is credited with building five Jantar Mantar observatories in India. These observatories were constructed in the early 18th century and were used to make astronomical observations and calculations.

The primary instrument used in ancient Indian observatories was the gnomon, a simple device consisting of a vertical rod or pillar. The gnomon was used to measure the angle of the Sun's rays at different times of the day and year, which allowed astronomers to determine the time, the seasons, and the positions of celestial bodies.

The Jantar Mantar observatory in Jaipur was used to observe the transit of Venus in 1769. This observation was significant because it allowed astronomers to accurately determine the distance between the Earth and the Sun.

Ancient Indian observatories were used for a variety of purposes, including studying the stars and planets, making astrological predictions, and determining the time and seasons. These observatories played a significant role in the development of Indian astronomy and astrology.

Aryabhata, an ancient Indian astronomer and mathematician, is known for his accurate calculations of the Earth's circumference. He calculated the Earth's circumference to be 4967 miles, which is remarkably close to the modern value of 40,075 miles.

The Man Mandir Observatory is an ancient Indian observatory located in the city of Varanasi. It was built in the 16th century and was used for astronomical observations and calculations.

The Delhi Observatory, also known as the Yantra Mahal, is known for its unique underground structure. It consists of a series of underground chambers that were used for astronomical observations.

The Vedhshala Observatory is an ancient Indian observatory located in the city of Mathura. It was built in the 16th century and was used for astronomical observations and calculations.

Bhaskara II, an ancient Indian astronomer and mathematician, is known for his work on planetary motion. He developed a model of planetary motion that was more accurate than the Ptolemaic model that was prevalent at the time.

The Yantra Mahal Observatory is an ancient Indian observatory located in the city of Delhi. It was built in the 16th century and was used for astronomical observations and calculations.

The Jaipur Observatory, also known as the Jantar Mantar, is known for its large collection of astronomical instruments. It contains a variety of instruments, including sundials, astrolabes, and quadrants, which were used for astronomical observations and calculations.

The Nalanda Observatory is an ancient Indian observatory located in the city of Nalanda. It was built in the 5th century and was used for astronomical observations and calculations.

Varahamihira, an ancient Indian astronomer and astrologer, is known for his work on eclipses. He developed a method for predicting eclipses that was more accurate than the methods used by his predecessors.

The Takshashila Observatory is an ancient Indian observatory located in the city of Takshashila. It was built in the 5th century and was used for astronomical observations and calculations.