The Nalanda University Observatory, located in present-day Bihar, is believed to have been built in the 5th century BCE and is considered the oldest known astronomical observatory in India.

The Jantar Mantar in Jaipur was constructed by Maharaja Sawai Jai Singh II in the early 18th century. It is a collection of architectural astronomical instruments that were used for various astronomical observations.

The primary purpose of an astronomical observatory is to observe celestial bodies and astronomical phenomena, such as the movement of stars, planets, and other objects in the sky.

The Sun Temple in Konark, Odisha, is known for its alignment with the Sun during the equinoxes. The temple's main entrance is positioned in such a way that the Sun's rays illuminate the sanctum sanctorum during sunrise on the spring and autumn equinoxes.

The Samrat Yantra in the Jantar Mantar is a large sundial that is used to measure the altitude of celestial bodies. It consists of a massive triangular structure with a curved quadrant and a gnomon, which casts a shadow on the quadrant to indicate the altitude of the Sun or other celestial objects.

The Brihadeeswarar Temple in Thanjavur, Tamil Nadu, is an ancient Indian observatory. It is known for its massive granite structure and its alignment with the Sun during the summer solstice.

The Jai Prakash in the Jantar Mantar is a large sundial that is used to measure the time of day. It consists of a hemispherical bowl with a gnomon at its center. The shadow of the gnomon falls on the inner surface of the bowl, indicating the time of day.

The Bhoj Shala in Dhar, Madhya Pradesh, is an ancient Indian observatory that is believed to have been built by King Bhoja in the 11th century. It is known for its collection of astronomical instruments, including a sundial, a quadrant, and a celestial globe.

The Rashivalaya in the Jantar Mantar is an instrument that is used to study the motion of the planets. It consists of a circular platform with twelve zodiac signs marked on it. The positions of the planets are plotted on the platform, allowing astronomers to track their movement over time.

The Sun Temple in Konark, Odisha, is an ancient Indian observatory. It is known for its intricate carvings and its alignment with the Sun during the equinoxes.

The Misra Yantra in the Jantar Mantar is an instrument that is used to measure the distance between celestial bodies. It consists of two pillars with a crossbar connecting them. The distance between the pillars is known, and the angle between the crossbar and the line connecting the two celestial bodies is measured. This allows astronomers to calculate the distance between the celestial bodies.

The Nalanda University Observatory is located in the state of Bihar. It is believed to have been built in the 5th century BCE and is considered the oldest known astronomical observatory in India.

The Samrat Yantra in the Jantar Mantar is also used to study the precession of the equinoxes. The precession of the equinoxes is the gradual shift in the position of the equinoxes over time. The Samrat Yantra allows astronomers to measure this shift and track its progress.

The Jantar Mantar in Jaipur, Rajasthan, is an ancient Indian observatory. It was constructed by Maharaja Sawai Jai Singh II in the early 18th century and is a collection of architectural astronomical instruments.

The Jai Prakash in the Jantar Mantar is also used to study the motion of the Moon. The Jai Prakash allows astronomers to track the Moon's position and velocity over time.