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In Nanjing, next to Lake Xuanwu (玄 武) in the middle of the city you can find the Purple Mountain (紫金山). The Purple Mountain is now surrounded by the city of Nanjing, but is not urbanised itself. This makes it an oasis between the old city on one side and the vast new districts on the other side of the mountain. On and around the mountain you will find the mausoleums of the first Ming (洪武) emperor, who makes Nanjing the capital, and Sun Yat-sen (孙中山) the first president of China. You can take a cable car to the top of the mountain and from there walk back to the city at the bottom of the mountain. You walk there between the trees and the view is beautiful.

The Nanjing Observatory is located on one of the peaks of 紫金山. The observatory is only used as part of the university. The telescopes are no longer used because the light from the city and air pollution make observations impossible. It was never an ideal place for an observatory. The climate is hot and humid in Nanjing and it is often cloudy.

Still, the observatory was used for observations and accurate timing until the twentieth century. There is another meridian viewer from after the revolution. A meridian is positioned exactly on the local meridian and is used to measure when a star, or the Sun, passes the local meridian. With this the local time can be calculated. The photo shows one of the meridian viewers. It is a small viewer because it was not intended to look deep into the universe, it is a measuring instrument to measure the time of the passage of a star. The viewer is therefore small, but very accurate.

The observatory was also close to power during the time when Nanjing was the capital. The observatory still contains a number of historical instruments from long ago. Astronomical observations were of great importance to the Chinese emperors. The emperor ruled with the mandate of heaven. Heaven, the position of the planets, set the correct date for affairs of state. So the calendar had to be correct. The latter was not so easy because in China they had a calendar that was based on both the movement of the Sun and the Moon. And the calendar was wrong. So the new Ming dynasty needed an observatory. A number of instruments were brought over from Kaifing and set up on Purple Mountain in Nanjing, close to the New Palace. The photos show two of the large instruments the Chinese used to measure the positions of the planets. Such a measuring instrument is called an armillary. The oldest were made in 1437, before the start of the Ming Dynasty. And these are copies of even older instruments.

When Matteo Ricci visited the observatory on his way to Beijing, he noticed that the instruments at the observatory were not at all suitable for that location. After all, they were made for Kaifeng, which is at a higher latitude. Because Matteo Ricci had also studied astronomy during his missionary training, he must have been familiar with an armillary and how to operate such an instrument. Armillaria were used in Europe to measure the positions of the planets, Sun and stars. He knew that the axis of an ecliptic armillary should point towards the pole star, so he quickly saw that there was something wrong with the armillary in Nanjing. This prompted him to write to Rome that a missionary astronomer in China might accomplish a lot. A calendar was essential for affairs of state, but an accurate calendar was a concern for Chinese astronomers.

Much later, one of Matteo Ricci's successors stood next to such a beautiful bronze instrument again, this time in Beijing. Ferdinand Verbiest had come to China long after Ricci. The Jesuits had already made a much improved calendar for the emperor, but intrigues at court had disgraced the Jesuits. Verbiest had one chance to show that the improved calendar was better than the Chinese calendar. He had to make a number of predictions, including the height of the sun on a given date. Height was measured with a gnomon. A gnomon is a type of sundial that is only used to measure the angle between the Sun and the horizon. Because a gnomon is positioned exactly on the local meridian (in a north-south direction), it shows the angle when it is greatest. That gnomon that Ferdinand Verbiest once stood next to to prove his calendar is right, later moved from Beijing to Nanjing. And now it is on the Purple Mountain, next to the other old bronze instruments and close to the modern version of the gnomon: the meridian viewer.