Radkan tower, built in the 13th century, is located in the Iranian province of Khorasan-Razavi. For many years Radkan tower was thought to be a tomb, but in recent years archaeologists have discovered that it is an ancient observatory designed by the eminent Iraqi astronomer Nasir ad-Din al-Tusi.
It is believed that he used it to calculate the diameter of the Earth and the movements of the planets in the solar system. The tower has 12 walls corresponding to the 12 months of the year, and each part of the wall has an angle of 30 degrees.
There are also 365 vertical grooves at the top of the wall, which divides the tower into 36 sectors, each of which is 10 degrees. The doors of the tower are built opposite the points of sunrise during the winter solstice and sunset during the summer solstice.
It is curious that Nasir ad-Din al-Tusi, in his time, put forward a new theory of the movement of stars and, in many respects, was ahead of European scientists. For example, as early as the 13th century, he believed that light consists of particles.