Radkan (East) Tower or Mil-i Sharq Radkan lies near Radkan Village, 25km away of Chenaran in Razavi Khorasan Province, Iran. According to Iranian archaeo-astronomer Manoochehr Arian, it was actually a highly sophisticated instrument for studying the stars built in AD 1261 by astronomers led by Nasruddin Tusi (Nasir Al-Tusi; 1201–74).
The round, conical-topped brick tower was designed so that the sun shines directly through its doors and niches on solstice and equinox days. It was possibly with data collected here and at his more famous observatory at Maraqeh that Tusi managed to calculate the earth’s diameter and explain discrepancies between Aristotle’s and Ptolemy’s theories of planetary movement.
Based on epigraphic remnants, German archaeologist and Iranologist Ernst Herzfeld, has argued that the tomb tower belongs to Amir Arghun Khan, a residence of Radkan who died in 1274. The tomb is cylindrical, with an octagonal burial chamber crowned by a conical dome. It is entered from two axial entrances facing southeast and northwest.
The thirty-six engaged columns enveloping its exterior between the base and the dome give the tomb a wavy outline. A spiraling stair encased within the monument’s walls gives access to the inner dome, of which only the base remains. The double dome construction of the roof has a long history in the tomb towers built in Iran during the Seljuk period (roughly 1050-1150) and before. Gunbad-i Qabus in Gorgan is the first example of a monumental tomb structure that employs a double dome construction with an outer conical roof covering an inner hemispherical one.
Sources: Lonely Planet | Radkan, Iran, ArchNet | Mil-i Sharq Radkan, Mehr News Agency, Facebook.com | Ariana Ahangary, citypedia.ir, The best travelled | Witold Repetowicz
The other Iran 
Thank you!!!! It is great that you brought the genius of this 1280-1300 building & current work of Manouchehr Arian to the forefront. I would like to meet anyone with a similar interests i solar buildings of Iran. There is another similar tower at Rey (south Tehran) called the Tuğrul Tower, which I have also visited. But Radkan is more complete & we have an in-house expert, although the building manager at Tuğrul was special too!. https://en.wikipedia.org/wiki/Tughrul_Tower
Thank you. I might cover Tuğrul Tower also at some point. I need to investigate it a bit more.