Come out of Jianguomen sta Exit C and turn right. Soon you'll see this wall, topped by some ancient naked eye astronomical instruments. The Ancient Observatory dates back to the fifteenth century, costs 20 Y to enter, and is a must-see for anyone with more than a passing interest in science. In its exhibit halls, English explanations are as voluminous as Chinese!
For the visitor, the site is best thought of in three parts: the grounds and gardens, the exhibit halls, and the observatory tower. The grounds are beautiful, at least in June, and include busts of important scientific figures, some measurement devices, and gardens, entered through a traditional circular gate. Those busts are of Ferdinandus Verbiest, a Belgian Jesuit missionary; Yi Xing, Tang Dynasty inventor of an armillary sphere; and Zu Chongzhi (429-500 AD), who calculated the value of pi to seven decimals and elucidated the concept of precession of the equinoxes.
There is a second garden with larger scale instruments:
The exhibit halls are each manned by a solo--and lonely--docent, who is friendly but mainly silent. One hall focused on astronomy per se and the heavenly bodies.
Some key ancient measurement devices are included and explained, including a cart measuring linear distance (the figure beats the drum after a specified distance has been covered) and a clepsydra (a water clock), actually first invented by the Egyptians, as far as we know.
But my favorite instrument is the one below. It's missing a key feature, though--each dragon should hold in its jaws a metal ball, perched above the those open-mouthed frogs. This is the first known seismograph, sometime in the second century: in the event of an earthquake, a ball or some balls will be shaken loose and drop into the frog's mouth. This tells the direction, distance and magnitude of the quake. Remarkable.
The observatory tower is no doubt the "money shot" of a trip to the Beijing Ancient Observatory. It was decommissioned in 1927 and is now a museum, but the platform still has several instruments including a celestial globe, armillary, and altazimuth. 103 steps lead up (pictured is the first flight) and the docent/guard certainly expected me to ask her to take my photo.
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