The arc of history

Only in the tropics — that is, only in the latitudes between the Tropic of Cancer and the Tropic of Capricorn — there are two days a year when the sun is directly overhead at noon and a vertical object, such as a flagpole, will cast no shadow. In Hawaii, where I live, one of those days occurs in May, when the sun is headed north to its summer solstice rendezvous with the Tropic of Cancer, and the other occurs in July, when the sun is headed back south to its winter solstice date with the Tropic of Capricorn. The astronomical term for the phenomenon is zenith passage.

Watch the shadow swing around its still point this year at

If you aren’t sure what you’re supposed to have seen, speed up the effect by sliding the cursor along the timeline and watching the shadow grow shorter and shorter on the west side of the foot of the stemware, then disappear, then reappear on the east side. The music is Bartók’s Music for Strings, Percussion and Celesta, movement 4, allegro molto. Seiji Ozawa, Berlin Philharmonic.