In addition to land and water, some globes are printed with a kind of map that looks like a skinny numeral 8. This is called the analemma, and it charts the apparent path of the sun up and down and across the sky as the seasons change. For the northern hemisphere, the analemma teaches you that in summer there’s a lot of sun and it soars high toward the north.
One more marking, right around the middle of the globe, shows a belt of latitudes called the tropics, bounded north of the Equator by the Tropic of Cancer and south of the Equator by the Tropic of Capricorn. The word “tropic” means “turning point,” and it’s at the two tropics where the sun changes its course along the analemma: from northbound to southbound when it’s directly overhead at the Tropic of Cancer on the first day of northern summer, and back again from south to north when it’s directly overhead at the Tropic of Capricorn on the first day of southern summer.
So in the tropics, and only in the tropics, there are two days in the year when the sun at noon will be directly overhead on its way up to and then back down from its boundary tropic. On those days, at the brief moment when the sun is at the zenith, a vertical object’s summer-short shadow will dwindle all the way down to nothing. In Hawaii, America’s only tropical state, the first of those days is always in May and the second is always in July. For today, May 26, 2022, the celestial event in Honolulu was this.
The moment of zero shadow is called the zenith passage or (specifically in Hawaii) Lahaina noon. Before that moment this month, the sun where I recorded this composite image was a little clouded over and the shadow of the boom on the pole wasn’t intense enough to be educational, but you’ve now seen the teaching aids from just at and then just after the climax. Notice how the invisible shadow of the speed limit sign becomes visible again as Hawaii’s sky swings back to ordinary. You were looking northeast.
To time the shadow, the preferred chronometer will be Frances Cornford’s (1886-1960) mood watch. It goes tock tick, not tick tock, and you see that that’s the correct right-to-left astronomical order.
It happens only in the tropics — that is, the latitudes between the Tropic of Cancer and the Tropic of Capricorn. Twice a year there — first when the sun is on its way north to the Tropic of Cancer, which it will reach at the summer solstice, then when it is on its way back south to the Tropic of Capricorn, which it will reach at the winter solstice — the sun at noon reaches a point directly overhead and a vertical object will cast no shadow. In Hawaii, where I live, the two dates will be in May and July. And the word tropic means turning point.
Today on Oahu, the still point (terms zenith passage or Lahaina noon) was predicted for 12:28, plus or minus a few minutes on different parts of the island. I set up my vertical object at 9:00 in brilliant sun and started shooting. The brilliant sun persisted until just about noon, but then the clouds rolled in. However, with the camera facing approximately north:
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.