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.