If you are within the tropics—but only if you are within the tropics—you will have the opportunity to see the sun pass directly overhead. The highest point in the sky is known as the zenith, so this event is known technically as solar zenith passage. Solar zenith passage will happen at local noon on two separate days in the year, but what those days are will depend upon your latitude on the earth. Right on the Tropics themselves, it happens on just one day in the year: the June solstice at the Tropic of Cancer in the northern hemisphere, and the December solstice at the Tropic of Capricorn in the south. A little south of the Tropic of Cancer, the dates of solar zenith passage will be a little before and after the June solstice: the farther south you are, the further apart these dates will be. At the equator, the dates occur exactly six months apart, at the two equinoxes. Moving farther south again, the two dates gradually converge on the December solstice.
One way of visualizing this is to imagine a tightly coiled spring wrapped around the earth with its open ends on the two Tropics, and its 183 coils not quite following lines of latitude. There is always some point on the earth where the sun is currently overhead, and from the June solstice to the December solstice this point will trace slowly along the coils of the spring, going around once every twenty-four hours and moving inexorably southwards along the coils, until the December solstice is reached and the sun starts to move back northwards again.
Times when the sun passes close to the zenith stand out, because people and other upright objects cease to have shadows. Such times have sacred and practical significance in a variety of cultures within the tropics. A number of legends identify it as a time when the way is open into the upper world. In some Guatemalan villages, for example, the two dates of zenith passage coincide with springtime and August rains, and are marked by ritual observances that still live on in Christian tradition in the form of parades. In ancient Hawai’i, the moment of solar zenith passage was a time with great mana, or sacred power. It was a time when a person’s shadow was no longer visible and was thought to have retreated directly into the brain through the top of the head; a person’s spirit could exit at this time.
The coming of the days when the sun will pass through the zenith may be recognized in various ways, for example, by observing the rising or setting position of the sun on the horizon, or by watching for the heliacal rise or set of certain stars or asterisms. Sometimes we have evidence of devices that marked the actual moment of solar zenith passage, often in a dramatic way. Zenith tubes, which allowed the light of the sun to pass down into a dark place only at the time of zenith passage, are known at the Mesoameri-can cities of Monte Alban and Xochicalco.
The zenith sun may hold one of the keys to the mystery of why Necker Island, a remote and uninhabitable rock lying well beyond the larger islands of the Hawaiian chain, is covered in temple platforms. It may have been an especially sacred place because it lay right on the Tropic of Cancer, at the very edge of the region where the sun reaches the zenith
