Zenith Sun and the Dashed Spotlight

Our previous post shows the day of no-shadow when the sun over Honolulu on July 15, 2024 is directly overhead at the zenith (see photo on the left). There was a similar solar event on May 27, 2024.

Earlier, Iwakage reported on the dashed spotlight of May 21, 2024 at the Kanayama Megaliths (photo on the right). There is a corresponding observation of the dashed spotlight on July 21, 2024.

What do these two pairs of solar observations have to do with each other? Does this image from page 48 of the guidebook help?

Recall that the summer solstice occurs on the day when the earth’s obliquity (the tilt of earth’s axis of rotation) is most tilted toward the sun. The summer solstice is indicated in the circular calendar on 6/21. The dashed spotlight at Kanayama Megaliths first appears on 5/21 when the sun in the sky is at just the right angle that it shines on the bumpy surface of the triangular panel. Then the sun moves higher in the sky and does not produce a dashed spotlight. The dashed spotlight reappears on 7/15 and makes a last appearance on 7/22.

It is the same phenomenon as the zenith sun over Hawaii and other tropical locations. On May 27, the sun was at the zenith over Honolulu, on its way north (so to speak). After June 21, the sun “headed south” and passed over Honolulu on July 15.

In a manner similar to that used by the ancient astronomers of Kanayama Megaliths, the astronomers of Hawaii could also use the zenith sun observations for their calendar. By my personal experience in viewing both types of solar observations, I would say that the spotlight observation is the more accurate of the two. It is rather difficult to position a upright pole and then to note when there is no shadow. Besides, the dashed spotlight makes quite a show!

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When the Sun is over your head: kau ka lā i ka lolo

The sun casts no shadow when it is directly overhead. Photo: Bishop Museum.

Places in the tropical zones of the Earth, between latitudes 23.44 degrees North and 23.44 degrees South enjoy the phenomenon of no noontime shadow twice a year when the sun is at the zenith. This is called the zenith passage of the sun, and the dates depend on the latitude of the location.

Here are some dates/times for places in Hawaii.

21.30 N May 27, 12:28; July 15, 12L37 Honolulu

19.7 N May 18, 12:16; July 23, 12:26 Hilo

18.9 N May 11, 12:19; July 27, 12:29 South Point, Hawaii Island

The time of this event in Hawaii is popularly called “lāhainā noon,” where the word lāhainā means cruel sun, perhaps because it can be scorchingly hot.

The proper Hawaiian term is kau ka lā i ka lolo, when the sun (ka lā) rests upon the brain (ka lolo).

Right at the equator, the zenith passage coincides with the equinoxes. At Chichén Itzá, latitude 20.68 degrees North, the zenith passage is experienced on May 26 and July 20, give or take a day. These zenith events apparently played a key role in the development of the Mayan calendar.

“When the sun passes overhead, the shadows disappear and the days when this happens assume a sacred significance for the people that live and lived within the tropics. ” From “Archaeoastronomy – The Zenith Passage of the Sun,” by Amelia Carolina Sparavigna.

July 15, 2024 12:37pm Honolulu. Photo by Iwakage.

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