Lāhainā Noon: Why Does Hawaii Experience ‘Zero Shadow Day’?

Historically, shadows have provided early evidence that light travels in a straight line. Unconsciously, humans use shadows to judge the shape of objects in their environment.

(Photo: Wikimedia Commons/ Daniel Ramirez)

The rotation of the Earth not only causes day and night but also causes the shadows to change throughout the day. A celestial phenomenon, however, causes vertical objects to lose shadow at noon during specific days of the year.

How are Shadows Formed?

Objects are classified into three types: transparent, translucent, and opaque. This classification is based on their ability to allow visible light to pass through them.

Shadows are formed when an opaque object or material is placed in the path of light rays. Since the opaque material does not let the light pass through it, the light rays that go past the edges of the materials outline the shadow.

Translucent objects like colored plastics, tinted glass, oil paper, and sunglasses form a lighter shadow as they block some light that falls on them and let the rest pass through. Meanwhile, transparent objects form the lightest shadow or do not form a shadow.

What is Lāhainā Noon?

In terms of latitude, there is a single point on Earth which is closest in distance to the Sun. Dubbed as a subsolar point, this place is known for having the Sun directly overhead with its rays striking the Earth's surface in an exactly perpendicular way.

Solar radiation is in part a function of the angle of incidence, or the angle at which the rays of the Sun strike the Earth. During the summer, the Sun's rays travel the shortest distance possible through the atmosphere before striking the Earth's surface. In winter, the Sun does not rise as high in the sky, so its rays travel a greater distance through the atmosphere before striking our planet.

Twice a year, the Sun passes directly over the Earth at a 90-degree angle in May and July. This happens in the tropics or places between 23.5 degrees north of the equator and 23.5 degrees south. In the U.S., Hawai'i is the only state in the tropics, hence the only state where this phenomenon occurs.

In 1990, the Bishop Museum held a contest to give a name to this celestial event, and "Lāhainā noon" was chosen as the winning name for the subsolar points. In Hawaiian, 'lā hainā' literally means "cruel sun". During the pre-contact era, the phenomenon was called "kau ka lā i ka lolo" which translates to "the sun rests on the brains". In traditional Hawaiian culture, kau ka lā i ka lolo was considered a time with great mana (power) because a person's shadow, which is not visible, was thought to have entered one's head.

Lāhainā noon is also known as a zero-shadow day. In this phenomenon, the Sun culminates at the zenith at solar noon, passing directly overhead. As a result, the Sun's ray falls exactly vertically relative to an object on the ground and does not cast an observable shadow.