Astronomical events Astronomy

Zero Shadow Day

Article by Mansha Kochar

The astronomical world is full of incredible and exciting events, from meteor showers to supernova explosions, eclipses to rare comet sightings, and many as the only list keeps growing.

One such event is Zero Shadow Day, which was observed in Chennai on August 18, 2023. A zero-shadow day is a remarkable occurrence in the sky when the sun is at its highest point and casts no shadow.

SO WHAT CAUSES THE ZERO SHADOW?

We all know that the plane or revolution and axis of rotation don’t align, but are inclined by 23.5 degrees. Yes, and this also indicates that the sun crosses between 23.5 degrees North and 23.5 degrees South, back and forth. When the latitude of a location in this region and the declination of the sun is equal, the sunrays fall exactly on top of the region. Hence one cannot find any shadow of a grounded object in such events.

WELL… WHAT HAPPENS DURING THE ZERO SHADOW DAY AND WHO CAN SEE IT?

Unlike its name, the zero-day does not last for an entire day; the duration is relatively small: ranging for only a few minutes, and occurs twice on a given day. Zero Shadow Day occurs during the equinoxes, which are the times of the year when the tilt of the Earth's axis is such that the plane of the Earth's equator passes through the centre of the Sun. This alignment causes the Sun to be directly above the equator, resulting in the phenomenon of zero shadows at locations near the Tropics. This phenomenon is observed twice in the region between the Tropic of Cancer and Capricorn at different dates and locations.

This is also called Lahaina Noon in Hawaii, where it happens four times a year due to its location near the equator

WHAT WERE ITS HISTORICAL SIGNIFICANCE

This particular phenomenon and its observations have been well-known and studied by ancient civilisations, like the Egyptians and Greeks. To the point where these observations were used in agricultural cycles and calendars.

It was also the foundation for measuring the circumference of the Earth using these observations which were groundbreaking at that time. A Greek mathematician, Eratosthenes formulated a relation between the length of the shadow and the circumference of the Earth using basic trigonometric relations and principles of geometry.

Eratosthenes set up a vertical object that cast a shadow in the neighbouring town of Alexandria, on a zero-shadow day in the Egyptian town of Syene.

$$\frac{\text{Angle of shadow in Alexandria}}{ \text{360 degrees}} = \frac{\text{Distance between Alexandria and Syene}} {\text{Circumference of Earth}}$$

A similar study is also seen in the Indian architectural structure: Jantar Mantar. It consists of a sundial, with a triangular gnomon whose hypotenuse is parallel to the Earth. The length of the shadows cast is used to calculate the time of the day and serves as the basis of calendars and calculations concerning various astronomical events.

WHAT ABOUT SCIENTIFIC SIGNIFICANCE?

Zero Shadow Day is a result of the Earth's axial tilt and its orbit around the Sun. As the Earth orbits the Sun, its axial tilt causes the Sun's position in the sky to change, resulting in variations in the length and direction of shadows cast by objects on the ground. The phenomenon of Zero Shadow Day provides a tangible demonstration of the Earth's tilt and its effect on the Sun's apparent path across the sky.

Another application of zero shadows is aiding in the orientation of solar panels in a direction to generate maximum energy. This analysis is also used in urban architecture and design to study the interaction of sunlight with buildings and open spaces, thus helping maximize natural light and energy efficiency.

CULTURAL SIGNIFICANCE?

In certain regions, these days are observed as festivals or cultural events, with various exhibitions and ceremonies.

Such events are also an attraction for photographers, as it serves as an opportunity for them to click pictures of objects floating without shadows, which are of high artistic value.

Hence, it can be inferred that Zero Shadow Day is quite an interesting and important astronomical event, from both a scientific and a cultural point of view. It allows us to explore the intricacies of the movement of the Earth around the Sun while also aiding and impacting a plethora of time-related aspects such as seasons and calendars in our daily lives.

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