Presentation Information
[O12-P35]Bringing Muroto to the World: Experimental Reproduction of "Daruma Sunset" as a Science Education Resource
*Mahiro Nagao1, *Saho Maeda1, Seiya Hatakenaka1, Reon Irikawa1 (1. Kochi Prefectural Muroto High School)
Keywords:
mirage,refraction,experimental reproduction,Daruma sunset (Omega sunset),educational material development,Kuroshio Current
1. Background
The “Daruma sunrise/sunset,” a type of inferior mirage also known as the Omega sunrise/sunset, is frequently observed along the Pacific coast of Japan (from Okinawa to Chiba) in winter (Fig. 1). This phenomenon occurs when cold seasonal winds cool the upper air, while the Kuroshio Current warms the air near the sea surface, forming a layered structure of warm air below and cold air above. This temperature difference produces a vertical refractive index gradient, causing sunlight to refract and form a virtual image beneath the real image near the horizon, resulting in a characteristic Daruma-shaped sun [1].Muroto City, Kochi Prefecture, is surrounded by the Pacific Ocean on three sides. Daruma sunrises are observed along the east coast, and sunsets along the west coast, with both visible at Cape Muroto. Thus, the area is known as a favorable observation site and is utilized as a tourism resource [2].
2. Objective
This study aims to establish a simple and reproducible experimental method to reproduce the Daruma sunset. Inspired by our observation of the phenomenon at our high school (Fig. 1), we also aim to share its appeal with a broader audience and apply the experiment to education and outreach.
3. Methods and Results
We first attempted to reproduce refraction using a stratified water system. A density-stratified structure was created using saltwater and freshwater, and the refraction of a laser beam was observed. Although refraction was confirmed (Fig. 2), the high-refractive-index layer could not be stably maintained above the lower layer, resulting in a structure opposite to that of the atmosphere. Attempts using agar failed due to insufficient transparency.
Next, we examined refraction caused by temperature gradients in air by observing a road mirage. A colored ball placed on a sun-heated concrete surface appeared vertically stretched in its lower part when observed from several meters away (Fig. 3).
We then generated a temperature gradient above a heated metal plate. Initially, a smartphone light produced a Daruma-like image (Fig. 4); however, a similar image appeared without heating, indicating reflection from the plate. To reduce this effect, the light source was replaced with a colored ball, and the distance between the object and the plate was set to ~250 cm (Fig. 5). Observing the ball through air above the heated plate (~60 cm distance) produced a distorted Daruma-shaped image at its lower part (Fig. 6), whereas no distortion appeared without heating. These results demonstrate successful reproduction of the Daruma sunset.
4. Discussion
In the successful experiment, air temperature was measured up to ~10 cm above the plate, and the refractive index distribution was estimated using Edlén’s equation [3]. The results showed a continuous increase in refractive index with height (Fig. 7). This gradient bends light rays upward, connecting a virtual image beneath the real image and forming the Daruma-shaped appearance. Thus, the experiment reproduces the essential optical mechanism of the phenomenon.
5. Conclusion and Applications
We established a reproducible method to simulate the Daruma sunset by combining an appropriate arrangement of a light source, heat source, and observer with a refractive index gradient above a heated plate. Based on this method, we produced an explanatory video, which has been used in presentations.
In the future, we aim to improve its presentation and promote it in educational and outreach settings to share the natural appeal of Muroto. This experiment can also deepen understanding of local climate and environmental conditions.
References
[1] Nakata, K., 2021. Quantitative analysis about the shape and mechanism of “DARUMA Sun”. Proc. Phys. Soc. Jpn., 76(1), 12aN1-5.
[2] Muroto City Tourism Association, 2020. Daruma Sunrise and Sunset. Available at: https://www.muroto-kankou.com/search_spot/nature/s_7/ (Accessed April 11, 2026).
[3] Edlén, B., 1966. The Refractive Index of Air. Metrologia, 2(2), 71–80.
The “Daruma sunrise/sunset,” a type of inferior mirage also known as the Omega sunrise/sunset, is frequently observed along the Pacific coast of Japan (from Okinawa to Chiba) in winter (Fig. 1). This phenomenon occurs when cold seasonal winds cool the upper air, while the Kuroshio Current warms the air near the sea surface, forming a layered structure of warm air below and cold air above. This temperature difference produces a vertical refractive index gradient, causing sunlight to refract and form a virtual image beneath the real image near the horizon, resulting in a characteristic Daruma-shaped sun [1].Muroto City, Kochi Prefecture, is surrounded by the Pacific Ocean on three sides. Daruma sunrises are observed along the east coast, and sunsets along the west coast, with both visible at Cape Muroto. Thus, the area is known as a favorable observation site and is utilized as a tourism resource [2].
2. Objective
This study aims to establish a simple and reproducible experimental method to reproduce the Daruma sunset. Inspired by our observation of the phenomenon at our high school (Fig. 1), we also aim to share its appeal with a broader audience and apply the experiment to education and outreach.
3. Methods and Results
We first attempted to reproduce refraction using a stratified water system. A density-stratified structure was created using saltwater and freshwater, and the refraction of a laser beam was observed. Although refraction was confirmed (Fig. 2), the high-refractive-index layer could not be stably maintained above the lower layer, resulting in a structure opposite to that of the atmosphere. Attempts using agar failed due to insufficient transparency.
Next, we examined refraction caused by temperature gradients in air by observing a road mirage. A colored ball placed on a sun-heated concrete surface appeared vertically stretched in its lower part when observed from several meters away (Fig. 3).
We then generated a temperature gradient above a heated metal plate. Initially, a smartphone light produced a Daruma-like image (Fig. 4); however, a similar image appeared without heating, indicating reflection from the plate. To reduce this effect, the light source was replaced with a colored ball, and the distance between the object and the plate was set to ~250 cm (Fig. 5). Observing the ball through air above the heated plate (~60 cm distance) produced a distorted Daruma-shaped image at its lower part (Fig. 6), whereas no distortion appeared without heating. These results demonstrate successful reproduction of the Daruma sunset.
4. Discussion
In the successful experiment, air temperature was measured up to ~10 cm above the plate, and the refractive index distribution was estimated using Edlén’s equation [3]. The results showed a continuous increase in refractive index with height (Fig. 7). This gradient bends light rays upward, connecting a virtual image beneath the real image and forming the Daruma-shaped appearance. Thus, the experiment reproduces the essential optical mechanism of the phenomenon.
5. Conclusion and Applications
We established a reproducible method to simulate the Daruma sunset by combining an appropriate arrangement of a light source, heat source, and observer with a refractive index gradient above a heated plate. Based on this method, we produced an explanatory video, which has been used in presentations.
In the future, we aim to improve its presentation and promote it in educational and outreach settings to share the natural appeal of Muroto. This experiment can also deepen understanding of local climate and environmental conditions.
References
[1] Nakata, K., 2021. Quantitative analysis about the shape and mechanism of “DARUMA Sun”. Proc. Phys. Soc. Jpn., 76(1), 12aN1-5.
[2] Muroto City Tourism Association, 2020. Daruma Sunrise and Sunset. Available at: https://www.muroto-kankou.com/search_spot/nature/s_7/ (Accessed April 11, 2026).
[3] Edlén, B., 1966. The Refractive Index of Air. Metrologia, 2(2), 71–80.