The daily disappearance of the Sun below the horizon, known as sunset, is one of the most consistently observed natural phenomena. This event marks the transition from day to night and is celebrated worldwide for the vivid colors that often accompany it. The mechanics of sunset involve a combination of celestial motion and the physics of light interacting with the Earth’s atmosphere.
The Apparent Motion of the Sun
The perception of the Sun sinking in the west is a direct consequence of the Earth’s continuous rotation. For an observer on the ground, the Sun appears to move from east to west across the sky over the course of the day. This apparent motion occurs because the observer’s location is spinning away from the light source as the Earth completes its 24-hour rotation. The western horizon is the line where the curvature of the Earth finally blocks the Sun’s light from reaching the viewer.
Why the Sky Changes Color
The spectacular shift in the sky’s color at sunset is explained by Rayleigh scattering, which describes how light interacts with tiny particles in the atmosphere. Sunlight is composed of a spectrum of colors, with violet and blue having the shortest wavelengths and red and orange having the longest. During the day, when the Sun is high, its light travels a relatively short distance, and the shorter-wavelength blue light is scattered efficiently by nitrogen and oxygen molecules, making the sky appear blue.
At sunset, the Sun is low on the horizon, forcing its light to travel a much greater distance through the atmosphere. As the light traverses this longer path, virtually all of the shorter-wavelength blue and violet light is scattered away and dispersed far from the observer’s line of sight. This leaves the longer-wavelength colors, primarily red, orange, and yellow, to continue along the direct path toward the viewer’s eyes. The presence of larger particles like dust, smoke, or water droplets can further intensify this effect, deepening the hue to a vivid red.
Atmospheric Refraction and Visual Effects
The moment the Sun appears to touch the horizon is not the true physical moment of sunset; this difference is caused by atmospheric refraction. As sunlight enters the Earth’s atmosphere at a shallow angle, the denser air acts like a weak prism and bends the light rays. This bending causes the Sun’s image to be lifted, making it appear higher in the sky than its true astronomical position.
The amount of refraction is greatest near the horizon, and this variation in bending across the Sun’s disk causes a noticeable visual distortion. The bottom edge of the Sun’s disk is refracted more than the top edge, which compresses the vertical axis and makes the Sun appear visibly flattened as it sets.
A related and much rarer phenomenon is the “Green Flash,” a moment when the upper rim of the setting Sun briefly flashes green. This effect is a result of the atmosphere dispersing the Sun’s light into its component colors. Just as the last sliver of the Sun disappears, the atmosphere absorbs or scatters the red and yellow light, while the more strongly refracted blue and violet light is also scattered away, leaving only a momentary view of the green light right at the horizon.