Sunset, the daily disappearance of the Sun below the horizon, is a complex optical phenomenon governed by the properties of Earth’s atmosphere. The spectacular progression of colors, from yellow to fiery red, is a direct result of sunlight interacting with the air molecules and particles that envelop our planet. Understanding the science of sunset requires looking at the precise astronomical timing and the way light is scattered through a long stretch of atmosphere.
Astronomical Definition of Sunset
From an astronomical perspective, sunset is defined as the moment the upper edge, or limb, of the Sun sinks below the true horizon. The apparent movement of the Sun is caused by the constant rotation of the Earth, which carries an observer away from the Sun’s direct light. Atmospheric refraction, where the Earth’s atmosphere acts like a lens and bends the light rays, causes the Sun to appear higher than its true geometric position. Consequently, when we observe the Sun touching the horizon, the solar disk is already entirely below the true horizon line. This refraction effectively delays the moment of sunset by approximately two minutes.
The Science Behind the Colors
The vivid colors of sunset are explained by Rayleigh scattering, which describes how light interacts with particles much smaller than its wavelength. Sunlight is composed of various colors, each corresponding to a different wavelength, with blue and violet being the shortest and red and orange the longest. During the day, when the Sun is high, its light travels a relatively short path through the atmosphere.
The tiny nitrogen and oxygen molecules in the air preferentially scatter the shorter-wavelength blue and violet light in all directions, making the sky appear blue. At sunset, the Sun’s light must travel through a significantly greater thickness of the atmosphere to reach the observer’s eye. This long journey amplifies the scattering effect, removing nearly all the short-wavelength colors from the direct line of sight. This leaves the longer-wavelength colors—yellows, oranges, and reds—to dominate the light that reaches the viewer.
Factors Affecting Intensity and Hue
The intensity and specific hue of a sunset are influenced by aerosols, which are tiny solid or liquid particles suspended in the air. These larger particles, such as dust, smoke, pollution, and volcanic ash, scatter light differently than air molecules through a process known as Mie scattering. Mie scattering is less dependent on wavelength, meaning it scatters all colors of light more uniformly.
A moderate concentration of fine aerosols can enhance the vividness of the sunset. These particles increase the overall scattering path and help filter out remaining traces of shorter wavelengths, further enriching the reds and oranges. For instance, fine dust injected high into the atmosphere from large forest fires or volcanic eruptions can lead to spectacular, deep crimson sunsets. Clean, dry air tends to produce brighter yellow and orange sunsets.
The Period After Sunset
The illumination of the sky continues after the solar disk disappears during the period known as twilight. Twilight is caused by the scattering of sunlight from the upper layers of the atmosphere, even though the Sun is below the horizon. This period is divided into three phases based on the Sun’s angular depression below the horizon.
Civil twilight lasts until the Sun is six degrees below the horizon, providing enough residual light for most outdoor activities. Nautical twilight continues until the Sun is twelve degrees down, at which point the horizon becomes difficult to discern at sea. Astronomical twilight ends when the Sun is eighteen degrees below the horizon, and the sky is considered fully dark. A rare optical phenomenon sometimes observed at the moment of the Sun’s final disappearance is the green flash, a brief green shimmer caused by the atmospheric separation of light colors.