Sunset

A sunset is the moment the Sun disappears below the horizon of an astronomical object. On Earth, this happens because our planet rotates on its axis. This rotation causes the Sun to appear to move along a path until it sinks out of view.

For most people, this phenomenon occurs approximately once every 24 hours. However, the experience changes significantly near the Earth's poles. In areas within the Arctic or Antarctic Circles, people can experience a polar day. This is a period where the Sun does not set for 24 hours. At latitudes within half a degree of the poles, the Sun may not rise or set on certain days. This happens because its angular elevation remains less than one degree between solar noon and midnight.

Astronomers define the exact moment of sunset as when the upper limb of the Sun disappears below the horizon. Interestingly, what we see is not always the true geometric position of the Sun. This is due to atmospheric refraction, which is the bending of light as it passes through the atmosphere. This refraction causes sunlight rays to distort as they approach the horizon. Because of this, the solar disk is actually about one diameter below the horizon when we observe the sunset. Refraction also changes the apparent shape of the Sun. It bends light from the bottom edge of the Sun more than light from the top. This makes the Sun appear wider than it is high, even though it is actually nearly spherical.

Once the Sun disappears, the sky enters a period called twilight. Twilight is not a single event but is divided into three distinct stages. The first stage is civil twilight, which begins immediately after the Sun disappears. It continues until the Sun reaches 6 degrees below the horizon. The second stage is nautical twilight, occurring when the Sun is between 6 and 12 degrees below the horizon.

The final stage is astronomical twilight, which lasts while the Sun is between 12 and 18 degrees below the horizon. Dusk marks the very end of astronomical twilight. This is the darkest moment of twilight just before the onset of true night. Night begins once the Sun reaches 18 degrees below the horizon and no longer illuminates the sky.

The timing and direction of a sunset vary based on your latitude, longitude, altitude, and time zone. The Sun's position on the horizon, known as the azimuth, changes with the seasons. During the equinoxes, the Sun sets due west for all viewers on Earth. In the Northern Hemisphere, the Sun sets to the northwest during spring and summer. It sets to the southwest during autumn and winter. These seasonal patterns are reversed in the Southern Hemisphere.

The timing of these sunsets is driven by the Earth's axial tilt and its elliptical orbit around the Sun. These factors, along with the Earth and Moon's revolutions, create small daily changes and larger semi-annual changes.

In the Northern Hemisphere, the latest sunset does not occur on the summer solstice of June 21. Instead, it happens in late June or early July. This delay is influenced by the Earth's movement near its aphelion. Similarly, the earliest sunset does not fall on the winter solstice. It usually occurs in late November or early December. This is due to the Earth's faster movement near its perihelion around January 3. These specific dates depend on the viewer's latitude. In the Southern Hemisphere, these seasonal timing patterns are reversed.

The vibrant colors of a sunset are caused by a process called Rayleigh scattering. Sunlight is composed of many different colors. As light travels through the atmosphere, air molecules and tiny particles scatter the light. Shorter wavelengths, such as blue and green, scatter more strongly. At sunset, the light must travel a much longer path through the atmosphere. This path is long enough to remove blue and green components almost completely.

This leaves the longer wavelength orange and red hues for us to see. Large cloud droplets can cause Mie scattering, which is not strongly dependent on wavelength. This process is responsible for the light scattered by clouds and the white daytime halo around the Sun.

Environmental factors can also dramatically change sunset colors. Volcanic eruptions can influence the appearance of the sky. If ash remains in the troposphere, it can mute the colors of a sunset. However, if volcanic ejecta is lofted into the stratosphere, it can create beautiful afterglows. These are caused by tiny sulfuric acid droplets that form thin clouds. These high-altitude clouds reflect reddened sunlight back down to the surface even after the Sun has set. Large eruptions, such as Mount Pinatubo in 1991 or Krakatoa in 1883, produced remarkable afterglows around the world.

Even the air quality can matter, as evening air often contains more particles than morning air, potentially making sunset colors more brilliant than sunrise colors.