Sunset occurs when the Sun’s upper limb disappears below the horizon. However, complete darkness does not follow instantly. A significant transitional period known as twilight occurs between daylight and true night. During this time, sunlight continues to illuminate the atmosphere, gradually fading as the Sun dips further below the horizon.
The Gradual Descent into Darkness
Darkness is a gradual process defined by different stages of twilight. This transitional phase occurs when the Sun is below the horizon but its rays still scatter through the atmosphere. Twilight is divided into three distinct categories, each marked by the Sun’s angular position relative to the horizon.
Civil twilight begins immediately after sunset and continues until the Sun is 6 degrees below the horizon. During this phase, the sky remains bright enough that artificial lighting is generally not needed. The horizon is clearly visible, and the brightest stars and planets become apparent. This period typically lasts 20 to 30 minutes, though it can be as short as 24 minutes near the equator.
Nautical twilight follows, occurring when the Sun is between 6 and 12 degrees below the horizon. The sky becomes noticeably dimmer, and artificial light is usually needed for outdoor activities. The horizon starts to become indistinct, but prominent stars are visible for celestial navigation, which is how this phase got its name. This period also lasts 20 to 30 minutes.
Astronomical twilight is the final stage, beginning when the Sun is 12 degrees below the horizon and ending when it reaches 18 degrees below. The sky appears almost completely dark to the unaided eye. Faint celestial objects, such as distant nebulae and galaxies, become visible to stargazers. This deepest twilight phase also lasts approximately 30 minutes, after which true darkness begins.
Key Factors Influencing Twilight
Twilight duration varies significantly depending on geographical and atmospheric factors. Latitude is a primary influence, dictating the angle at which the Sun descends below the horizon. Near the equator, the Sun sets almost perpendicularly, causing rapid transitions and shorter twilight periods.
At higher latitudes, the Sun sets at a much shallower angle. This extended angular path means the Sun takes longer to pass through each 6-degree band of twilight, resulting in longer durations. For instance, civil twilight can last up to 48 minutes in places like Greenwich, England (51.5°N). At the poles, twilight can extend for several weeks, leading to “white nights” where it never gets truly dark in summer.
Season also plays a role in twilight duration due to Earth’s axial tilt. This tilt affects how sunlight reaches different parts of the planet throughout the year. At mid-latitudes, twilight tends to be longer during the summer months and shorter in winter.
Atmospheric conditions further influence the perceived brightness and length of twilight. Cloud cover, atmospheric haze, and pollution can scatter or block sunlight. Heavy cloud cover can make the sky appear darker sooner, while atmospheric particles can prolong the perception of light.
Observing the Night Sky
Understanding twilight stages provides practical insights for activities dependent on natural light. For stargazing, wait until astronomical twilight has fully ended, when the Sun is more than 18 degrees below the horizon. This ensures optimal viewing of faint celestial objects.
For photographers, the twilight periods offer unique lighting opportunities. The “golden hour” typically refers to civil twilight, characterized by warm, soft light. The “blue hour” often corresponds to nautical twilight, providing deep blue tones. These transitional times are ideal for capturing landscapes and cityscapes as natural light blends with artificial illumination.
Twilight timing also affects outdoor lighting. Streetlights turn on during civil twilight as natural light diminishes. As the sky progresses through nautical and astronomical twilight, the need for artificial illumination becomes more pronounced for continued outdoor activities. The scientific definition of “darkness” aligns with the end of astronomical twilight, marking the point where the sky receives virtually no direct or scattered sunlight.