Twilight is the gradual transition from day to night. The time it takes to get truly dark after sunset varies, influenced by several factors and how “dark” is defined. Understanding the distinct phases of twilight clarifies this progression.
Stages of Twilight
Twilight is divided into three distinct phases, each defined by the sun’s position below the horizon. Civil twilight begins at sunset and continues until the sun is 6 degrees below the horizon. During this time, there is enough natural light for outdoor activities without artificial illumination, and the horizon remains clearly visible. Brightest planets and stars become visible.
Next, nautical twilight occurs when the sun’s center is between 6 and 12 degrees below the horizon. The sky darkens, making the horizon indistinct and requiring artificial lights for most outdoor activities. Brighter stars become visible, which historically allowed sailors to use them for navigation.
The final stage is astronomical twilight, spanning from when the sun is 12 degrees below the horizon until it reaches 18 degrees below. The sky is dark enough for most astronomical observations, with faint stars and celestial objects becoming visible. True night begins once the sun drops more than 18 degrees below the horizon, as its light no longer contributes to sky illumination.
Why Twilight Duration Varies
Twilight duration varies, primarily influenced by two factors: latitude and the time of year. At the equator, the sun sets at a nearly perpendicular angle to the horizon, causing it to descend quickly through the twilight stages, resulting in shorter periods, often 20-30 minutes per phase.
Conversely, at higher latitudes, the sun sets at a shallower angle to the horizon. This means the sun takes longer to drop through each 6-degree increment, leading to extended periods of diminishing light. For example, in New York (approximately 40° North), civil twilight can last about 28 minutes, while in Oslo (around 60° North), it can extend to about 43 minutes.
Season also plays a role due to Earth’s axial tilt. During summer at higher latitudes, the sun’s path causes it to dip less steeply below the horizon, making twilight last longer. In some polar regions during summer, the sun may not even go 18 degrees below the horizon, resulting in continuous twilight without true darkness. In winter, the sun sets at a steeper angle, leading to shorter twilight periods.
Everyday Relevance of Twilight
Twilight’s phases have practical applications. For driving and safety, artificial lighting (headlights, streetlights) becomes necessary during civil twilight due to reduced natural light. As twilight progresses, visibility further decreases, impacting activities reliant on natural light.
Outdoor enthusiasts (hikers, photographers, stargazers) often plan activities around twilight. Photographers favor the “golden hour” (civil twilight) for its warm, soft light, and the “blue hour” (nautical twilight) for its cool tones. For optimal stargazing, true darkness after astronomical twilight is preferred to minimize sky glow and allow fainter celestial objects to be seen.
Knowing twilight duration aids planning for outdoor pursuits requiring specific light conditions, like fishing. The light transition affects animal behavior; many crepuscular animals become active, increasing wildlife encounters (e.g., deer) during morning and evening twilight.