The process of the sky losing light after sunset is gradual, a phenomenon known as twilight. Twilight occurs because the atmosphere continues to scatter sunlight even after the sun dips below the horizon. This period is not uniform across the globe or throughout the year, as it depends on Earth’s geometry and position relative to the sun. To determine when darkness falls in a location like Texas, one must understand how astronomers define the different phases of evening light.
Defining Darkness and the Stages of Twilight
The transition from day to night is divided into three distinct phases, defined by the solar depression angle (how far the sun’s center is below the horizon). The end of this process marks the moment the sky is considered fully “dark” for the general public.
The first phase is Civil Twilight, which begins at sunset and ends when the sun reaches six degrees below the horizon. During this time, the horizon is clearly distinguishable, and there is enough ambient light for most outdoor activities to proceed without artificial lighting. Only the brightest planets and stars are typically visible at this stage.
Following this is Nautical Twilight, which lasts while the sun is between six and twelve degrees below the horizon. The horizon becomes difficult to discern at this point, which is why this phase earned its name, as sailors would use the visible bright stars and the faint horizon line for celestial navigation. Although some natural light remains, detailed outdoor tasks require supplemental illumination.
The final stage is Astronomical Twilight, which continues until the sun is eighteen degrees below the horizon. This boundary marks the point where the sun’s illumination is minimal and no longer scatters enough light to interfere with astronomical observation. For most observers, the sky is effectively “dark” once astronomical twilight ends.
How Latitude and Season Impact Twilight Duration
The total length of time it takes for the sky to progress through these three twilight phases is variable, depending on a location’s latitude and the time of year. This variation occurs because the angle at which the sun descends below the horizon changes across the globe.
Near the equator (low latitudes), the sun’s path is nearly perpendicular to the horizon at sunset, causing it to drop quickly to the required 18 degrees below the horizon. This steep angle means the total twilight period is short, often lasting only about 72 minutes from sunset to the end of astronomical twilight around the equinoxes.
Moving toward the poles (higher latitudes), the sun sets at a much shallower angle to the horizon. Because the sun is moving more horizontally relative to the observer, it takes a longer time to achieve the 18-degree depression necessary to end astronomical twilight. This phenomenon explains why northern regions experience twilight lasting for many hours during the summer months.
The seasonal impact is due to the Earth’s axial tilt, which changes the sun’s declination throughout the year. Twilight is shortest near the equinoxes, when the sun is directly over the equator, and longest near the solstices when the sun is farthest north or south. This tilt causes the sun to follow a shallower path in the sky during summer and a steeper path during winter.
Applying the Concepts to Texas
Texas spans a significant range of latitudes, which directly influences the duration of evening twilight. The state extends from approximately 25.9 degrees North near Brownsville to about 36.5 degrees North in the Panhandle region near Amarillo. This difference in latitude creates a measurable variance in the time it takes for the sky to achieve astronomical darkness.
In the southern part of the state, such as Brownsville, the sun sets at a steeper angle because the location is closer to the equator. During the month of January, for example, astronomical twilight ends approximately 81 minutes after sunset. This represents the shorter end of the twilight duration scale for Texas.
Conversely, in the northern Panhandle city of Amarillo, the sun’s setting path is shallower due to the higher latitude. This shallower angle means the sun takes longer to drop 18 degrees below the horizon. The total twilight period in Amarillo during the same time in January extends to about 90 minutes after sunset.
Therefore, for anyone in Texas seeking to know how long after sunset it gets truly dark, the answer is not fixed but generally falls within a range. Depending on the specific location’s latitude and the time of year, the sky reaches full astronomical darkness between 75 and 90 minutes after the official sunset time. The northern regions consistently experience a longer period of twilight than the southern regions.