This period of time usually lasts around 30 minutes, however it might be longer or shorter depending on the time of year and your location on the planet. During civil twilight, you may also be able to view some of the brightest stars in the sky, as well as planets such as Venus. After total darkness has fallen, only light from the moon or nearby stars is visible.
The color of sunlight gradually fades after it hits the Earth's atmosphere because red light is more easily absorbed than blue light. The amount of fading varies depending on the weather at your location - if it's clear with no clouds present, then more sunlight reaches the ground surface to start with. If it's foggy or overcast, then less reaches the ground.
On average, the color of sunsets lasts for about 30 minutes in mid-latitudes and less near the poles. However, this period can extend up to an hour in tropical regions like Hawaii and Amazonia. Sunsets also tend to be brighter near the horizon and darker as you go higher into the sky.
After several minutes without seeing any color change, it's safe to assume that the sunset has ended. This is when you should start looking for new sights to see during sunset season!
The color of sunrises is generally much stronger than that of sunsets because they're seen against a dark background rather than daylight itself.
Civil twilight is essentially similar to Lighting-up Time, when the Sun's center is 6 degrees below the horizon. It is between 30 and 60 minutes after sunset in the United Kingdom. The brightest stars are visible, and the horizon is well defined at sea. Civil twilight ends when it gets completely dark outside.
Nautical twilight starts when the sun is 12 degrees below the horizon. At this point you can still see daylight colors on the sky but it is dark enough to see moderate detail on a clear night with no clouds or fog present. Nautical twilight ends when it gets completely dark outside. During nautical twilight, you need artificial light to see what is happening on the ground.
Astronomical twilight occurs when there is only one star visible to the naked eye from any given location. This happens approximately half way between civil and nautical twilight. Astronomical twilight ends when it gets completely dark outside. Until then, you can still use artificial lights to explore during these hours.
Solar twilight occurs when the Sun is up but not directly above you. During solar twilight, you need sunlight with some degree of illumination to see what is happening on the ground. Solar twilight ends when it gets completely dark outside. You cannot see anything during this time except under certain conditions with electric lights.
The length of twilight varies with latitude and season since it is determined by the angle of the sun's travel over the horizon at sunset and sunrise. The longest twilight occurs near the poles, lasting roughly six weeks before and after the yearly sunrise and sunset. The shortest twilight occurs near the equator, only about four hours long when the sun is due south or north. In between these two extremes are several other periods of twilight each year depending on the location of the sun.
As one moves away from the poles, the length of twilight decreases until it reaches a minimum near the equator. There the day is not completely dark even at noon because the sun is so far south that its rays reach all the way across the globe to illuminate the opposite side!
As one moves further from the equator, the length of daylight increases again until it reaches a maximum near the poles. Here night lasts for nearly five months following the sun past the horizon.
Since the distance that light travels in one second (the speed of light) is always constant, this means that the period of twilight will lengthen as you go farther from the equator. For example, at 45 degrees north latitude in northern Canada there is almost no twilight during the summer months because the sun is high in the sky then, leaving very little time before sunrise and after sunset when darkness can be seen against the backdrop of the sky.
Extending from one day to the next Twilight can extend from sunset until sunrise for latitudes higher than 48 degrees North or South on days near the summer solstice, since the sun can not descend more than 18 degrees below the horizon, hence full darkness does not occur even at solar midnight. At lower latitudes and in the winter, twilight is shorter, usually ending around midnight.
In terms of hours, twilight lasts for about six months starting just after midnight and ending just before dawn. It then repeats itself again six months later.
The length of twilight is dependent on the angle between the Earth and the Sun, so it varies over the year. In addition, the color of daylight changes as well; during mid-summer there is less blue light and more green light coming from the sky compared to fall or spring when the opposite is true. All together this means that daytime turns out to be darker and cooler as we move further away from the summer solstice.
Furthermore, because the rotation of the Earth is not a perfect circle but takes 24 hours 56 minutes, seasons affect how much time there is of sunlight each day. The amount of daylight decreases when you go from the equator towards the poles. By comparison, at the same latitude but in a place like Alaska where there is much less daily variation in temperature, there is always 12 hours of daylight throughout the year.