The three phases of twilight—civil, nautical, and astronomical—are defined by astronomers based on the Sun's elevation, which is the angle formed by the geometric center of the Sun with the horizon. Varying degrees of twilight When the sun is less than 6 degrees below the horizon, it is considered civil twilight. From this point until midnight, the sky is dark enough to see stars but not completely black. Nautical twilight begins at 12 degrees below the horizon and lasts for an hour and a half. During this time, there are sufficient shadows beneath boats that you could use them as guides while hiking or skiing. Astronomical twilight ends when the Sun reaches its highest point in the sky, which is called sunset. The night becomes fully dark again when the Sun drops below the horizon.
Civil twilight ends when the Sun is directly over the horizon, which is usually around noon during the summer months. At this point, sunlight is directly overhead, so all points on the compass are equally illuminated. Nautical twilight ends when the last bit of daylight disappears under the horizon. Astronomical twilight ends when the first star appears in the east. This marks the beginning of true night, when only faint lights from cities are visible before dawn.
During a total solar eclipse, the Earth's atmosphere refracts light from the Sun, causing the edge of the Moon to appear reddish. As the eclipse progresses, this effect becomes more prominent; by the time totality comes around, the Moon has taken on a red color.
Astronomers, on the other hand, divide twilight into three stages: civil twilight, nautical twilight, and astronomical twilight. Civil twilight ends when there are less than 5 minutes after sunset or before sunrise. Nautical twilight starts when there is less than a half-hour of daylight at mid-northern latitudes (about 55 degrees north and south). Astronomical twilight begins when night falls completely. The three states of twilight can occur either simultaneously or in turn. For example, civil twilight might end while nautical twilight remains, or vice versa.
This occurs only during certain times of the year because of solar elevation above the horizon. At these times, sunsets happen more quickly and disappear below the horizon while dawns take longer to come up because it takes time for enough sunlight to reach the horizon to make up for what's gone down hill since sunset. These events are called "half moons" or "new moons". New moon means the moon is directly opposite the earth from the center of earth's orbit. When this happens, no part of the moon is facing toward the earth, so it doesn't reflect any light back to us.
The Civil Twilight Civil twilight is the brightest phase of twilight and lasts from the moment the sun descends below the horizon until the sun's center is 6 degrees below the horizon (or from the time the sun is 6 degrees below the horizon until it hits the horizon in the morning). During this time, only the highest clouds are still visible. As night falls, the sky becomes increasingly darker, so by the time civil twilight ends, it is dark enough to need a light source such as a flashlight or headlamp.
Civil twilight is used by astronomers to estimate the time when stars can first be seen with the unaided eye, which is about an hour after sunset or before sunrise. Because daylight savings time moves our clocks one hour forward at 2am on March 31st, then back one hour at 2am on November 3rd, these dates vary year to year. Astronomers use the word "solstice" to describe the point when sunlight is at its greatest distance from the earth during the day and opposite the equator; at the northern hemisphere's mid-winter solstice, the sun is at its farthest north, while at the southern hemisphere's mid-summer solstice, it is at its farthest south.
Sunset is distinct from twilight, which is divided into three stages: the first is civil twilight, which begins once the sun has disappeared below the horizon and continues until it descends to 6 degrees below the horizon; the second is nautical twilight, which occurs between 6 and 12 degrees below the horizon; and the third is nocturnal twilight, which occurs between 6 and 12 degrees below the horizon. Sunset is the moment when light leaves the sky at the end of the day.
The Sun goes down at its highest point in the sky each night and rises at its lowest point the next morning. Thus, it can be said that the Sun moves across the sky as it sets and rises. Have you ever watched the Sun set over the ocean or a large body of water? You will see that it is surrounded by many colors, including red, orange, yellow, green, blue, indigo, and violet. These are called sunset colors because they are visible only at sunset or during very dark nights.
You may have also seen that the Sun does not always go down at its highest point in the sky and rise at its lowest point the next morning. If you watch the Sun during a total solar eclipse, for example, you will see that it stays in the same place in the sky while a shadow falls across it. This is because Earth gets in the way of the Sun's light as it travels through space. Only the part of the light beam traveling along our line of sight reaches the surface.