Twilight, dawn and dusk are terms used to describe different times of the day. These phenomena of light are experienced during sunrise and sunset. They also vary with seasons especially during equinoxes. During these times of the day, the sky is filled with unique colors. Most of these occurrences mark the beginning and end of activities like sailing or military operations. Both three times of the day are divided into three categories depending on the angle formed between the sun and Earth’s horizon.
Twilight is time between night and day when the sun is below the horizon but the sun is still outside. The sky is sometimes filled with magical colors. Twilight begins and ends with dawn and dusk respectively. This time is characterized by spectacular colors. This occurs when the upper part of the atmosphere scatters and reflects light from the sun which is illuminated onto the lower atmosphere. There are three types of twilight namely; civil, nautical and astronomical twilights. These three stages are defined with relation to suns elevation in relation to the earth. This is the angle between the earth’s horizon and geometric center of the sun.
Civil twilight occurs when the sun is 6 degrees below the horizon at most. In the morning its starts and ends at sunrise. In the evening its starts when the sun reaches six degrees below horizon and ends at sunset. Nautical twilight occurs when suns’ geometrical center is between 6 and twelve degrees under the horizon. Astronomical is the time of day when the sun is 18 degrees below earth’s horizon. The sky is usually dark at this time. Civil twilight is therefore the brightest form of twilight. Natural light is enough for non-nocturnal animals including man. Brightest celestial objects can be observed at this time. This term has been used to make hunting and aviation laws.
This is the time of day which marks the beginning of twilight before sunrise or a period for sunrise. The time is usually characterized by weak sunlight when the sun is still insignificant under the horizon. Types of dawn are also divided into astronomical, nautical and civil. Astronomical dawn starts when the sky ceases to be dark. The sun is 18 degrees under the horizon at morning hours. Both astronomical dawn and dusk are usually dark even in absence of clouds.
Nautical twilight starts at time when there is enough light for sailing. It is easier to tell between the horizon and the sea. However, artificial light may be required to carry out activities outdoors. This time is usually dark and is considered as night. Civil dawn occurs when the sun is 6 degrees below the horizon in the morning. Enough light is available to distinguish objects but the sky is slightly dark.
This is the darkest moment of twilight, precisely, the very end of astronomical twilight. This happens after sunset and before nightfall. This is preceded by intermediate twilight where there is enough light under the sky. The human eye can read texts without relying on artificial light.
Civil dusk occurs when the center of the sun is 6 degrees below the horizon in the evening. Nautical dusk is time of day when the sun’s center is 12 degrees below horizon in the evening. Stars in the sky can be seen with naked eyes. This term is very common among sailors and the military. For instance, the morning and evening nautical twilight is used by United States military to plan tactical operations. Astronomical dusk is when the geometrical center of the sun is 18 degrees immediately below the horizon. This is the onset of darkness and light is no longer in the sky.
Polar and Equatorial Twilight
The duration of an equinox depends on latitude. Longer twilights are experienced longer on higher latitudes than equatorial and tropical regions. Astronomical twilight during sunrise resembles the one experienced at sunset during summer months. This happens when the angle between the sun and the horizon is less than 18 degrees also known as solar elevation angle. Higher altitudes may also experience an extended nautical twilight.
During the March equinox, there is no astronomical or nautical twilight. In turn, a longer period of civil twilight is experienced during this equinox. Sun is prevalent on the north pole until the equinox in September.During this period no twilight is experienced, a phenomena described as midnight sun or polar day. Just after the September equinox, the civil twilight starts showing up followed by days of nautical twilight then astronomical twilight. This transition ends by October as the sinks below 18 degrees. During this time, the pole experiences polar night. Astronomical twilight manifests itself on the north pole. From there nautical twilight is observed as the sun moves up. The same is observed on the South Pole at equally opposite times.