What Is Dawn

What Is Dawn

Dawn refers to the transition from night to day as the sky gets brighter and the morning twilight heralds the beginning of a new day. The term has different colloquial meanings, but there is also a scientific definition. Dawn on Santorini island, Greece.

Scientific Definition

Science specifies dawn as a particular moment defined by the solar elevation angle, which is the position of the Sun in relation to the horizon. As with dusk, astronomers differentiate between 3 different stages of dawn, each marking the beginning of 1 of the 3 phases of morning twilight:

1. Astronomical dawn – the geometric center of the Sun's disk is 18 degrees below the horizon.
2. Nautical dawn – the geometric center of the Sun's disk is 12 degrees below the horizon.
3. Civil dawn – the geometric center of the Sun's disk is 6 degrees below the horizon.

Astronomical Dawn

The 3 stages of dawn and morning twilight.
When the Sun passes the elevation angle of -18 degrees as it ascends towards the horizon before sunrise, a very small portion of its rays begin to permeate the firmament. The Earth's upper atmosphere begins to scatter and reflect the sunlight, illuminating the lower atmosphere.
However, at this point, the twilight is so faint that it is generally indistinguishable from night, especially in areas with light pollution. Astronomers may be unable to observe some of the fainter stars and galaxies as the Sun passes this mark.
Astronomical dawn marks the beginning of astronomical twilight, which lasts until nautical dawn.

What is “False Dawn”?
Nautical Dawn

At nautical dawn, the geometric center of the Sun's disk reaches an angle of 12 degrees below the horizon. The sunlight reflected by the atmosphere is now generally sufficient to distinguish the sky from land or water in clear weather conditions. As the horizon becomes faintly visible during this stage, seafarers can use it as a reference point when navigating by the stars.

Nautical dawn rings in the period of nautical twilight, which ends at civil dawn.

Civil Dawn

Civil dawn, the brightest instance of dawn, occurs when the geometric center of the Sun's disc is 6° below the horizon. If the sky is clear, it is now enveloped in bright orange and yellow colors. At this point, only the brightest stars and planets, like Venus and Jupiter, are visible to the naked eye.
Find the planets in the night sky
Even on a cloudy morning, there is now probably enough indirect natural light to make it possible to carry out most outdoor activities without artificial lighting.
This moment marks the beginning of civil twilight, the period of daybreak just before sunrise. It ends as the upper edge of the Sun touches the horizon.

Popular Usage

Colloquially, dawn is used to denote daybreak in general. It is also often seen as a synonym for the period of morning twilight when the Sun is still below the horizon, but its rays are already reflected by the Earth's atmosphere, lighting up the sky.
The term is also sometimes used as another word for sunrise – the moment when the upper edge of the Sun's disc appears above the horizon and the first direct sunlight of the day becomes visible.

Religious Significance

As the beginning of a new day, dawn has a special significance in many of the world's religions. However, the definition of the term varies from one faith and religious community to another.
For example, Muslims are required to offer the Fajr prayer, which is 1 of the 5 obligatory daily prayers in Islam and constitutes one of the Five Pillars of the Islamic faith, during the morning twilight period. Even within Islam, however, there are differing opinions about the correct moment for the prayer.
The Jewish Holy Scripture also dictates dawn as a time for prayer. The Talmud defines dawn as the moment 72 minutes before sunrise. However, some Jewish communities claim that this definition pertains only to the situation in Mesopotamia on the days of the equinoxes. These groups calculate the correct moment of dawn for each day and location, based on a solar elevation angle of 16.1 degrees below the horizon.

How is the time of sunrise calculated?

This seems simple but is actually a very interesting question. The first thing that you have to decide upon is your definition of when the sun is rising. Is it when the middle of the Sun crosses the horizon, or the top edge, or the bottom edge. Also do you take the horizon to be sea level or do you take into account the topography of the location you are at. The Earth's atmosphere can also have an effect, refracting (or bending) the light so that the Sun appears to rise a few moments earlier/later than it would if there were no atmosphere. The US Naval Observatory define sunrise/sunset as follows (in their FAQ):

"Sunrise and sunset conventionally refer to the times when the upper edge of the disk of the Sun is on the horizon, considered unobstructed relative to the location of interest. Atmospheric conditions are assumed to be average, and the location is in a level region on the Earth's surface."

As the Earth rotates about its axis celestial objects appear to rise and set. Objects at declination zero (or directly "above" the Earth's equator (see our explanation of right ascension and declination) are up for twelve hours and down for twelve hours. When the Sun is at declination zero (at the vernal and autumnal equinoxes) it is therefore up for twelve hours and and down for twelve hours. The point at which is it at its highest (when it crosses the meridian) is defined to be local noon. For observers that are in the northern hemisphere, objects that are north of the celestial equator are up for longer than twelve hours (to the point that some are circumpolar and never set). Objects south of the celestial equator are up for less than twelve hours (some never rise). In winter (in the Northern hemisphere) the Sun is south of the celestial equator so is up for less than twelve hours - the exact length of time depends on its exact declination. Local noon is still the time when it reaches its highest point.

So, using local time it is theoretically easy to figure out when the Sun rises. You work out its declination from the time of the year it is. Then you work out how long an object at that declination is up in the sky for give your latitude on Earth. Then you subtract approximately half (see first paragraph for why approximately) of that time from local noon to find the local time at which the sun rises. Of course we don't use local time - we have time zones in which the time is defined to be the same for a certain area of the Earth. Only in the exact centre of that time zone is the time equal to local time. So you also need to know your longitude to know the difference between your local time and the time you use on your watch.

Reference Link

Different types of dawn and the difference between dawn and sunrise and the meaning of twilight

Dawn occurs before sunrise, before the top of the Sun reaches the horizon. Astronomical Dawn is the point at which it becomes possible to detect light in the sky, when the sun is 18° below the horizon. Nautical Dawn occurs at 12° below the horizon, when it becomes possible to see the horizon properly and distinguish some objects. Civil Dawn occurs when the sun is 6° below the horizon and there is enough light for activities to take place without artificial lighting.
Dusk occurs after sunset, once the top of the Sun has passed the horizon. As with dawn there is astronomical dusk, nautical dusk and civil dusk, occurring at 18°, 12° and 6° below the horizon respectively.
Twilight is the name given to the period between dawn and sunrise, or between sunset and dusk, when light is still visible in the sky due to sunlight scattering off the atmosphere. It can also be separated in astronomical, nautical and civil sections by how far below the horizon the Sun is.
Sunrise and sunset are the points at which the top edge of the Sun reaches the horizon; the only difference between them is the direction in which the Sun is moving at the time. It actually occurs when the top of the Sun is 0.57° below the horizon due to refraction of the Sun’s light by the atmosphere.

Reference Link