verkle wrote:Regarding the comment "Earth dwellers experienced 'nearly' 12 hours of daylight and 12 hours of darkness..." seems rather arbitrary to me to define sunrise as the FIRST appearance of sunlight and sunset and the LAST appearance of sunlight. According to this definition, the word used here should be rather "slightly more than 12 hours", not "nearly 12 hours", since everyone on earth enjoys more than 12 hours on equinox day. Isn't there a more scientific definition of sunrise and sunset that defines it based on the geometric center of the sun?
http://en.wikipedia.org/wiki/Equinox wrote:
<<Although the word equinox is often understood to mean "equal [day and] night"
this is not strictly true. An equinox happens each year
at two specific moments in time when there is a location on the Earth's Equator where the centre of the Sun can be observed to be vertically overhead.
For most locations on earth, there are two distinct identifiable days per year when the length of day and night are closest to being equal; those days are commonly referred to as the "equiluxes" to distinguish them from the equinoxes. Equinoxes are points in time, but equiluxes are days. By convention, equiluxes are the days where sunrise and sunset are closest to being exactly 12 hours apart.>>
Equilux for
Tihany will occur around
October 13th.
http://www.youtube.com/watch?v=t0rGIyCf ... re=related
In the tropics between about 15º S and 15º N day is always longer than night.
http://en.wikipedia.org/wiki/Civil_twilight#Civil_twilight wrote:
<<Twilight is the time between dawn and sunrise, and the time between sunset and dusk. Sunlight scattered in the upper atmosphere illuminates the lower atmosphere, and the surface of the Earth is neither completely lit nor completely dark. The sun itself is not actually visible because it is below the horizon. Twilight is technically defined as the period before sunrise and again after sunset during which there is natural light provided by the upper atmosphere, which does receive direct sunlight and reflects part of it toward the Earth's surface.
Twilight is defined according to the position of the sun (its centre) relative to the horizon. There are three established and widely accepted subcategories of twilight: civil twilight (brightest), nautical twilight and astronomical twilight (darkest).
- Definition Position of sun
----------------------------------------
Night more than 18°
Astronomical twilight 12 – 18°
Nautical twilight 6 – 12°
Civil twilight less than 6°
Day (sun above the horizon)
----------------------------------------
(For these definitions, an ideal horizon 90° from the zenith is used. The altitudes of the sun below the horizon are "true geometric" altitudes; that is, refraction by the atmosphere and other small factors influencing the experiential position of the sun are not to be accounted for.) Note that if the sun is 8½ degrees below the horizon, it provides the same level of illumination to the surface of the Earth as a full moon directly overhead.
Morning civil twilight begins when the geometric center of the sun is 6° below the horizon (the point of civil dawn), and ends at sunrise. Evening civil twilight begins at sunset and ends when the center of the sun reaches 6° below the horizon (the point of civil dusk). The brightest stars appear during civil twilight, as well as planets, such as Venus, which is known as the 'morning star' and/or 'evening star'. During this period there is enough light from the sun that artificial sources of light may not be needed to carry on outdoor activities. This concept is sometimes enshrined in laws, for example, when drivers of automobiles must turn on their headlights, when pilots may exercise the rights to fly aircraft, or if the crime of burglary is to be treated as nighttime burglary, which carries stiffer penalties in some jurisdictions. A fixed period (most commonly 30 minutes after sunset or before sunrise) is typically used in such statutes, rather than how many degrees the sun is below the horizon. Civil twilight can also be described as the limit at which twilight illumination is sufficient, under good weather conditions, for terrestrial objects to be clearly distinguished; at the beginning of morning civil twilight, or end of evening civil twilight, the horizon is clearly defined and the brightest stars are visible under good atmospheric conditions.>>
[quote="verkle"]Regarding the comment "Earth dwellers experienced 'nearly' 12 hours of daylight and 12 hours of darkness..." seems rather arbitrary to me to define sunrise as the FIRST appearance of sunlight and sunset and the LAST appearance of sunlight. According to this definition, the word used here should be rather "slightly more than 12 hours", not "nearly 12 hours", since everyone on earth enjoys more than 12 hours on equinox day. Isn't there a more scientific definition of sunrise and sunset that defines it based on the geometric center of the sun?[/quote]
[quote=" http://en.wikipedia.org/wiki/Equinox"]
<<Although the word equinox is often understood to mean "equal [day and] night" [b]this is not strictly true[/b]. An equinox happens each year [b]at two specific moments in time when there is a location on the Earth's Equator where the centre of the Sun can be observed to be vertically overhead[/b].
For most locations on earth, there are two distinct identifiable days per year when the length of day and night are closest to being equal; those days are commonly referred to as the "equiluxes" to distinguish them from the equinoxes. Equinoxes are points in time, but equiluxes are days. By convention, equiluxes are the days where sunrise and sunset are closest to being exactly 12 hours apart.>>[/quote]
Equilux for [url=http://en.wikipedia.org/wiki/Tihany][b]Tihany[/b][/url] will occur around [url=http://en.wikipedia.org/wiki/File:TwilightLength.png][b]October 13th[/b][/url].
[b] http://www.youtube.com/watch?v=t0rGIyCfOsQ&feature=related
In the tropics between about 15º S and 15º N day is always longer than night.[/b]
[quote=" http://en.wikipedia.org/wiki/Civil_twilight#Civil_twilight"]
<<Twilight is the time between dawn and sunrise, and the time between sunset and dusk. Sunlight scattered in the upper atmosphere illuminates the lower atmosphere, and the surface of the Earth is neither completely lit nor completely dark. The sun itself is not actually visible because it is below the horizon. Twilight is technically defined as the period before sunrise and again after sunset during which there is natural light provided by the upper atmosphere, which does receive direct sunlight and reflects part of it toward the Earth's surface.
Twilight is defined according to the position of the sun (its centre) relative to the horizon. There are three established and widely accepted subcategories of twilight: civil twilight (brightest), nautical twilight and astronomical twilight (darkest).
[list][list]Definition Position of sun
----------------------------------------
Night more than 18°
Astronomical twilight 12 – 18°
Nautical twilight 6 – 12°
Civil twilight less than 6°
Day (sun above the horizon)
----------------------------------------[/list]
(For these definitions, an ideal horizon 90° from the zenith is used. The altitudes of the sun below the horizon are "true geometric" altitudes; that is, refraction by the atmosphere and other small factors influencing the experiential position of the sun are not to be accounted for.) Note that if the sun is 8½ degrees below the horizon, it provides the same level of illumination to the surface of the Earth as a full moon directly overhead.[/list]
Morning civil twilight begins when the geometric center of the sun is 6° below the horizon (the point of civil dawn), and ends at sunrise. Evening civil twilight begins at sunset and ends when the center of the sun reaches 6° below the horizon (the point of civil dusk). The brightest stars appear during civil twilight, as well as planets, such as Venus, which is known as the 'morning star' and/or 'evening star'. During this period there is enough light from the sun that artificial sources of light may not be needed to carry on outdoor activities. This concept is sometimes enshrined in laws, for example, when drivers of automobiles must turn on their headlights, when pilots may exercise the rights to fly aircraft, or if the crime of burglary is to be treated as nighttime burglary, which carries stiffer penalties in some jurisdictions. A fixed period (most commonly 30 minutes after sunset or before sunrise) is typically used in such statutes, rather than how many degrees the sun is below the horizon. Civil twilight can also be described as the limit at which twilight illumination is sufficient, under good weather conditions, for terrestrial objects to be clearly distinguished; at the beginning of morning civil twilight, or end of evening civil twilight, the horizon is clearly defined and the brightest stars are visible under good atmospheric conditions.>>[/quote]