................lets break down day.......
From Wikipedia, the free encyclopedia
A day (symbol d) is a unit of time equivalent to approximately 24 hours. It is not an SI unit but it is accepted for use with SI. The SI unit of time is the second.
The word 'day' can also refer to the (roughly) half of the day that is not night, also known as 'daytime'. Both refer to a length of time. Within these meanings, several definitions can be distinguished. 'Day' may also refer to a day of the week or to a calendar date, as in answer to the question "On which day?".
The term comes from the Old English dæg, with similar terms common in all other Indo-European languages, such as Tag in German, dies in Latin, dydd in Welsh or dive in Sanskrit.
 International System of Units (SI)
A day is defined as 86,400 seconds.
A day on the UTC time scale can include a negative or positive leap second, and can therefore have a length of 86,399 or 86,401 seconds.
The International Bureau of Weights and Measures (BIPM) currently defines a second as
… the duration of 9 192 631 770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium 133 atom.
This makes the SI-based day last exactly 794,243,384,928,000 of those periods.
In the 19th century it had also been suggested to make a decimal fraction (1⁄10,000 or 1⁄100,000) of an astronomic day the base unit of time. This was an afterglow of decimal time and calendar, which had been given up already.
A day of exactly 86,400 SI seconds is the fundamental unit of time in astronomy.
For a given planet, there are two types of day defined in astronomy:
- 1 apparent sidereal day - a single rotation of a planet with respect to the distant stars (for Earth it is 23.934 hours);
- 1 solar day - a single rotation of a planet with respect to its star.
In astronomy, the sidereal day is also used; it is about 3 minutes 56 seconds shorter than the solar day, and close to the actual rotation period of the Earth, as opposed to the Sun's apparent motion. In fact, the Earth spins 366 times about its axis during a 365-day year, because the Earth's revolution about the Sun removes one apparent turn of the Sun about the Earth.
The word refers to various relatedly defined ideas, including the following:
- 24 hours (exactly)
- the period of light when the Sun is above the local horizon (i.e., the time period from sunrise to sunset);
- the full day covering a dark and a light period, beginning from the beginning of the dark period or from a point near the middle of the dark period;
- a full dark and light period, sometimes called a nychthemeron in English, from the Greek for night-day;
- the time period from 6:00 AM to 6:00 PM or 9:00 PM or some other fixed clock period overlapping or set off from other time periods such as "morning", "evening", or "night".
The word day is used for several different units of time based on the rotation of the Earth around its axis. The most important one follows the apparent motion of the Sun across the sky (solar day). The reason for this apparent motion is the rotation of the Earth around its axis, as well as the revolution of the Earth in its orbit around the Sun.
A day, as opposed to night, is commonly defined as the period during which sunlight directly reaches the ground, assuming that there are no local obstacles. Two effects make days on average longer than nights. The Sun is not a point, but has an apparent size of about 32 minutes of arc. Additionally, the atmosphere refracts sunlight in such a way that some of it reaches the ground even when the Sun is below the horizon by about 34 minutes of arc. So the first light reaches the ground when the centre of the Sun is still below the horizon by about 50 minutes of arc. The difference in time depends on the angle at which the Sun rises and sets (itself a function of latitude), but amounts to almost seven minutes at least.
Ancient custom has a new day start at either the rising or setting of the Sun on the local horizon (Italian reckoning, for example) The exact moment of, and the interval between, two sunrises or two sunsets depends on the geographical position (longitude as well as latitude), and the time of year. This is the time as indicated by ancient hemispherical sundials.
A more constant day can be defined by the Sun passing through the local meridian, which happens at local noon (upper culmination) or midnight (lower culmination). The exact moment is dependent on the geographical longitude, and to a lesser extent on the time of the year. The length of such a day is nearly constant (24 hours ± 30 seconds). This is the time as indicated by modern sundials.
A further improvement defines a fictitious mean Sun that moves with constant speed along the celestial equator; the speed is the same as the average speed of the real Sun, but this removes the variation over a year as the Earth moves along its orbit around the Sun (due to both its velocity and its axial tilt).
The Earth's day has increased in length over time. The original length of one day, when the Earth was new about 4.5 billion years ago, was about six hours as determined by computer simulation. It was 21.9 hours 620 million years ago as recorded by rhythmites (alternating layers in sandstone). This phenomenon is due to tides raised by the Moon which slow Earth's rotation. Because of the way the second is defined, the mean length of a day is now about 86,400.002 seconds, and is increasing by about 1.7 milliseconds per century (an average over the last 2,700 years). See tidal acceleration for details.
 Civil day
For civil purposes a common clock time has been defined for an entire region based on the mean local solar time at some central meridian. Such time zones began to be adopted about the middle of the 19th century when railroads with regular schedules came into use, with most major countries having adopted them by 1929. For the whole world, 40 such time zones are now in use. The main one is "world time" or Coordinated Universal Time (UTC).
The present common convention has the civil day starting at midnight, which is near the time of the lower culmination of the mean Sun on the central meridian of the time zone. A day is commonly divided into 24 hours of 60 minutes of 60 seconds each.
 Leap seconds
To keep the civil day aligned with the apparent movement of the Sun, positive or negative leap seconds may be inserted.
A civil clock day is typically 86,400 SI seconds long, but will be 86,401 s or 86,399 s long in the event of a leap second.
Leap seconds are announced in advance by the International Earth Rotation and Reference Systems Service which measures the Earth's rotation and determines whether a leap second is necessary. Leap seconds occur only at the end of a UTC month, and have only ever been inserted at the end of June 30 or December 31.
 Boundaries of the day
For most diurnal animals, the day naturally begins at dawn and ends at sunset. Humans, with our cultural norms and scientific knowledge, have supplanted Nature with several different conceptions of the day's boundaries. The Jewish day begins at either sunset or at nightfall (when three second-magnitude stars appear). Medieval Europe followed this tradition, known as Florentine reckoning: in this system, a reference like "two hours into the day" meant two hours after sunset and thus times during the evening need to be shifted back one calendar day in modern reckoning. Days such as Christmas Eve, Halloween, and the Eve of Saint Agnes are the remnants of the older pattern when holidays began the evening before. Present common convention is for the civil day to begin at midnight, that is 00:00 (inclusive), and last a full twenty-four hours until 24:00 (exclusive).
The bible defines day as either "light" or as "Evening and morning were the first day"(Gen 1:5). In ancient Egypt, the day was reckoned from sunrise to sunrise. Muslims fast from daybreak to sunset each day of the month of Ramadan. The "Damascus Document", copies of which were also found among the Dead Sea scrolls, states regarding Sabbath observance that "No one is to do any work on Friday from the moment that the sun's disk stands distant from the horizon by the length of its own diameter," presumably indicating that the monastic community responsible for producing this work counted the day as ending shortly before the sun had begun to set.
In the United States, nights are named after the previous day, e.g. "Friday night" usually means the entire night between Friday and Saturday. This is the opposite of the Jewish pattern. This difference from the civil day often leads to confusion. Events starting at midnight are often announced as occurring the day before. TV-guides tend to list nightly programs at the previous day, although programming a VCR requires the strict logic of starting the new day at 00:00 (to further confuse the issue, VCRs set to the 12-hour clock notation will label this "12:00 AM"). Expressions like "today", "yesterday" and "tomorrow" become ambiguous during the night.
Validity of tickets, passes, etc., for a day or a number of days may end at midnight, or closing time, when that is earlier. However, if a service (e.g. public transport) operates from e.g. 6:00 to 1:00 the next day (which may be noted as 25:00), the last hour may well count as being part of the previous day (also for the arrangement of the timetable). For services depending on the day ("closed on Sundays", "does not run on Fridays", etc.) there is a risk of ambiguity. As an example, for the Nederlandse Spoorwegen (Dutch Railways), a day ticket is valid 28 hours, from 0:00 to 28:00 (i.e. 4:00 the next day). To give another example, the validity of a pass on London Regional Transport services is until the end of the "transport day" -- that is to say, until 4:30 am on the day after the "expiry" date stamped on the pass.
 Metaphorical days
In the Bible, as a way to describe that time is immaterial to God, one day is described as being like one thousand years (Psalms 90:4, 2 Peter 3:8) to him. Also in 2 Peter 3:8, one thousand years is described as being like one day. However, some Bible experts interpret this more literally as a way to understand some prophecies like those in Book of Daniel and others (like the Book of Revelation) where are mentioned days in form of weeks and years.
 24 hours vs daytime
To distinguish between a full day and daytime, the English word nychthemeron may be used for the former, or more colloquially the term 24 hours. In other languages, the latter is also often used. Other languages also have a separate word for a full day, such as in Hebrew, dygn in Swedish, etmaal in Dutch and in Russian. In Spanish, singladura is used, but only as a marine unit of length, being the distance covered in 24 hours.
 See also
 Notes and references
- ^ NISI Guide to the SI
- ^ Resolution 1 of the 13th meeting of the CGPM (1967/68)
- ^ "singladura - Definición". WordReference.com. http://www.wordreference.com/definicion/singladura. Retrieved on 2009-03-22.
 External links
............now lets go deeper....
|day·time [ dáy tm ]
|sunlit part of day: the part of the day when there is natural light
|of or for daytime: occurring, done, or used during the daytime
From Wikipedia, the free encyclopedia
On Earth, daytime is roughly the period on any given point of the planet's surface during which it experiences natural illumination from indirect or (especially) direct sunlight.
Other planets that rotate in relation to a luminous primary, such as a local star, also experience daytime of a sort, but this article primarily discusses daytime on Earth.
Approximately half of the Earth is illuminated at any given time by sunlight. The area subjected to direct illumination is almost exactly half the planet; but because of atmospheric and other effects that extend the reach of indirect illumination, the area of the planet covered by either direct or indirect illumination amounts to slightly more than half the surface.
The hemisphere of the Earth experiencing daytime at any given instant changes continuously as the planet rotates on its own axis. The axis of the Earth's rotation is not exactly perpendicular to the plane of its orbit around the Sun (which is parallel with the direction of sunlight), and so the length of the daytime period varies from one point on the planet to another. Additionally, since the axis of rotation is relatively fixed in comparison to the stars, it moves with respect to the Sun as the planet orbits the star. This creates seasonal variations in the length of the daytime period at most points on the planet's surface.
The period of daytime from the standpoint of a surface observer is roughly defined as the period between sunrise, when the Earth's rotation towards the east first causes the Sun's disc to appear above the horizon, to sunset, when the continuing rotation of the Earth causes the Sun's disc to disappear below the horizon to the west. Because the Sun' is a luminous disc as seen from the Earth, rather than a point source of light, sunrise and sunset are not instantaneous and the exact definition of both can vary with context. Additionally, the Earth's atmosphere further diffuses light from the Sun and lengthens the period of sunrise and sunset. For a certain period after sunset and before sunrise, indirect light from the Sun lightens the sky on Earth; this period is often referred to as twilight. Certain groups, such as Earthly astronomers, do not consider daytime to be truly ended until the Sun's disc is actually well below the Earth's horizon, because of this indirect illumination.
 Daytime variations with latitude and seasons
Earth daylight Northern summer
Given that the Earth's own axis of rotation is inclined by about 23.5 degrees from the perpendicular (as compared to its orbital plane), the length of the daytime period varies with seasons on the planet's surface, depending on the observer's latitude. Areas experiencing summer are tilted toward the sun. Their tilt toward the sun leads to over half of the day being in daylight and warmer temperatures due to the increased directness of the sun's rays. While increased daylight can have some effect on the increased temperature in the summer, most of the increase in temperature is due to the directness of the sun, not the increased daylight. The high (near 90 degrees) angles of the sun is what causes the tropics to be warm while low (barely above the horizon) angles at the poles is what causes then to be cold. Hours of daylight having little effect on temperature can can be seen with the poles still being cold in their respective summers despite 24 hours of daylight, while the equator is warm with only 12 hours of daylight.
Although the length of the daytime period is always twelve hours at the Equator, in all seasons, at all other latitudes the length varies with the season. During the winter, the daytime period is shorter than twelve hours; during the summer, it is longer than 12 hours. When it is winter north of the Equator, it is summer south of the Equator, and vice versa.
Earth daylight Northern winter
 At the Equator
At the Equator, the daytime period is always almost twelve hours in length, no matter what the season. The sun always rises nearly perpendicular to the horizon. During the summer months it rises a bit north of east, and sets a bit north of west. During the winter months it rises a bit south of east and sets a bit south of west. The path of the Sun lies entirely in the northern half of the sky for the summer half of the year and is entirely in the southern half in the winter part of the year, with the Sun passing directly overhead at noon on the equinoxes.
The fact that the Sun is always so close to the vertical at noon on the Equator (never being more than 23.5 degrees north or south) explains why equatorial regions are the hottest regions on the planet overall. Additionally, sunrise and sunset always occur very quickly at the Equator, because the Sun's path across the sky is so nearly vertical with respect to the horizon; at the equinox, the Sun requires only two minutes to traverse the horizon at sunrise and sunset.
 In the tropics
The tropics occupy a band of the Earth's surface between 23.5° north latitude and 23.5° south latitude. Within this band, the Sun will pass almost directly overhead on at least one day per year. The line of 23.5° north latitude is called the Tropic of Cancer, because the Sun passes overhead at the time of year when it is roughly within the zodiac sign of Cancer. The equivalent line of south latitude is called the Tropic of Capricorn, for similar reasons.
On the Tropics of Cancer and Capricorn, the Sun is directly overhead only once per year, on the corresponding solstice. At latitudes closer to the Equator and on the Equator itself, it will be overhead twice per year (on the equinoxes in the case of the Equator). Outside the tropics, the Sun never passes directly overhead.
 Near the poles
Near the poles, which coincide with the rotational axis of the Earth as it passes through the surface, the seasonal variations in the length of daytime are extreme. In fact, within 23.5° latitude of the poles, there will be at least some days each year during which the sun never goes below the horizon. In there will be days where the sun never rises above the horizon. This number will be fewer, but close to the number of days in the summer where the sun doesn't set (for example the sunrise is usually a few days before the spring equinox and extends a few days past the fall equinox, even WITHOUT accounting for twilight). This phenomenon of more daylight than night is not unique to the poles. In fact, at any given time slightly more than half of the earth is in daylight. The 24 hours of summer daylight is known as the “Midnight Sun” that is famous in some northern and southern countries. To the north, the Arctic Circle marks this 23.5° boundary. To the south, the Antarctic Circle marks the boundary. These boundaries correspond to 66.5° north or south latitude, respectively. Because the Sun's disc itself is about half a degree in diameter and is very bright, truly dark days during which the sun never seems to rise are only seen beyond 67° north or south latitude.
At and near the poles, the sun never rises very far above the horizon, even in summer, which is why these regions of the world are consistently cold in all seasons. Even at the summer solstice, when the sun reaches its highest point above the horizon at noon, it is still only 23.5° above the horizon at the poles. Additionally, as one approaches the poles, the apparent path of the Sun through the sky each day diverges more and more from the vertical, and more and more closely resembles an inclined circle. As summer approaches, the Sun rises more and more to the northeast, and sets more and more to the northwest; at the poles, the path of the Sun is indeed a circle, which is roughly equidistant above the horizon for the entire duration of the daytime period on any given day. The circle gradually sinks below the horizon as winter approaches, and gradually rises above it as summer approaches. At the poles, then, “sunrise” and “sunset” may last for several days.
 Middle latitudes
At middle latitudes, far from both the Equator and the North and South Poles, variations in the length of daytime are moderate. At high middle latitudes, such as those of Montréal or Paris, the difference in the length of the day from summer to winter can be very noticeable: the sky may still be lit at 10 PM in summer, but may be dark at 5 PM in winter. At low middle latitudes, such as southern California or Egypt, the seasonal difference may be quite small and only slightly noticeable to the locals.
In middle latitudes, the seasonal climate variations produced by changes in the length of daytime are the most marked, with very distinct periods of cold and heat, and other secondary seasonal changes such as snow and ice in winter that disappear in summer, and so on. At high latitudes, it is cold most of the time, with constant snow and ice, so the seasons are less obvious; and in the tropics, it is hot most of the time, with no snow or ice at all, so again the seasons are less obvious.
 Variations in solar noon
The exact instant of solar noon, when the sun reaches its highest elevation in the sky, varies with the seasons everywhere except at the poles and on the Equator. This variation is called the equation of time, and the magnitude of the variation is usually in the range of minutes over the course of a year, depending on the observer's latitude.
..................for if we remain in motion........is it still impossible for us to remain clear of deaths shadow....
..................for what is the natural boundary of light once we leave the surface.....????..?