https://www.time.gov/
Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, into the future.[1][2][3] It is a component quantity of various measurements used to sequence events, to compare the duration of events or the intervals between them, and to quantify rates of change of quantities in material reality or in the conscious experience.[4][5][6][7] Time is often referred to as a fourth dimension, along with three spatial dimensions.[8]
Time has long been an important subject of study in religion, philosophy, and science, but defining it in a manner applicable to all fields without circularity has consistently eluded scholars.[7][9] Nevertheless, diverse fields such as business, industry, sports, the sciences, and the performing arts all incorporate some notion of time into their respective measuring systems.[10][11][12]
https://en.wikipedia.org/wiki/Time
A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A date is the designation of a single and specific day within such a system. A calendar is also a physical record (often paper) of such a system. A calendar can also mean a list of planned events, such as a court calendar or a partly or fully chronological list of documents, such as a calendar of wills.
Periods in a calendar (such as years and months) are usually, though not necessarily, synchronized with the cycle of the sun or the moon. The most common type of pre-modern calendar was the lunisolar calendar, a lunar calendar that occasionally adds one intercalary month to remain synchronized with the solar year over the long term.
https://en.wikipedia.org/wiki/Calendar
The term calendar is taken from kalendae, the term for the first day of the month in the Roman calendar, related to the verb calare 'to call out', referring to the "calling" of the new moon when it was first seen.[1] Latin calendarium meant 'account book, register' (as accounts were settled and debts were collected on the calends of each month). The Latin term was adopted in Old French as calendier and from there in Middle English as calender by the 13th century (the spelling calendar is early modern).
https://en.wikipedia.org/wiki/Calendar
A lunisolar calendar is a calendar in many cultures, combining lunar calendars and solar calendars. The date of Lunisolar calendars therefore indicates both the Moon phase and the time of the solar year, that is the position of the Sun in the Earth's sky. If the sidereal year (such as in a sidereal solar calendar) is used instead of the solar year, then the calendar will predict the constellation near which the full moon may occur. As with all calendars which divide the year into months there is an additional requirement that the year have a whole number of months. In some case ordinary years consist of twelve months but every second or third year is an embolismic year, which adds a thirteenth intercalary,[1] embolismic, or leap month.
Their months are based on the regular cycle of the Moon's phases. So lunisolar calendars are lunar calendars with – in contrast to them – additional intercalation rules being used to bring them into a rough agreement with the solar year and thus with the seasons.
The main other type of calendar is a solar calendar.
Examples
The Chinese, Buddhist, Burmese, Assyrian, Hebrew, Jain and Kurdish as well as the traditional Hindu, Japanese, Korean, Mongolian, Tibetan, and Vietnamese calendars (in the East Asian Chinese cultural sphere), plus the ancient Hellenic, Coligny, and Babylonian calendars are all lunisolar. Also, some of the ancient pre-Islamic calendars in south Arabia followed a lunisolar system.[2] The Chinese, Coligny and Hebrew[3] lunisolar calendars track more or less the tropical year whereas the Buddhist and Hindu lunisolar calendars track the sidereal year. Therefore, the first three give an idea of the seasons whereas the last two give an idea of the position among the constellations of the full moon. The Tibetan calendar was influenced by the Buddhist calendar. The Germanic peoples also used a lunisolar calendar before their conversion to Christianity.
https://en.wikipedia.org/wiki/Lunisolar_calendar
Intercalation or embolism in timekeeping is the insertion of a leap day, week, or month into some calendar years to make the calendar follow the seasons or moon phases.[1] Lunisolar calendars may require intercalations of both days and months.
https://en.wikipedia.org/wiki/Intercalation_(timekeeping)
Solar calendars
The solar or tropical year does not have a whole number of days (it is about 365.24 days), but a calendar year must have a whole number of days. The most common way to reconcile the two is to vary the number of days in the calendar year.
In solar calendars, this is done by adding to a common year of 365 days, an extra day ("leap day" or "intercalary day") about every four years, causing a leap year to have 366 days (Julian, Gregorian and Indian national calendars).
The Decree of Canopus, which was issued by the pharaoh Ptolemy III Euergetes of Ancient Egypt in 239 BCE, decreed a solar leap day system; an Egyptian leap year was not adopted until 25 BC, when the Roman Emperor Augustus successfully instituted a reformed Alexandrian calendar.
In the Julian calendar, as well as in the Gregorian calendar, which improved upon it, intercalation is done by adding an extra day to February in each leap year. In the Julian calendar this was done every four years. In the Gregorian, years divisible by 100 but not 400 were exempted in order to improve accuracy. Thus, 2000 was a leap year; 1700, 1800, and 1900 were not.
Epagomenal[2] days are days within a solar calendar that are outside any regular month. Usually five epagomenal days are included within every year (Egyptian, Coptic, Ethiopian, Mayan Haab' and French Republican Calendars), but a sixth epagomenal day is intercalated every four years in some (Coptic, Ethiopian and French Republican calendars).
The Solar Hijri calendar, used in Iran, is based on solar calculations and is similar to the Gregorian calendar in its structure, and hence the intercalation, with the exception that its epoch the Hijrah.[3]
The Bahá'í calendar includes enough epagomenal days (usually 4 or 5) before the last month (علاء, ʿalāʾ) to ensure that the following year starts on the March equinox. These are known as the Ayyám-i-Há.
https://en.wikipedia.org/wiki/Intercalation_(timekeeping)
Lunar calendars
In principle, lunar calendars do not employ intercalation because they do not seek to synchronise with the seasons and the motion of the moon is astronomically predictable. However, religious lunar calendars rely on actual observation.
The Lunar Hijri calendar, the purely lunar calendar observed by most of Islam, depends on actual observation of the first crescent of the moon and consequently does not have any intercalation. Each month still has either 29 or 30 days, but due to the variable method of observations employed, there is usually no discernible order in the sequencing of either 29 or 30-day month lengths. Traditionally, the first day of each month is the day (beginning at sunset) of the first sighting of the hilal (crescent moon) shortly after sunset. If the hilal is not observed immediately after the 29th day of a month (either because clouds block its view or because the western sky is still too bright when the moon sets), then the day that begins at that sunset is the 30th.[4]
The tabular Islamic calendar, used in Iran, has 12 lunar months that usually alternate between 30 and 29 days every year, but an intercalary day is added to the last month of the year 12 times within a 33-year cycle. Some historians also linked the pre-Islamic practice of Nasi' to intercalation.
Leap seconds
The International Earth Rotation and Reference Systems Service can insert or remove leap seconds from the last day of any month (June and December are preferred). These are sometimes described as intercalary.[5]
Other uses
ISO 8601 includes a specification for a 52/53-week year. Any year that has 53 Thursdays has 53 weeks; this extra week may be regarded as intercalary.
The xiuhpōhualli (year count) system of the Aztec calendar had five intercalary days after the eighteenth and final month, the nēmontēmi, in which the people reflect on the past year and do fasting.
https://en.wikipedia.org/wiki/Intercalation_(timekeeping)
The International Earth Rotation and Reference Systems Service (IERS), formerly the International Earth Rotation Service, is the body responsible for maintaining global time and reference frame standards, notably through its Earth Orientation Parameter (EOP) and International Celestial Reference System (ICRS) groups.
https://en.wikipedia.org/wiki/International_Earth_Rotation_and_Reference_Systems_Service
See also
- Astrometry
- International Atomic Time
- Coordinated Universal Time
- International Terrestrial Reference System and Frame
- International Celestial Reference System and its realizations
- Earth rotation, ΔT
- IERS Reference Meridian
- List of astronomical societies
References
- "IERS Conventions Centre". IERS. Retrieved 5 July 2022.
- "Earth Orientation Center". Observatoire de Paris. Retrieved 2 August 2016.
https://en.wikipedia.org/wiki/International_Earth_Rotation_and_Reference_Systems_Service
International Atomic Time (abbreviated TAI, from its French name temps atomique international[1]) is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid.[2] TAI is a weighted average of the time kept by over 450 atomic clocks in over 80 national laboratories worldwide.[3] It is a continuous scale of time, without leap seconds, and it is the principal realisation of Terrestrial Time (with a fixed offset of epoch). It is the basis for Coordinated Universal Time (UTC), which is used for civil timekeeping all over the Earth's surface and which has leap seconds.
UTC deviates from TAI by a number of whole seconds. As of 1 January 2017, when another leap second was put into effect,[4] UTC is currently exactly 37 seconds behind TAI. The 37 seconds result from the initial difference of 10 seconds at the start of 1972, plus 27 leap seconds in UTC since 1972.
TAI may be reported using traditional means of specifying days, carried over from non-uniform time standards based on the rotation of the Earth. Specifically, both Julian days and the Gregorian calendar are used. TAI in this form was synchronised with Universal Time at the beginning of 1958, and the two have drifted apart ever since, due primarily to the slowing rotation of the Earth.
https://en.wikipedia.org/wiki/International_Atomic_Time
https://en.wikipedia.org/wiki/IERS_Reference_Meridian
Coordinated Universal Time or UTC is the primary time standard by which the world regulates clocks and time. It is within about one second of mean solar time (such as UT1) at 0° longitude (at the IERS Reference Meridian as the currently used prime meridian) and is not adjusted for daylight saving time. It is effectively a successor to Greenwich Mean Time (GMT).
The coordination of time and frequency transmissions around the world began on 1 January 1960. UTC was first officially adopted as CCIR Recommendation 374, Standard-Frequency and Time-Signal Emissions, in 1963, but the official abbreviation of UTC and the official English name of Coordinated Universal Time (along with the French equivalent) were not adopted until 1967.[1]
The system has been adjusted several times, including a brief period during which the time-coordination radio signals broadcast both UTC and "Stepped Atomic Time (SAT)" before a new UTC was adopted in 1970 and implemented in 1972. This change also adopted leap seconds to simplify future adjustments. This CCIR Recommendation 460 "stated that (a) carrier frequencies and time intervals should be maintained constant and should correspond to the definition of the SI second; (b) step adjustments, when necessary, should be exactly 1 s to maintain approximate agreement with Universal Time (UT); and (c) standard signals should contain information on the difference between UTC and UT."[2]
https://en.wikipedia.org/wiki/Coordinated_Universal_Time
https://en.wikipedia.org/wiki/International_Telecommunication_Union
https://en.wikipedia.org/wiki/Coordinated_Universal_Time
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The geographic coordinate system (GCS) is a spherical or geodetic coordinates system for measuring and communicating positions directly on the Earth as latitude and longitude.[1] It is the simplest, oldest and most widely used of the various spatial reference systems that are in use, and forms the basis for most others. Although latitude and longitude form a coordinate tuple like a cartesian coordinate system, the geographic coordinate system is not cartesian because the measurements are angles and are not on a planar surface.[2][self-published source?]
A full GCS specification, such as those listed in the EPSG and ISO 19111 standards, also includes a choice of geodetic datum (including an Earth ellipsoid), as different datums will yield different latitude and longitude values for the same location.[3]
https://en.wikipedia.org/wiki/Geographic_coordinate_system
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A spatial reference system (SRS) or coordinate reference system (CRS) is a framework used to precisely measure locations on the surface of the Earth as coordinates. It is thus the application of the abstract mathematics of coordinate systems and analytic geometry to geographic space. A particular SRS specification (for example, "Universal Transverse Mercator WGS 84 Zone 16N") comprises a choice of Earth ellipsoid, horizontal datum, map projection (except in the geographic coordinate system), origin point, and unit of measure. Thousands of coordinate systems have been specified for use around the world or in specific regions and for various purposes, necessitating transformations between different SRS.
Although they date to the Hellenic Period, spatial reference systems are now a crucial basis for the sciences and technologies of Geoinformatics, including cartography, geographic information systems, surveying, remote sensing, and civil engineering. This has led to their standardization in international specifications such as the EPSG codes[1] and ISO 19111:2007 Geographic information—Spatial referencing by coordinates, prepared by ISO/TC 211, also published by the Open Geospatial Consortium as Abstract Specification, Topic 2: Spatial referencing by coordinate.[2]
https://en.wikipedia.org/wiki/Spatial_reference_system
Types of systems
The thousands of spatial reference systems used today are based on a few general strategies, which have been defined in the EPSG, ISO, and OGC standards:[1][2]
- Geographic coordinate system (or geodetic)
- A spherical coordinate system measuring locations directly on the Earth (modeled as a sphere or ellipsoid) using latitude (degrees north or south of the equator) and longitude (degrees west or east of a prime meridian).
- Geocentric coordinate system (or Earth-centered Earth-fixed)
- A three-dimensional cartesian coordinate system that models the Earth as a three-dimensional object, measuring locations from a center point, usually the center of mass of the Earth, along x, y, and z axes aligned with the equator and the prime meridian. This system is commonly used to track the orbits of satellites, because they are based on the center of mass. Thus, this is the internal coordinate system used by Satellite navigation systems such as GPS to compute locations using multilateration.
- Projected coordinate system (or planar, grid)
- Layout of a UTM coordinate system.
- A standardized cartesian coordinate system that models the Earth (or more commonly, a large region thereof) as a plane, measuring locations from an arbitrary origin point along x and y axes more or less aligned with the cardinal directions. Each of these systems is based on a particular Map projection to create a planar surface from the curved Earth surface. These are generally defined and used strategically to minimize the distortions inherent to projections. Common examples include the Universal transverse mercator (UTM) and national systems such as the British National Grid, and State Plane Coordinate System (SPCS).
- Engineering coordinate system (or local, custom)
- A cartesian coordinate system (2-D or 3-D) that is created bespoke for a small area, often a single engineering project, over which the curvature of the Earth can be safely approximated as flat without significant distortion. Locations are typically measured directly from an arbitrary origin point using surveying techniques. These may or may not be aligned with a standard projected coordinate system. Local tangent plane coordinates are a type of local coordinate system used in aviation and marine vehicles.
These standards acknowledge that standard reference systems also exist for measuring elevation using vertical datums and time (e.g. ISO 8601), which may be combined with a spatial reference system to form a compound coordinate system for representing three-dimensional and/or spatio-temporal locations. There are also internal systems for measuring location within the context of an object, such as the rows and columns of pixels in a raster image, Linear referencing measurements along linear features (e.g., highway mileposts), and systems for specifying location within moving objects such as ships. The latter two are often classified as subcategories of engineering coordinate systems.
Components
The goal of any spatial reference system is to create a common reference frame in which locations can be measured precisely and consistently as coordinates, which can then be shared unambiguously, so that any recipient can identify the same location that was originally intended by the originator.[3] To accomplish this, any coordinate reference system definition needs to be composed of several specifications:
- A coordinate system, an abstract framework for measuring locations. Like any mathematical coordinate system, its definition consists of a measurable space (whether a plane, a three-dimension void, or the surface of an object such as the Earth), an origin point, a set of axis vectors emanating from the origin, and a unit of measure.
- A horizontal datum, which binds the abstract coordinate system to the real space of the Earth. A horizontal datum can be defined as a precise reference framework for measuring geographic coordinates (latitude and longitude). Examples include the World Geodetic System and the 1927 and 1983 North American Datum. A datum generally consists of an estimate of the shape of the Earth (usually an ellipsoid), and one or more anchor points or control points, established locations (often marked by physical monuments) for which the measurement is documented.
- A definition for a projected CRS must also include a choice of map projection to convert the spherical coordinates specified by the datum into cartesian coordinates on a planar surface.
Thus, a CRS definition will typically consist of a "stack" of dependent specifications, as exemplified in the following table:
| EPSG Code | Name | Ellipsoid | Horizontal Datum | CS Type | Projection | Origin | Axes | Unit of Measure |
|---|---|---|---|---|---|---|---|---|
| 4326 | GCS WGS 84 | GRS 80 | WGS 84 | ellipsoidal (lat, lon) | N/A | equator/prime meridian | equator, prime meridian | degree of arc |
| 26717 | UTM Zone 17N NAD 27 | Clarke 1866 | NAD 27 | cartesian (x,y) | Transverse Mercator: central meridian 81°W, scaled 0.9996 | 500km west of (81°W, 0°N) | equator, 81°W meridian | meter |
| 6576 | SPCS Tennessee Zone NAD 83 (2011) ftUS | GRS 80 | NAD 83 (2011 epoch) | cartesian (x,y) | Lambert Conformal Conic: center 86°W, 34°20'N, standard parallels 35°15'N, 36°25'N | 600km grid west of center point | grid east at center point, 86°W meridian | US survey foot |
Examples by continent
Examples of systems around the world are:
Asia
- Chinese Global Navigation Grid Code, China
- Israeli Cassini Soldner, Israel
- Israeli Transverse Mercator, Israel
- Jordan Transverse Mercator, Jordan
Europe
- British national grid reference system, Britain
- Lambert-93 (fr), the official projection in Metropolitan France
- Hellenic Geodetic Reference System 1987, Greece
- Irish grid reference system, Ireland
- Irish Transverse Mercator, Ireland
- SWEREF 99 (sv), Sweden
North America
Worldwide
- Universal Transverse Mercator coordinate system
- Lambert conformal conic projection
- International mapcode system
- Military Grid Reference System
Identifiers
A Spatial Reference System Identifier (SRID) is a unique value used to unambiguously identify projected, unprojected, and local spatial coordinate system definitions. These coordinate systems form the heart of all GIS applications.
Virtually all major spatial vendors have created their own SRID implementation or refer to those of an authority, such as the EPSG Geodetic Parameter Dataset.
SRIDs are the primary key for the Open Geospatial Consortium (OGC) spatial_ref_sys metadata table for the Simple Features for SQL Specification, Versions 1.1 and 1.2, which is defined as follows:
CREATE TABLE SPATIAL_REF_SYS
(
SRID INTEGER NOT NULL PRIMARY KEY,
AUTH_NAME CHARACTER VARYING(256),
AUTH_SRID INTEGER,
SRTEXT CHARACTER VARYING(2048)
)
In spatially enabled databases (such as IBM Db2, IBM Informix, Ingres, Microsoft SQL Server, MonetDB, MySQL, Oracle RDBMS, Teradata, PostGIS, SQL Anywhere and Vertica), SRIDs are used to uniquely identify the coordinate systems used to define columns of spatial data or individual spatial objects in a spatial column (depending on the spatial implementation). SRIDs are typically associated with a well-known text (WKT) string definition of the coordinate system (SRTEXT, above). Here are two common coordinate systems with their EPSG SRID value followed by their WKT:
UTM, Zone 17N, NAD27 — SRID 2029:
PROJCS["NAD27(76) / UTM zone 17N",
GEOGCS["NAD27(76)",
DATUM["North_American_Datum_1927_1976",
SPHEROID["Clarke 1866",6378206.4,294.9786982138982,
AUTHORITY["EPSG","7008"]],
AUTHORITY["EPSG","6608"]],
PRIMEM["Greenwich",0,
AUTHORITY["EPSG","8901"]],
UNIT["degree",0.01745329251994328,
AUTHORITY["EPSG","9122"]],
AUTHORITY["EPSG","4608"]],
UNIT["metre",1,
AUTHORITY["EPSG","9001"]],
PROJECTION["Transverse_Mercator"],
PARAMETER["latitude_of_origin",0],
PARAMETER["central_meridian",-81],
PARAMETER["scale_factor",0.9996],
PARAMETER["false_easting",500000],
PARAMETER["false_northing",0],
AUTHORITY["EPSG","2029"],
AXIS["Easting",EAST],
AXIS["Northing",NORTH]]
WGS84 — SRID 4326
GEOGCS["WGS 84",
DATUM["WGS_1984",
SPHEROID["WGS 84",6378137,298.257223563,
AUTHORITY["EPSG","7030"]],
AUTHORITY["EPSG","6326"]],
PRIMEM["Greenwich",0,
AUTHORITY["EPSG","8901"]],
UNIT["degree",0.01745329251994328,
AUTHORITY["EPSG","9122"]],
AUTHORITY["EPSG","4326"]]
SRID values associated with spatial data can be used to constrain spatial operations — for instance, spatial operations cannot be performed between spatial objects with differing SRIDs in some systems, or trigger coordinate system transformations between spatial objects in others.
See also
- Engineering datum
- Geodesy
- Geodetic datum
- Georeferencing
- Geographic coordinate systems
- Geographic information system (GIS).
- Grid reference
- List of National Coordinate Reference Systems
References
- A guide to coordinate systems in Great Britain (PDF), D00659 v2.3, Ordnance Survey, 2020, p. 11, archived from the original (PDF) on 24 September 2015, retrieved 2021-12-16
External links
- spatialreference.org – A website that defines spatial reference systems, in a variety of formats.
- OpenGIS Specifications (Standards) Archived 2004-12-13 at the Wayback Machine
- OpenGIS Simple Features Specification for CORBA (99-054)
- OpenGIS Simple Features Specification for OLE/COM (99-050)
- OpenGIS Simple Features Specification for SQL (99-054, 05-134, 06-104r3)
- OGR Archived 2006-04-22 at the Wayback Machine — library implementing relevant OGC standards
- EPSG Geodetic Parameter Registry Archived 2020-08-09 at the Wayback Machine - search engine for EPSG defined reference systems
- EPSG.io/ - Full text search indexing over 6000 coordinate systems
- Galdos Systems INdicio CRS Registry
https://en.wikipedia.org/wiki/Spatial_reference_system
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