Time is the indefinite continued progress of existence and events that occur in irreversible succession through the past, in the present, the future. Time is a component quantity of various measurements used to sequence events, to compare the duration of events or the intervals between them, to quantify rates of change of quantities in material reality or in the conscious experience. Time is referred to as a fourth dimension, along with three spatial dimensions. Time has long been an important subject of study in religion and science, but defining it in a manner applicable to all fields without circularity has eluded scholars. Diverse fields such as business, sports, the sciences, the performing arts all incorporate some notion of time into their respective measuring systems. Time in physics is unambiguously operationally defined as "what a clock reads". See Units of Time. Time is one of the seven fundamental physical quantities in both the International System of Units and International System of Quantities.
Time is used to define other quantities – such as velocity – so defining time in terms of such quantities would result in circularity of definition. An operational definition of time, wherein one says that observing a certain number of repetitions of one or another standard cyclical event constitutes one standard unit such as the second, is useful in the conduct of both advanced experiments and everyday affairs of life; the operational definition leaves aside the question whether there is something called time, apart from the counting activity just mentioned, that flows and that can be measured. Investigations of a single continuum called spacetime bring questions about space into questions about time, questions that have their roots in the works of early students of natural philosophy. Temporal measurement has occupied scientists and technologists, was a prime motivation in navigation and astronomy. Periodic events and periodic motion have long served as standards for units of time. Examples include the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum, the beat of a heart.
The international unit of time, the second, is defined by measuring the electronic transition frequency of caesium atoms. Time is of significant social importance, having economic value as well as personal value, due to an awareness of the limited time in each day and in human life spans. Speaking, methods of temporal measurement, or chronometry, take two distinct forms: the calendar, a mathematical tool for organising intervals of time, the clock, a physical mechanism that counts the passage of time. In day-to-day life, the clock is consulted for periods less than a day whereas the calendar is consulted for periods longer than a day. Personal electronic devices display both calendars and clocks simultaneously; the number that marks the occurrence of a specified event as to hour or date is obtained by counting from a fiducial epoch – a central reference point. Artifacts from the Paleolithic suggest that the moon was used to reckon time as early as 6,000 years ago. Lunar calendars were among the first to appear, with years of either 13 lunar months.
Without intercalation to add days or months to some years, seasons drift in a calendar based on twelve lunar months. Lunisolar calendars have a thirteenth month added to some years to make up for the difference between a full year and a year of just twelve lunar months; the numbers twelve and thirteen came to feature prominently in many cultures, at least due to this relationship of months to years. Other early forms of calendars originated in Mesoamerica in ancient Mayan civilization; these calendars were religiously and astronomically based, with 18 months in a year and 20 days in a month, plus five epagomenal days at the end of the year. The reforms of Julius Caesar in 45 BC put the Roman world on a solar calendar; this Julian calendar was faulty in that its intercalation still allowed the astronomical solstices and equinoxes to advance against it by about 11 minutes per year. Pope Gregory XIII introduced a correction in 1582. During the French Revolution, a new clock and calendar were invented in attempt to de-Christianize time and create a more rational system in order to replace the Gregorian calendar.
The French Republican Calendar's days consisted of ten hours of a hundred minutes of a hundred seconds, which marked a deviation from the 12-based duodecimal system used in many other devices by many cultures. The system was abolished in 1806. A large variety of devices have been invented to measure time; the study of these devices is called horology. An Egyptian device that dates to c. 1500 BC, similar in shape to a bent T-square, measured the passage of time from the shadow cast by its crossbar on a nonlinear rule. The T was oriented eastward in the mornings. At noon, the device was turned around so. A sundial uses a gnomon to cast a shadow on a set of markings calibrated to the hour; the position of the shadow marks the hour in local time. The idea to separate the day into smaller parts is credited to Egyptians because of their sundials, which operated on a duodecimal system; the importance of the number 12 is due to the number of lunar cycles in a year and the number of stars used to count the passage of night.
The most precise timekeeping device of the ancient
In geology and related fields, a stratum is a layer of sedimentary rock or soil, or igneous rock that were formed at the Earth's surface, with internally consistent characteristics that distinguish it from other layers. The "stratum" is the fundamental unit in a stratigraphic column and forms the basis of the study of stratigraphy; each layer is one of a number of parallel layers that lie one upon another, laid down by natural processes. They may extend over hundreds of thousands of square kilometers of the Earth's surface. Strata are seen as bands of different colored or differently structured material exposed in cliffs, road cuts and river banks. Individual bands may vary in thickness from a few millimeters to a kilometer or more. A band may represent a specific mode of deposition: river silt, beach sand, coal swamp, sand dune, lava bed, etc. Geologists categorize them by the material of beds; each distinct layer is assigned to the name of sheet based on a town, mountain, or region where the formation is exposed and available for study.
For example, the Burgess Shale is a thick exposure of dark fossiliferous, shale exposed high in the Canadian Rockies near Burgess Pass. Slight distinctions in material in a formation may be described as "members". Formations are collected into "groups" while groups may be collected into "supergroups". Archaeological horizon Geologic formation Geologic map Geologic unit Law of superposition Bed GeoWhen Database
A committee is a body of one or more persons, subordinate to a deliberative assembly. The assembly sends matters into a committee as a way to explore them more than would be possible if the assembly itself were considering them. Committees may have different functions and their type of work differ depending on the type of the organization and its needs. A deliberative assembly may form a committee consisting of one or more persons to assist with the work of the assembly. For larger organizations, much work is done in committees. Committees can be a way to formally draw together people of relevant expertise from different parts of an organization who otherwise would not have a good way to share information and coordinate actions, they may have the advantage of sharing out responsibilities. They can be appointed with experts to recommend actions in matters that require specialized knowledge or technical judgment. Committees can serve several different functions: Governance In organizations considered too large for all the members to participate in decisions affecting the organization as a whole, a smaller body, such as a board of directors, is given the power to make decisions, spend money, or take actions.
A governance committee is formed as a separate committee to review the performance of the board and board policy as well as nominate candidates for the board. Coordination and administration A large body may have smaller committees with more specialized functions. Examples are an audit committee, an elections committee, a finance committee, a fundraising committee, a program committee. Large conventions or academic conferences are organized by a coordinating committee drawn from the membership of the organization. Research and recommendations Committees may be formed to do research and make recommendations on a potential or planned project or change. For example, an organization considering a major capital investment might create a temporary working committee of several people to review options and make recommendations to upper management or the board of directors. Discipline A committee on discipline may be used to handle disciplinary procedures on members of the organization; as a tactic for indecision As a means of public relations by sending sensitive, inconvenient, or irrelevant matters to committees, organizations may bypass, stall, or disacknowledge matters without declaring a formal policy of inaction or indifference.
However, this could be considered a dilatory tactic. Committees are required to report to their parent body. Committees do not have the power to act independently unless the body that created it gives it such power; when a committee is formed, a chairman is designated for the committee. Sometimes a vice-chairman is appointed, it is common for the committee chairman to organize its meetings. Sometimes these meetings are held through videoconferencing or other means if committee members are not able to attend in person, as may be the case if they are in different parts of the country or the world; the chairman is responsible for running meetings. Duties include keeping the discussion on the appropriate subject, recognizing members to speak, confirming what the committee has decided. Using Roberts Rules of Order Newly Revised, committees may follow informal procedures; the level of formality depends on the size and type of committee, in which sometimes larger committees considering crucial issues may require more formal processes.
Minutes are a record of the decisions at meetings. They can be taken by a person designated as the secretary. For most organizations, committees are not required to keep formal minutes. However, some bodies require that committees take minutes if the committees are public ones subject to open meeting laws. Committees may meet on a regular basis, such as weekly or more or meetings may be called irregularly as the need arises; the frequency of the meetings depends on the needs of the parent body. When the committee completes its work, it provides the results in a report to its parent body; the report may include the methods used, the facts uncovered, the conclusions reached, any recommendations. If the committee is not ready to report, it may provide a partial report or the assembly may discharge the committee of the matter so that the assembly can handle it. If members of the committee are not performing their duties, they may be removed or replaced by the appointing power. Whether the committee continues to exist after presenting its report depends on the type of committee.
Committees established by the bylaws or the organization's rules continue to exist, while committees formed for a particular purpose go out of existence after the final report. In parliamentary procedure, the motion to commit is used to refer another motion—usually a main motion—to a committee. A motion to commit should specify to which committee the matter is to be referred, if the committee is a special committee appointed for purposes of the referred motion, it should specify the number of committee members and the method of their selection, unless, specified in the bylaws. Any proposed amendments to the main motion that are pending at the time the motion is referred to a committee go to the committee as well. Once referred, but before the committee reports its recommendations back to the assembly, the referred motion may be removed from the committee's consideration by the motion to discharge a committee. In the United States House of Representatives, a motion to recommit
Geochronology is the science of determining the age of rocks and sediments using signatures inherent in the rocks themselves. Absolute geochronology can be accomplished through radioactive isotopes, whereas relative geochronology is provided by tools such as palaeomagnetism and stable isotope ratios. By combining multiple geochronological indicators the precision of the recovered age can be improved. Geochronology is different in application from biostratigraphy, the science of assigning sedimentary rocks to a known geological period via describing and comparing fossil floral and faunal assemblages. Biostratigraphy does not directly provide an absolute age determination of a rock, but places it within an interval of time at which that fossil assemblage is known to have coexisted. Both disciplines work together hand in hand, however, to the point where they share the same system of naming rock layers and the time spans utilized to classify layers within a stratum; the science of geochronology is the prime tool used in the discipline of chronostratigraphy, which attempts to derive absolute age dates for all fossil assemblages and determine the geologic history of the Earth and extraterrestrial bodies.
By measuring the amount of radioactive decay of a radioactive isotope with a known half-life, geologists can establish the absolute age of the parent material. A number of radioactive isotopes are used for this purpose, depending on the rate of decay, are used for dating different geological periods. More decaying isotopes are useful for longer periods of time, but less accurate in absolute years. With the exception of the radiocarbon method, most of these techniques are based on measuring an increase in the abundance of a radiogenic isotope, the decay-product of the radioactive parent isotope. Two or more radiometric methods can be used in concert to achieve more robust results. Most radiometric methods are suitable for geological time only, but some such as the radiocarbon method and the 40Ar/39Ar dating method can be extended into the time of early human life and into recorded history; some of the used techniques are: Radiocarbon dating. This technique measures the decay of carbon-14 in organic material and can be best applied to samples younger than about 60,000 years.
Uranium–lead dating. This technique measures the ratio of two lead isotopes to the amount of uranium in a mineral or rock. Applied to the trace mineral zircon in igneous rocks, this method is one of the two most used for geologic dating. Monazite geochronology is another example of U–Pb dating, employed for dating metamorphism in particular. Uranium–lead dating is applied to samples older than about 1 million years. Uranium–thorium dating; this technique is used to date speleothems, corals and fossil bones. Its range is from a few years to about 700,000 years. Potassium–argon dating and argon–argon dating; these techniques date metamorphic and volcanic rocks. They are used to date volcanic ash layers within or overlying paleoanthropologic sites; the younger limit of the argon–argon method is a few thousand years. Electron spin resonance dating A series of related techniques for determining the age at which a geomorphic surface was created, or at which surficial materials were buried. Exposure dating uses the concentration of exotic nuclides produced by cosmic rays interacting with Earth materials as a proxy for the age at which a surface, such as an alluvial fan, was created.
Burial dating uses the differential radioactive decay of 2 cosmogenic elements as a proxy for the age at which a sediment was screened by burial from further cosmic rays exposure. Luminescence dating techniques observe'light' emitted from materials such as quartz, diamond and calcite. Many types of luminescence techniques are utilized in geology, including optically stimulated luminescence, cathodoluminescence, thermoluminescence. Thermoluminescence and optically stimulated luminescence are used in archaeology to date'fired' objects such as pottery or cooking stones and can be used to observe sand migration. Incremental dating techniques allow the construction of year-by-year annual chronologies, which can be fixed or floating. Dendrochronology Ice cores Lichenometry Varves A sequence of paleomagnetic poles, which are well defined in age, constitutes an apparent polar wander path; such a path is constructed for a large continental block. APWPs for different continents can be used as a reference for newly obtained poles for the rocks with unknown age.
For paleomagnetic dating, it is suggested to use the APWP in order to date a pole obtained from rocks or sediments of unknown age by linking the paleopole to the nearest point on the APWP. Two methods of paleomagnetic dating have been suggested Rotation method. First method is used for paleomagnetic dating of rocks inside of the same continental block; the second method is used for the folded areas. Magnetostratigraphy determines age from the pattern of magnetic polarity zones in a series of bedded sedimentary and/or volcanic rocks by comparison to the magnetic polarity timescale; the polarity timescale has been determined by dating of seafloor magnetic anomalies, radiometrically dating volcanic rocks within magnetostratigraphic sections, astronomically dating magnetostratigraphic sections. Global trends in isotope compositions Carbon 13 and strontium isotopes, can be used to corr
The Pliocene Epoch is the epoch in the geologic timescale that extends from 5.333 million to 2.58 million years BP. It is the youngest epoch of the Neogene Period in the Cenozoic Era; the Pliocene is followed by the Pleistocene Epoch. Prior to the 2009 revision of the geologic time scale, which placed the four most recent major glaciations within the Pleistocene, the Pliocene included the Gelasian stage, which lasted from 2.588 to 1.806 million years ago, is now included in the Pleistocene. As with other older geologic periods, the geological strata that define the start and end are well identified but the exact dates of the start and end of the epoch are uncertain; the boundaries defining the Pliocene are not set at an identified worldwide event but rather at regional boundaries between the warmer Miocene and the cooler Pliocene. The upper boundary was set at the start of the Pleistocene glaciations. Charles Lyell gave the Pliocene its name in Principles of Geology; the word pliocene comes from the Greek words πλεῖον and καινός and means "continuation of the recent", referring to the modern marine mollusc fauna.
H. W. Fowler called the term Pliocene a "regrettable barbarism" and an indication that "a good classical scholar" such as Lyell should have requested a philologist's help when coining words. To summarize the usage of these "regrettable barbarisms" in the labelling of the Cenozoic era: with the understanding that these are all new relative to the Mesozoic and Paleozoic eras. In the official timescale of the ICS, the Pliocene is subdivided into two stages. From youngest to oldest they are: Piacenzian Zanclean The Piacenzian is sometimes referred to as the Late Pliocene, whereas the Zanclean is referred to as the Early Pliocene. In the system of North American Land Mammal Ages include Hemphillian, Blancan; the Blancan extends forward into the Pleistocene. South American Land Mammal Ages include Montehermosan and Uquian. In the Paratethys area the Pliocene contains the Romanian stages; as usual in stratigraphy, there are many other local subdivisions in use. In Britain the Pliocene is divided into the following stages: Gedgravian, Pre-Ludhamian, Thurnian, Bramertonian or Antian, Pre-Pastonian or Baventian and Beestonian.
In the Netherlands the Pliocene is divided into these stages: Brunssumian C, Reuverian A, Reuverian B, Reuverian C, Tiglian A, Tiglian B, Tiglian C1-4b, Tiglian C4c, Tiglian C5, Tiglian C6 and Eburonian. The exact correlations between these local stages and the ICS stages is still a matter of detail; the global average temperature in the mid-Pliocene was 2–3 °C higher than today, carbon dioxide levels were the same as today, global sea level was 25 m higher. The northern hemisphere ice sheet was ephemeral before the onset of extensive glaciation over Greenland that occurred in the late Pliocene around 3 Ma; the formation of an Arctic ice cap is signaled by an abrupt shift in oxygen isotope ratios and ice-rafted cobbles in the North Atlantic and North Pacific ocean beds. Mid-latitude glaciation was underway before the end of the epoch; the global cooling that occurred during the Pliocene may have spurred on the disappearance of forests and the spread of grasslands and savannas. Continents continued to drift, moving from positions as far as 250 km from their present locations to positions only 70 km from their current locations.
South America became linked to North America through the Isthmus of Panama during the Pliocene, making possible the Great American Interchange and bringing a nearly complete end to South America's distinctive large marsupial predator and native ungulate faunas. The formation of the Isthmus had major consequences on global temperatures, since warm equatorial ocean currents were cut off and an Atlantic cooling cycle began, with cold Arctic and Antarctic waters dropping temperatures in the now-isolated Atlantic Ocean. Africa's collision with Europe formed the Mediterranean Sea, cutting off the remnants of the Tethys Ocean; the border between the Miocene and the Pliocene is the time of the Messinian salinity crisis. Sea level changes exposed the land bridge between Asia. Pliocene marine rocks are well exposed in the Mediterranean and China. Elsewhere, they are exposed near shores. During the Pliocene parts of southern Norway and southern Sweden, near sea level rose. In Norway this rise elevated the Hardangervidda plateau to 1200 m in the Early Pliocene.
In Southern Sweden similar movements elevated the South Swedish highlands leading to a deflection of the ancient Eridanos river from its original path across south-central Sweden into a course south of Sweden. The change to a cooler, seasonal climate had considerable impacts on Pliocene vegetation, reducing tropical species worldwide. Deciduous forests proliferated, coniferous forests and tundra covered much of the north, grasslands spread on all continents. Tropical forests were limited to a tight band around the equator, in addition to dry savannahs, deserts appeared in Asia and Africa. Both marine and co
The geologic record in stratigraphy and other natural sciences refers to the entirety of the layers of rock strata — deposits laid down by volcanism or by deposition of sediment derived from weathering detritus including all its fossil content and the information it yields about the history of the Earth: its past climate, geography and the evolution of life on its surface. According to the law of superposition and volcanic rock layers are deposited on top of each other, they harden over time to become a solidified rock column, that may be intruded by igneous rocks and disrupted by tectonic events. At a certain locality on the Earth's surface, the rock column provides a cross section of the natural history in the area during the time covered by the age of the rocks; this is sometimes called the rock history and gives a window into the natural history of the location that spans many geological time units such as ages, epochs, or in some cases multiple major geologic periods—for the particular geographic region or regions.
The geologic record is in no one place complete for where geologic forces one age provide a low-lying region accumulating deposits much like a layer cake, in the next may have uplifted the region, the same area is instead one, weathering and being torn down by chemistry, wind and water. This is to say that in a given location, the geologic record can be and is quite interrupted as the ancient local environment was converted by geological forces into new landforms and features. Sediment core data at the mouths of large riverine drainage basins, some of which go 7 miles deep support the law of superposition; however using broadly occurring deposited layers trapped within differently located rock columns, geologists have pieced together a system of units covering most of the geologic time scale using the law of superposition, for where tectonic forces have uplifted one ridge newly subject to erosion and weathering in folding and faulting the strata, they have created a nearby trough or structural basin region that lies at a relative lower elevation that can accumulate additional deposits.
By comparing overall formations, geologic structures and local strata, calibrated by those layers which are widespread, a nearly complete geologic record has been constructed since the 17th century. Correcting for discordancies can be done in a number of ways and utilizing a number of technologies or field research results from studies in other disciplines. In this example, the study of layered rocks and the fossils they contain is called biostratigraphy and utilizes amassed geobiology and paleobiological knowledge. Fossils can be used to recognize rock layers of the same or different geologic ages, thereby coordinating locally occurring geologic stages to the overall geologic timeline; the pictures of the fossils of monocellular algae in this USGS figure were taken with a scanning electron microscope and have been magnified 250 times. In the U. S. state of South Carolina three marker species of fossil algae are found in a core of rock whereas in Virginia only two of the three species are found in the Eocene Series of rock layers spanning three stages and the geologic ages from 37.2–55.8 MA.
Comparing the record about the discordance in the record to the full rock column shows the non-occurrence of the missing species and that portion of the local rock record, from the early part of the middle Eocene is missing there. This is one form of discordancy and the means geologists use to compensate for local variations in the rock record. With the two remaining marker species it is possible to correlate rock layers of the same age in both South Carolina and Virginia, thereby "calibrate" the local rock column into its proper place in the overall geologic record; as the picture of the overall rock record emerged, discontinuities and similarities in one place were cross-correlated to those in others, it became useful to subdivide the overall geologic record into a series of component sub-sections representing different sized groups of layers within known geologic time, from the shortest time span stage to the largest thickest strata eonothem and time spans eon. Concurrent work in other natural science fields required a time continuum be defined, earth scientists decided to coordinate the system of rock layers and their identification criteria with that of the geologic time scale.
This gives the pairing between the physical layers of the left column and the time units of the center column in the table at right
In stratigraphy and geology, an eonothem is the totality of rock strata laid down in the stratigraphic record deposited during a certain eon of the continuous geologic timescale. The eonothem is not to be confused with the eon itself, a corresponding division of geologic time spanning a specific amount of years, during which rocks were formed that are classified within the eonothem. In practice, the rock column is discontinuous: Eonothems, despite discontinuities, can be compared to others where the rock record is more complete and by correlation of points of correspondence be fixed appropriately within the eon. Eonothems are therefore useful as a broad chronostratigraphic unit, specifying approximate age within the timelines within the rock column. Eonothems are subdivided into erathems and their smaller subdivisions within geology and paleobiology and their sub-fields, a whole system of cross-disciplinary classification by strata is in place with oversight by the International Commission on Stratigraphy.
Since oldest rocks are deposited first and lowest in a stratigraphic section, whether one is discussing the rock record is clear in context. Eonothems are not used in practice as expert dating estimates can be and are specified into the more refined timelines of smaller chronostratigraphic units, which can be subdivided in turn down to the many defined stages, the smallest units used in dating. Eonothems have the same names as their corresponding eons, which means during the history of the Earth only four eonothems were formed. Oldest to newest these are the Hadean, Archean and Phanerozoic. GSSAs are defined by the International Commission on Stratigraphy and are used for time dating rock layers older than 630 million years ago, before a good fossil record exists; the record becomes spotty at about 542 mya, the ICS may well have resort to defining additional GSSA's between the two dates. For more recent periods, a Global Boundary Stratotype Section and Point based on research progress in geobiology and improved methods of fossil dating is used to define such boundaries.
In contrast to GSSAs, GSSPs are based on important events and transitions within a particular stratigraphic section. In older sections, there is insufficient fossil record or well preserved sections to identify the key events necessary for a GSSP so GSSAs are defined based on fixed dates. Eonothem derives from eon, a Latin transliteration from the koine Greek word ὁ αἰών from the archaic αἰϝών, thema, "that, placed or laid down", "subject of a discourse". Chronostratigraphy Lithostratigraphy Geologic record Body form Fauna Type locality Hedberg, H. D. International stratigraphic guide: A guide to stratigraphic classification and procedure, New York, John Wiley and Sons, 1976 International Stratigraphic Chart from the International Commission on Stratigraphy USA National Park Service Washington State University Web Geological Time Machine Eon or Aeon, Math Words - An alphabetical index The Global Boundary Stratotype Section and Point: overview Chart of The Global Boundary Stratotype Sections and Points: chart Geotime chart displaying geologic time periods compared to the fossil record