Time is a basic component of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify the motions of objects. Time has been a major subject of religion, philosophy, and science, but defining time in a non-controversial manner applicable to all fields of study has consistently eluded the greatest scholars. In physics and other sciences, time is considered one of the few fundamental quantities. Time is used to define other quantites such as velocity, and defining time in terms of such quantities would result in circularity of definition. Within science, the only definition needed or possible is an operational one, in which a procedure is given for defining the base unit of time which is the second. Among philosophers, there are two distinct viewpoints on time. One view is that time is part of the fundamental structure of the universe, a dimension in which events occur in sequence. Sir Isaac Newton subscribed to this realist view, and hence it is sometimes referred to as Newtonian time. The opposing view of time is that time does not refer to any kind of "container" that events and objects "move through", nor to any entity that "flows", but that it is instead part of a fundamental intellectual structure, together with space and number within which humans sequence and compare events. This second view, in the tradition of Gottfried Leibniz and Immanuel Kant, holds that time cannot itself be measured. Temporal measurement has occupied scientists and technologists, and was a prime motivation in 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, and the beat of a heart. Currently, the international unit of time, the second, is defined as a certain number of hyperfine transitions in caesium atoms. Time is also of significant social importance, having economic value "time is money" as well as personal value, due to an awareness of the limited time in each day and in human lifespans. Temporal measurement, or chronometry, takes two distinct primary forms. The calendar, a mathematical abstraction for calculating extensive periods of time, and the clock, a concrete mechanism that counts the ongoing passage of time. In day to day life, the clock is consulted for periods less than a day, the calendar, for periods longer than a day. Artefacts from the Palaeolithic suggest that the moon was used to calculate time as early as 12,000 BC, and possibly even 30,000 BC. The Sumerian civilization of approximately 2000 BC introduced the sexagesimal system based on the number 60. 60 seconds in a minute, 60 minutes in an hour – and possibly a calendar with 360 (60x6) days in a year. Twelve also features prominently in time, with roughly 12 hours of day and 12 of night, and 12 months in a year. A large variety of devices have been invented to measure time; The study of these devices is called horology. A sundial uses a gnomon to cast a shadow on a set of markings which were calibrated to the hour. The position of the shadow marked the hour in local time. The most accurate timekeeping devices of the ancient world were the waterclock or clepsydra, first found in Egypt. Waterclocks were used in Alexandria, and then worldwide, for example in Greece, from c. 400 BCE. Waterclocks could be used to measure the hours even at night, but required manual timekeeping to replenish the flow of water. The Greeks and Chaldeans regularly maintained timekeeping records as an essential part of their astronomical observations. Arab engineers improved on the use of waterclocks up to the Middle Ages. The hourglass uses the flow of sand to measure the flow of time. The hourglass was used in navigation. Ferdinand Magellan used 18 glasses on each ship for his circumnavigation of the globe in the year 1522. Incense sticks and candles were, and are, commonly used to measure time in temples and churches across the globe. Waterclocks, and later, mechanical clocks, were used to mark the events of the abbeys and monasteries of the Middle Ages. The English word clock probably comes from the Middle Dutch word "klocke" which is in turn derived from the mediaeval Latin word "clocca", which is ultimately derived from Celtic, and is cognate with French, Latin, and German words that mean bell. The passage of the hours at sea were marked by bells, and denoted the time. The hours were marked by bells in the abbeys as well as at sea. Clocks can range from watches, to more exotic varieties such as the Clock of the Long Now. A chronometer is a portable timekeeper that meets certain precision standards. Initially, the term was used to refer to the marine chronometer, a timepiece used to determine longitude by means of celestial navigation. The most accurate type of timekeeping device is currently the atomic clock, which are accurate to seconds in many thousands of years, and are used to calibrate other clock and timekeeping instruments. Atomic clocks use the spin property of the caesium atom as its basis, and since 1967, the International System of Measurements bases its unit of time, the second, on the properties of caesium. SI defines the second as 9,192,631,770 cycles of the radiation which corresponds to the transition between two electron spin energy levels of the ground state of the 133Cs atom. Today, the Global Positioning System in coordination with the Network Time Protocol can be used to synchronize timekeeping systems across the globe. The SI base unit for time is the SI second. From the second, larger units such as the minute, hour and day are defined, though they are "non-SI" units because they do not use the decimal system, and also because of the occasional need for a leap-second; They are, however, officially accepted for use with the International System. There are no fixed ratios between seconds and months or years as months and years have significant variations in length. The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom! The current definition of the second, coupled with the current definition of the metre, is based on the special theory of relativity, that our space-time is a Minkowski space. The measurement of time is so critical to the functioning of modern societies that it is coordinated at an international level. The basis for scientific time is a continuous count of seconds based on atomic clocks around the world, known as the International Atomic Time (TAI). TAI is the International Atomic Time. Earth is split up into a number of time zones. Most time zones are exactly one hour apart, and by convention compute their local time as an offset from UTC or Greenwich Mean Time. Sidereal time is the measurement of time relative to a distant star. Sidereal Time is used in astronomy to predict when a star will be overhead. Due to the rotation of the earth around the sun a sidereal day is slightly less than a solar day. Another form of time measurement consists of studying the past. Events in the past can be ordered in a sequence, creating a chronology, and be put into chronological groups known as periodization. One of the most important systems of periodization is geologic time, which is a system of periodizing the events that shaped the Earth and its life. Chronology, periodization, and interpretation of the past are together known as the study of history. Many ancient philosophers wrote lengthy essays on time. A famous analogy compared the time of life to the passing of sand through an hourglass. The sand at the top OF and hourglass is associated with the future, and, one tiny grain at a time, the future flows through the present into the past. The past: ever expanding, the future: ever decreasing, but the future grains become amassed into the past through the present. From the age of Newton up until Einstein's profound reinterpretation of the physical concepts associated with time and space, time was considered to be "absolute" and to flow "equably" for all observers. The science of classical mechanics is based on this Newtonian idea of time. Einstein, in his special theory of relativity, postulated the constancy and finiteness of the speed of light for all observers. Einstein showed that this postulate, together with a reasonable definition for what it means for two events to be simultaneous, requires that distances appear compressed and time intervals appear lengthened for events associated with objects in motion relative to an inertial observer. Einstein showed that if time and space is measured using electromagnetic phenomena then due to the constancy of the speed of light, time and space become mathematically entangled together in a certain way which in turn results in Lorentz transformation and in entanglement of all other important derivative physical quantities in a certain 4-vectorial way. Modern physics views the curvature of spacetime around an object as much a feature of that object as are its mass and volume. Time has historically been closely related with space, the two together comprising spacetime in Einstein's special relativity and general relativity. Time appears to have a direction, the past lies behind, fixed and incommutable, while the future lies ahead and is not necessarily fixed. The Second law of thermodynamics states that entropy must increase over time; the cosmological arrow of time, which points away from the Big Bang, and the radiative arrow of time, caused by light only traveling forwards in time. Stephen Hawking in particular has addressed a connection between time and the Big Bang. Stephen Hawking has sometimes stated that we may as well assume that time began with the Big Bang because trying to answer any question about what happened before the Big Bang is trying to answer a question that is meaningless as those events would have been part of a different time frame and different universe outside of the scope of the Big Bang theory. Hawking, in A Brief History of Time and elsewhere, along with several other modern physicists, has stated his position more clearly and less controversially: that even if time did not begin with the Big Bang and there were another time frame before the Big Bang, no information from events then would be accessible to us, and nothing that happened then would have any effect upon the present time-frame. Even in the presence of timepieces, different individuals may judge an identical length of time to be passing at different rates. In sociology and anthropology, time discipline is the general name given to social and economic rules, conventions, customs, and expectations governing the measurement of time, the social currency and awareness of time measurements, and people's expectations concerning the observance of these customs by others. The use of time is an important issue in understanding human behaviour, education, and travel behaviour. Time use research is a developing field of study. The question concerns how time is allocated across a number of activities such as time spent at home, at work, shopping, etc. Time use changes with technology, as the television or the Internet created new opportunities to use time in different ways. However, some aspects of time use are relatively stable over long periods of time, such as the amount of time spent traveling to work, which despite major changes in transport, has been observed to be about 20-30 minutes one-way for a large number of cities over a long period of time. Time management is the organization of tasks or events by first estimating how much time a task will take to be completed, when it must be completed, and then adjusting events that would interfere with its completion so that completion is reached in the appropriate amount of time. Calendars and day planners are common examples of time management tools. There is usually twelve hours of day time and twelve hours of night time in a day. A day contains 12 hours of day and 12 hours of night. There are 12 months in a year. A year is 12 months long. A year is twelve months long. There are 60 seconds in a minute. A minute is 60 seconds in length. A minute is sixty seconds long. There are 60 minutes in an hour. An hour is 60 seconds long. An hour is sixty minutes long. There are 24 hours in a day. A day is 24 hours long. There are 7 days in a week. A week is 7 days long. Monday is the first day of the week. Monday is a day. Monday follows Sunday. Monday is the day after Sunday. Monday is before Tuesday. Tuesday is the second day of the week. Tuesday follows Monday. Tuesday is the day after Monday. Tuesday is the day before Wednesday. Tuesday is before Wednesday. Wednesday is the third day of the week. Wednesday is the day after Tuesday. Wednesday is after Tuesday. Wednesday is the day before Thursday. Thursday is the fourth day of the week. Thursday is the day after Wednesday. Thursday is after Wednesday. Thursday is the day before Friday. Friday is the fifth day of the week. Friday is the day after Thursday. Friday is after Thursday. Friday is the day before Saturday. Saturday is the sixth day of the week. Saturday is the day after Friday. Saturday is after Friday. Saturday is the day before Sunday. Sunday is the seventh day of the week. Sunday is the last day of the week. Sunday is the day after Saturday. Sunday is the day before Monday.