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98-557: A collection of sequential days is organized into calendars as dates , almost always into weeks , months and years . A solar calendar organizes dates based on the Sun's annual cycle, giving consistent start dates for the four seasons from year to year. A lunar calendar organizes dates based on the Moon 's lunar phase . In common usage, a day starts at midnight , written as 00:00 or 12:00 am in 24- or 12-hour clocks , respectively. Because

196-413: A prime meridian , is mathematically related to time differences up to 12 hours by a factor of 15. Thus, a time differential (in hours) between two points is multiplied by 15 to obtain a longitudinal difference (in degrees). Historically, times used for calculating longitude have included apparent solar time , local mean time , and ephemeris time , with mean time being the one most used for navigation of

294-596: A business. It is used for budgeting, keeping accounts, and taxation. It is a set of 12 months that may start at any date in a year. The US government's fiscal year starts on 1 October and ends on 30 September. The government of India's fiscal year starts on 1 April and ends on 31 March. Small traditional businesses in India start the fiscal year on Diwali festival and end the day before the next year's Diwali festival. Longitude Longitude ( / ˈ l ɒ n dʒ ɪ tj uː d / , AU and UK also / ˈ l ɒ ŋ ɡ ɪ -/ )

392-548: A calendar based on the reign of their current sovereign. For example, the year 2006 in Japan is year 18 Heisei, with Heisei being the era name of Emperor Akihito . An astronomical calendar is based on ongoing observation; examples are the religious Islamic calendar and the old religious Jewish calendar in the time of the Second Temple . Such a calendar is also referred to as an observation-based calendar. The advantage of such

490-437: A calendar is that it is perfectly and perpetually accurate. The disadvantage is that working out when a particular date would occur is difficult. An arithmetic calendar is one that is based on a strict set of rules; an example is the current Jewish calendar . Such a calendar is also referred to as a rule-based calendar. The advantage of such a calendar is the ease of calculating when a particular date occurs. The disadvantage

588-435: A calendar provides a way to determine when to start planting or harvesting, which days are religious or civil holidays , which days mark the beginning and end of business accounting periods, and which days have legal significance, such as the day taxes are due or a contract expires. Also, a calendar may, by identifying a day, provide other useful information about the day such as its season. Calendars are also used as part of

686-544: A civil day can be either 86,401 or 86,399 SI seconds long on such a day. Other than the two-millisecond variation from tidal deceleration, other factors minutely affect the day's length , which creates an irregularity in the placement of leap seconds. Leap seconds are announced in advance by the International Earth Rotation and Reference Systems Service (IERS) , which measures the Earth's rotation and determines whether

784-417: A complete timekeeping system: date and time of day together specify a moment in time . In the modern world, timekeepers can show time, date, and weekday. Some may also show the lunar phase. The Gregorian calendar is the de facto international standard and is used almost everywhere in the world for civil purposes. The widely used solar aspect is a cycle of leap days in a 400-year cycle designed to keep

882-428: A difference of 15° longitude corresponds to a one-hour difference in local time, due to the differing position in relation to the Sun. Comparing local time to an absolute measure of time allows longitude to be determined. Depending on the era, the absolute time might be obtained from a celestial event visible from both locations, such as a lunar eclipse, or from a time signal transmitted by telegraph or radio. The principle

980-405: A distant star (assumed to be fixed). Measuring a day as such is used in astronomy . A sidereal day is about 4 minutes less than a solar day of 24 hours (23 hours 56 minutes and 4.09 seconds), or 0.99726968 of a solar day of 24 hours. There are about 366.2422 stellar days in one mean tropical year (one stellar day more than the number of solar days). Besides a stellar day on Earth , other bodies in

1078-623: A few of the more prevalent ones. Longitude is given as an angular measurement with 0° at the Prime Meridian , ranging from −180° westward to +180° eastward. The Greek letter λ (lambda) is used to denote the location of a place on Earth east or west of the Prime Meridian. Each degree of longitude is sub-divided into 60 minutes , each of which is divided into 60 seconds . A longitude is thus specified in sexagesimal notation as, for example, 23° 27′ 30″ E. For higher precision,

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1176-484: A full calendar system; neither is a system to name the days within a year without a system for identifying the years. The simplest calendar system just counts time periods from a reference date. This applies for the Julian day or Unix Time . Virtually the only possible variation is using a different reference date, in particular, one less distant in the past to make the numbers smaller. Computations in these systems are just

1274-410: A leap second is necessary. Discovered by paleontologist John W. Wells , the day lengths of geological periods have been estimated by measuring sedimentation rings in coral fossils , due to some biological systems being affected by the tide. The length of a day at the Earth's formation is estimated at 6 hours. Arbab I. Arbab plotted day lengths over time and found a curved line. Arbab attributed this to

1372-534: A lunar calendar. Other marked bones may also represent lunar calendars. Similarly, Michael Rappenglueck believes that marks on a 15,000-year-old cave painting represent a lunar calendar. A lunisolar calendar is a lunar calendar that compensates by adding an extra month as needed to realign the months with the seasons. Prominent examples of lunisolar calendar are Hindu calendar and Buddhist calendar that are popular in South Asia and Southeast Asia . Another example

1470-412: A matter of addition and subtraction. Other calendars have one (or multiple) larger units of time. Calendars that contain one level of cycles: Calendars with two levels of cycles: Cycles can be synchronized with periodic phenomena: Very commonly a calendar includes more than one type of cycle or has both cyclic and non-cyclic elements. Most calendars incorporate more complex cycles. For example,

1568-468: A method of determining longitude by comparing the local time of a lunar eclipse at two different places, thus demonstrating an understanding of the relationship between longitude and time. Claudius Ptolemy (2nd century CE) developed a mapping system using curved parallels that reduced distortion. He also collected data for many locations, from Britain to the Middle East. He used a prime meridian through

1666-427: A new day starting at either the rising or setting of the Sun on the local horizon (Italian reckoning, for example, being 24 hours from sunset, old style). The exact moment of, and the interval between, two sunrises or sunsets depends on the geographical position ( longitude and latitude, as well as altitude), and the time of year (as indicated by ancient hemispherical sundials ). A more constant day can be defined by

1764-513: 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. Various decimal or metric time proposals have been made, but do not redefine the day, and use the day or sidereal day as a base unit. Metric time uses metric prefixes to keep time. It uses

1862-484: 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

1960-648: A total of 27 leap seconds that have been inserted, so roughly once every other year). The term comes from the Old English term dæġ ( /dæj/ ), with its cognates such as dagur in Icelandic , Tag in German , and dag in Norwegian , Danish , Swedish and Dutch – all stemming from a Proto-Germanic root *dagaz . Several definitions of this universal human concept are used according to context, need, and convenience. Besides

2058-549: Is cos φ decreases from 1 at the equator to 0 at the poles, which measures how circles of latitude shrink from the equator to a point at the pole, so the length of a degree of longitude decreases likewise. This contrasts with the small (1%) increase in the length of a degree of latitude (north–south distance), equator to pole. The table shows both for the WGS84 ellipsoid with a = 6 378 137 .0 m and b = 6 356 752 .3142 m . The distance between two points 1 degree apart on

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2156-521: Is a geographic coordinate that specifies the east – west position of a point on the surface of the Earth , or another celestial body. It is an angular measurement , usually expressed in degrees and denoted by the Greek letter lambda (λ). Meridians are imaginary semicircular lines running from pole to pole that connect points with the same longitude. The prime meridian defines 0° longitude; by convention

2254-562: Is a lunar calendar consisting of 12 lunar months in a year of 354 or 355 days. It is used to date events in most of the Muslim countries (concurrently with the Gregorian calendar) and used by Muslims everywhere to determine the proper day on which to celebrate Islamic holy days and festivals. Its epoch is the Hijra (corresponding to AD 622). With an annual drift of 11 or 12 days, the seasonal relation

2352-604: Is based on the prohibition of intercalation ( nasi' ) by Muhammad , in Islamic tradition dated to a sermon given on 9 Dhu al-Hijjah AH 10 (Julian date: 6 March 632). This resulted in an observation-based lunar calendar that shifts relative to the seasons of the solar year. There have been several modern proposals for reform of the modern calendar, such as the World Calendar , the International Fixed Calendar ,

2450-470: Is generally known as intercalation . Even if a calendar is solar, but not lunar, the year cannot be divided entirely into months that never vary in length. Cultures may define other units of time, such as the week, for the purpose of scheduling regular activities that do not easily coincide with months or years. Many cultures use different baselines for their calendars' starting years. Historically, several countries have based their calendars on regnal years ,

2548-423: Is imperfect accuracy. Furthermore, even if the calendar is very accurate, its accuracy diminishes slowly over time, owing to changes in Earth's rotation. This limits the lifetime of an accurate arithmetic calendar to a few thousand years. After then, the rules would need to be modified from observations made since the invention of the calendar. The early Roman calendar , created during the reign of Romulus , lumped

2646-419: Is near the time of the lower culmination of the Sun on the central meridian of the time zone. Such a day may be called a calendar day . A day is commonly divided into 24 hours, with each hour being made up of 60 minutes, and each minute composed of 60 seconds. A sidereal day or stellar day is the span of time it takes for the Earth to make one entire rotation with respect to the celestial background or

2744-425: Is practiced, and each year there will be one 23-hour civil day and one 25-hour civil day. Due to slight variations in the rotation of the Earth, there are rare times when a leap second will get inserted at the end of a UTC day, and so while almost all days have a duration of 86,400 seconds, there are these exceptional cases of a day with 86,401 seconds (in the half-century spanning 1972 through 2022, there have been

2842-672: Is repeated approximately every 33 Islamic years. Various Hindu calendars remain in use in the Indian subcontinent, including the Nepali calendars , Bengali calendar , Malayalam calendar , Tamil calendar , Vikrama Samvat used in Northern India, and Shalivahana calendar in the Deccan states. The Buddhist calendar and the traditional lunisolar calendars of Cambodia , Laos , Myanmar , Sri Lanka and Thailand are also based on an older version of

2940-402: Is straightforward, but in practice finding a reliable method of determining longitude took centuries and required the effort of some of the greatest scientific minds. A location's north–south position along a meridian is given by its latitude , which is approximately the angle between the equatorial plane and the normal from the ground at that location. Longitude is generally given using

3038-459: Is the Hebrew calendar, which uses a 19-year cycle . Nearly all calendar systems group consecutive days into "months" and also into "years". In a solar calendar a year approximates Earth's tropical year (that is, the time it takes for a complete cycle of seasons ), traditionally used to facilitate the planning of agricultural activities. In a lunar calendar , the month approximates the cycle of

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3136-558: Is used in Iran and some parts of Afghanistan . The Assyrian calendar is in use by the members of the Assyrian community in the Middle East (mainly Iraq, Syria, Turkey, and Iran) and the diaspora. The first year of the calendar is exactly 4750 years prior to the start of the Gregorian calendar. The Ethiopian calendar or Ethiopic calendar is the principal calendar used in Ethiopia and Eritrea , with

3234-514: The Hellenistic period they gave rise to the ancient Roman calendar and to various Hindu calendars . Calendars in antiquity were lunisolar , depending on the introduction of intercalary months to align the solar and the lunar years. This was mostly based on observation, but there may have been early attempts to model the pattern of intercalation algorithmically, as evidenced in the fragmentary 2nd-century Coligny calendar . The Roman calendar

3332-525: The Holocene calendar , and the Hanke–Henry Permanent Calendar . Such ideas are mooted from time to time, but have failed to gain traction because of the loss of continuity and the massive upheaval that implementing them would involve, as well as their effect on cycles of religious activity. A full calendar system has a different calendar date for every day. Thus the week cycle is by itself not

3430-585: The International Reference Meridian for the Earth passes near the Royal Observatory in Greenwich , south-east London on the island of Great Britain . Positive longitudes are east of the prime meridian, and negative ones are west. Because of the Earth's rotation , there is a close connection between longitude and time measurement . Scientifically precise local time varies with longitude:

3528-554: The Oromo calendar also in use in some areas. In neighboring Somalia , the Somali calendar co-exists alongside the Gregorian and Islamic calendars. In Thailand , where the Thai solar calendar is used, the months and days have adopted the western standard, although the years are still based on the traditional Buddhist calendar . A fiscal calendar generally means the accounting year of a government or

3626-521: 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. Thus, daytime is on average around 7 minutes longer than 12 hours. Daytime is further divided into morning , afternoon , and evening . Morning occurs between sunrise and noon . Afternoon occurs between noon and sunset , or between noon and

3724-445: 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). In terms of Earth's rotation, the average day length is about 360.9856°. A day lasts for more than 360° of rotation because of the Earth's revolution around the Sun. With a full year being slightly more than 360 days,

3822-522: The geodetic normal or the gravity direction . The astronomical longitude can differ slightly from the ordinary longitude because of vertical deflection , small variations in Earth's gravitational field (see astronomical latitude ). The concept of longitude was first developed by ancient Greek astronomers. Hipparchus (2nd century BCE) used a coordinate system that assumed a spherical Earth, and divided it into 360° as we still do today. His prime meridian passed through Alexandria . He also proposed

3920-469: The solar year over the long term. 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. Latin calendarium meant 'account book, register' (as accounts were settled and debts were collected on the calends of each month). The Latin term

4018-532: The 12th century, astronomical tables were prepared for a number of European cities, based on the work of al-Zarqālī in Toledo . The lunar eclipse of September 12, 1178 was used to establish the longitude differences between Toledo, Marseilles , and Hereford . Christopher Columbus made two attempts to use lunar eclipses to discover his longitude, the first in Saona Island , on 14 September 1494 (second voyage), and

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4116-530: The 61 days of the winter period them together as simply "winter." Over time, this period became January and February; through further changes over time (including the creation of the Julian calendar ) this calendar became the modern Gregorian calendar, introduced in the 1570s. The primary practical use of a calendar is to identify days: to be informed about or to agree on a future event and to record an event that has happened. Days may be significant for agricultural, civil, religious, or social reasons. For example,

4214-632: The British parliament in 1714. It offered two levels of rewards, for solutions within 1° and 0.5°. Rewards were given for two solutions: lunar distances, made practicable by the tables of Tobias Mayer developed into an nautical almanac by the Astronomer Royal Nevil Maskelyne ; and for the chronometers developed by the Yorkshire carpenter and clock-maker John Harrison . Harrison built five chronometers over more than three decades. This work

4312-751: The Canary Islands, so that all longitude values would be positive. While Ptolemy's system was sound, the data he used were often poor, leading to a gross over-estimate (by about 70%) of the length of the Mediterranean. After the fall of the Roman Empire, interest in geography greatly declined in Europe. Hindu and Muslim astronomers continued to develop these ideas, adding many new locations and often improving on Ptolemy's data. For example al-Battānī used simultaneous observations of two lunar eclipses to determine

4410-516: The Earth's rotational period is slowing. Because of the way the second is defined, the mean length of a solar day is now about 86,400.002 seconds, and is increasing by about 2 milliseconds per century. Since the rotation rate of the Earth is slowing, the length of a SI second fell out of sync with a second derived from the rotational period. This arose the need for leap seconds , which insert extra seconds into Coordinated Universal Time (UTC) . Although typically 86,400 SI seconds in duration,

4508-515: The Earth's daily orbit around the Sun is slightly less than 1°, so the day is slightly less than 361° of rotation. Elsewhere in the Solar System or other parts of the universe , a day is a full rotation of other large astronomical objects with respect to its star. For civil purposes, a common clock time is typically defined for an entire region based on the local mean solar time at a central meridian. Such time zones began to be adopted about

4606-508: The Eve of Saint Agnes are remnants of the older pattern when holidays began during the prior evening. The common convention among the ancient Romans , ancient Chinese and in modern times is for the civil day to begin at midnight, i.e. 00:00, and to last a full 24 hours until 24:00, i.e. 00:00 of the next day. The International Meridian Conference of 1884 resolved That the Conference expresses

4704-723: The Hindu calendar. Most of the Hindu calendars are inherited from a system first enunciated in Vedanga Jyotisha of Lagadha, standardized in the Sūrya Siddhānta and subsequently reformed by astronomers such as Āryabhaṭa (AD 499), Varāhamihira (6th century) and Bhāskara II (12th century). The Hebrew calendar is used by Jews worldwide for religious and cultural affairs, also influences civil matters in Israel (such as national holidays ) and can be used business dealings (such as for

4802-582: The Old Calendar) and the Revised Julian Calendar (often called the New Calendar). The Revised Julian Calendar is nearly the same as the Gregorian calendar, with the addition that years divisible by 100 are not leap years , except that years with remainders of 200 or 600 when divided by 900 remain leap years, e.g. 2000 and 2400 as in the Gregorian calendar. The Islamic calendar or Hijri calendar

4900-489: The Poles. Also the discontinuity at the ± 180° meridian must be handled with care in calculations. An example is a calculation of east displacement by subtracting two longitudes, which gives the wrong answer if the two positions are on either side of this meridian. To avoid these complexities, some applications use another horizontal position representation . The length of a degree of longitude (east–west distance) depends only on

4998-533: The Roman Rite of the Catholic Church, and generally include the liturgical seasons of Advent , Christmas , Ordinary Time (Time after Epiphany ), Lent , Easter , and Ordinary Time (Time after Pentecost ). Some Christian calendars do not include Ordinary Time and every day falls into a denominated season. The Eastern Orthodox Church employs the use of 2 liturgical calendars; the Julian calendar (often called

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5096-574: The Roman calendar contained remnants of a very ancient pre-Etruscan 10-month solar year. The first recorded physical calendars, dependent on the development of writing in the Ancient Near East , are the Bronze Age Egyptian and Sumerian calendars. During the Vedic period India developed a sophisticated timekeeping methodology and calendars for Vedic rituals. According to Yukio Ohashi,

5194-635: The Solar System have day times, the durations of these being: In the International System of Units (SI), a day not an official unit, but is accepted for use with SI . A day, with symbol d, is defined using SI units as 86,400 seconds; the second is the base unit of time in SI units . In 1967–68, during the 13th CGPM (Resolution 1), the International Bureau of Weights and Measures (BIPM) redefined

5292-451: 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

5390-978: The Vedanga calendar in ancient India was based on astronomical studies during the Vedic Period and was not derived from other cultures. A large number of calendar systems in the Ancient Near East were based on the Babylonian calendar dating from the Iron Age , among them the calendar system of the Persian Empire, which in turn gave rise to the Zoroastrian calendar and the Hebrew calendar . A great number of Hellenic calendars were developed in Classical Greece , and during

5488-465: The West Indies, and as far as Japan and China in the years 1874–90. This contributed greatly to the accurate mapping of these areas. While mariners benefited from the accurate charts, they could not receive telegraph signals while under way, and so could not use the method for navigation. This changed when wireless telegraphy (radio) became available in the early 20th century. Wireless time signals for

5586-548: The advantages that both the observations and the calculations were simpler, and as they became cheaper in the early 19th century they started to replace lunars, which were seldom used after 1850. The first working telegraphs were established in Britain by Wheatstone and Cooke in 1839, and in the US by Morse in 1844. It was quickly realised that the telegraph could be used to transmit a time signal for longitude determination. The method

5684-483: The annual flooding of the Nile River. They built a calendar with 365 days, divided into 12 months of 30 days each, with 5 extra days at the end of the year. However, they did not include the extra bit of time in each year, and this caused their calendar to slowly become inaccurate. Not all calendars use the solar year as a unit. A lunar calendar is one in which days are numbered within each lunar phase cycle. Because

5782-493: The change of water volume present affecting Earth's rotation. For most diurnal animals, the day naturally begins at dawn and ends at sunset. Humans, with their cultural norms and scientific knowledge, have employed several different conceptions of the day's boundaries. In the Hebrew Bible , Genesis 1:5 defines a day in terms of "evening" and "morning" before recounting the creation of the Sun to illuminate it: "And God called

5880-724: The convention of negative for east is also sometimes seen, most commonly in the United States ; the Earth System Research Laboratories used it on an older version of one of their pages, in order "to make coordinate entry less awkward" for applications confined to the Western Hemisphere . They have since shifted to the standard approach. The longitude is singular at the Poles and calculations that are sufficiently accurate for other positions may be inaccurate at or near

5978-697: The dating of cheques ). Followers of the Baháʼí Faith use the Baháʼí calendar . The Baháʼí Calendar, also known as the Badi Calendar was first established by the Bab in the Kitab-i-Asma. The Baháʼí Calendar is also purely a solar calendar and comprises 19 months each having nineteen days. The Chinese , Hebrew , Hindu , and Julian calendars are widely used for religious and social purposes. The Iranian (Persian) calendar

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6076-434: The day as the base unit, and smaller units being fractions of a day: a metric hour ( deci ) is 1 ⁄ 10 of a day; a metric minute ( milli ) is 1 ⁄ 1000 of a day; etc. Similarly, in decimal time, the length of a day is static to normal time. A day is also split into 10 hours, and 10 days comprise a décade – the equivalent of a week. 3 décades make a month. Various decimal time proposals which do not redefine

6174-444: The day do not have set times; they can vary by lifestyle or hours of daylight in a given place. Daytime is the part of the day during which sunlight directly reaches the ground, assuming that there are no obstacles. The length of daytime averages slightly more than half of the 24-hour day. Two effects make daytime on average longer than night. The Sun is not a point but has an apparent size of about 32 minutes of arc . Additionally,

6272-424: The day of 24 hours (86,400 seconds), the word day is used for several different spans of time based on the rotation of the Earth around its axis. An important one is the solar day , the time it takes for the Sun to return to its culmination point (its highest point in the sky). Due to an orbit's eccentricity, the Sun resides in one of the orbit's foci instead of the middle. Consequently, due to Kepler's second law ,

6370-523: The day: Henri de Sarrauton's proposal kept days, and subdivided hours into 100 minutes; in Mendizábal y Tamborel's proposal, the sidereal day was the basic unit, with subdivisions made upon it; and Rey-Pailhade's proposal divided the day 100 cés. The word refers to various similarly defined ideas, such as: Mainly due to tidal deceleration – the Moon's gravitational pull slowing down the Earth's rotation –

6468-421: The development of telescopes and pendulum clocks until the mid-18th century saw a steady increase in the number of places whose longitude had been determined with reasonable accuracy, often with errors of less than a degree, and nearly always within 2° to 3°. By the 1720s errors were consistently less than 1°. At sea during the same period, the situation was very different. Two problems proved intractable. The first

6566-509: The difference in longitude between Antakya and Raqqa with an error of less than 1°. This is considered to be the best that can be achieved with the methods then available: observation of the eclipse with the naked eye, and determination of local time using an astrolabe to measure the altitude of a suitable "clock star". In the later Middle Ages, interest in geography revived in the west, as travel increased, and Arab scholarship began to be known through contact with Spain and North Africa. In

6664-418: The duration of the year aligned with the solar year . There is a lunar aspect which approximates the position of the moon during the year, and is used in the calculation of the date of Easter . Each Gregorian year has either 365 or 366 days (the leap day being inserted as 29 February), amounting to an average Gregorian year of 365.2425 days (compared to a solar year of 365.2422 days). The Gregorian calendar

6762-409: The early 17th century. Initially an observation device, developments over the next half century transformed it into an accurate measurement tool. The pendulum clock was patented by Christiaan Huygens in 1657 and gave an increase in accuracy of about 30 fold over previous mechanical clocks. These two inventions would revolutionise observational astronomy and cartography. On land, the period from

6860-408: The established method for commercial shipping until replaced by GPS in the early 1990s. The main conventional methods for determining longitude are listed below. With one exception (magnetic declination), they all depend on a common principle, which is to determine the time for an event or measurement and to compare it with the time at a different location. Longitude, being up to 180° east or west of

6958-443: The evening twilight begins at sunset and ends at dusk . Both periods of twilight can be divided into civil twilight , nautical twilight , and astronomical twilight . Civil twilight is when the sun is up to 6 degrees below the horizon; nautical when it is up to 12 degrees below, and astronomical when it is up to 18 degrees below. Night is the period in which the sky is dark ; the period between dusk and dawn when no light from

7056-481: The hope that as soon as may be practicable the astronomical and nautical days will be arranged everywhere to begin at midnight. In ancient Egypt the day was reckoned from sunrise to sunrise. Prior to 1926, Turkey had two time systems: Turkish , counting the hours from sunset, and French , counting the hours from midnight. Humans have divided the day in rough periods, which can have cultural implications, and other effects on humans' biological processes. The parts of

7154-460: The interval between two such successive events may be allowed to vary slightly during the year, or it may be averaged into a mean solar day . Other types of calendar may also use a solar day. The Egyptians appear to have been the first to develop a solar calendar, using as a fixed point the annual sunrise reappearance of the Dog Star— Sirius , or Sothis—in the eastern sky, which coincided with

7252-488: The length of the lunar month is not an even fraction of the length of the tropical year , a purely lunar calendar quickly drifts against the seasons, which do not vary much near the equator. It does, however, stay constant with respect to other phenomena, notably tides . An example is the Islamic calendar . Alexander Marshack, in a controversial reading, believed that marks on a bone baton ( c.  25,000 BC ) represented

7350-535: The light Day, and the darkness he called Night. And the evening and the morning were the first day." The Jewish day begins at either sunset or nightfall (when three second- magnitude stars appear). Medieval Europe also 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 (“All Hallows’ Eve”), and

7448-419: The middle of the 19th century when railroads with regularly occurring schedules came into use, with most major countries having adopted them by 1929. As of 2015, throughout the world, 40 such zones are now in use: the central zone, from which all others are defined as offsets, is known as UTC+00 , which uses Coordinated Universal Time (UTC) . The most common convention starts the civil day at midnight : this

7546-424: The moon phase. Consecutive days may be grouped into other periods such as the week. Because the number of days in the tropical year is not a whole number, a solar calendar must have a different number of days in different years. This may be handled, for example, by adding an extra day in leap years . The same applies to months in a lunar calendar and also the number of months in a year in a lunisolar calendar. This

7644-450: The observation of religious feast days. While the Gregorian calendar is itself historically motivated to the calculation of the Easter date , it is now in worldwide secular use as the de facto standard. Alongside the use of the Gregorian calendar for secular matters, there remain several calendars in use for religious purposes. Western Christian liturgical calendars are based on the cycle of

7742-552: The planet travels at different speeds at various positions in its orbit, and thus a solar day is not the same length of time throughout the orbital year. Because the Earth moves along an eccentric orbit around the Sun while the Earth spins on an inclined axis, this period can be up to 7.9 seconds more than (or less than) 24 hours. In recent decades, the average length of a solar day on Earth has been about 86,400.002 seconds (24.000 000 6 hours). There are currently about 365.2421875 solar days in one mean tropical year . Ancient custom has

7840-454: The radius of a circle of latitude. For a sphere of radius a that radius at latitude φ is a cos φ , and the length of a one-degree (or ⁠ π / 180 ⁠ radian ) arc along a circle of latitude is When the Earth is modelled by an ellipsoid this arc length becomes where e , the eccentricity of the ellipsoid, is related to the major and minor axes (the equatorial and polar radii respectively) by An alternative formula

7938-414: The same circle of latitude, measured along that circle of latitude, is slightly more than the shortest ( geodesic ) distance between those points (unless on the equator, where these are equal); the difference is less than 0.6 m (2 ft). A geographical mile is defined to be the length of one minute of arc along the equator (one equatorial minute of longitude) therefore a degree of longitude along

8036-423: The same day. In specific applications, the definition of a day is slightly modified, such as in the SI day (exactly 86,400 seconds ) used for computers and standards keeping , local mean time accounting of the Earth's natural fluctuation of a solar day , and stellar day and sidereal day (using the celestial sphere ) used for astronomy . In most countries outside of the tropics , daylight saving time

8134-428: The sea. See also the equation of time for details on the differences. With the exception of magnetic declination, all proved practicable methods. Developments on land and sea, however, were very different. Several newer methods for navigation, location finding, and the determination of longitude exist. Radio navigation , satellite navigation , and Inertial navigation systems , along with celestial navigation , are

8232-552: The second in Jamaica on 29 February 1504 (fourth voyage). It is assumed that he used astronomical tables for reference. His determinations of longitude showed large errors of 13° and 38° W respectively. Randles (1985) documents longitude measurement by the Portuguese and Spanish between 1514 and 1627 both in the Americas and Asia. Errors ranged from 2° to 25°. The telescope was invented in

8330-473: The seconds are specified with a decimal fraction . An alternative representation uses degrees and minutes, and parts of a minute are expressed in decimal notation, thus: 23° 27.5′ E. Degrees may also be expressed as a decimal fraction: 23.45833° E. For calculations, the angular measure may be converted to radians , so longitude may also be expressed in this manner as a signed fraction of π ( pi ), or an unsigned fraction of 2 π . For calculations,

8428-423: The start of evening. This period of time sees human's highest body temperature , an increase of traffic collisions , and a decrease of productivity . Evening begins around 5 or 6 pm, or when the sun sets, and ends when one goes to bed. Twilight is the period before sunrise and after sunset in which there is natural light but no direct sunlight. The morning twilight begins at dawn and ends at sunrise, while

8526-427: The sun is visible. Light pollution during night can impact human and animal life, for example by disrupting sleep. Calendar 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

8624-512: The time of midnight varies between locations, time zones are set up to facilitate the use of a uniform standard time . Other conventions are sometimes used, for example the Jewish religious calendar counts days from sunset to sunset, so the Jewish Sabbath begins at sundown on Friday . In astronomy , a day begins at noon so that observations throughout a single night are recorded as happening on

8722-648: The use of ships were transmitted from Halifax, Nova Scotia , starting in 1907 and from the Eiffel Tower in Paris from 1910. These signals allowed navigators to check and adjust their chronometers frequently. Radio navigation systems came into general use after World War II . The systems all depended on transmissions from fixed navigational beacons. A ship-board receiver calculated the vessel's position from these transmissions. They allowed accurate navigation when poor visibility prevented astronomical observations, and became

8820-414: The vast majority of them track years, months, weeks and days. The seven-day week is practically universal, though its use varies. It has run uninterrupted for millennia. Solar calendars assign a date to each solar day . A day may consist of the period between sunrise and sunset , with a following period of night , or it may be a period between successive events such as two sunsets. The length of

8918-466: The west/east suffix is replaced by a negative sign in the western hemisphere . The international standard convention ( ISO 6709 )—that east is positive—is consistent with a right-handed Cartesian coordinate system , with the North Pole up. A specific longitude may then be combined with a specific latitude (positive in the northern hemisphere ) to give a precise position on the Earth's surface. Confusingly,

9016-448: Was Greece, in 1923. The calendar epoch used by the Gregorian calendar is inherited from the medieval convention established by Dionysius Exiguus and associated with the Julian calendar. The year number is variously given as AD (for Anno Domini ) or CE (for Common Era or Christian Era ). The most important use of pre-modern calendars is keeping track of the liturgical year and

9114-517: 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). The course of the Sun and the Moon are the most salient regularly recurring natural events useful for timekeeping , and in pre-modern societies around the world lunation and the year were most commonly used as time units. Nevertheless,

9212-539: Was introduced in 1582 as a refinement to the Julian calendar , that had been in use throughout the European Middle Ages, amounting to a 0.002% correction in the length of the year. During the Early Modern period, its adoption was mostly limited to Roman Catholic nations, but by the 19th century it had become widely adopted for the sake of convenience in international trade. The last European country to adopt it

9310-429: Was reformed by Julius Caesar in 46 BC. His "Julian" calendar was no longer dependent on the observation of the new moon, but followed an algorithm of introducing a leap day every four years. This created a dissociation of the calendar month from lunation . The Gregorian calendar , introduced in 1582, corrected most of the remaining difference between the Julian calendar and the solar year. The Islamic calendar

9408-662: Was soon in practical use for longitude determination, especially in North America, and over longer and longer distances as the telegraph network expanded, including western Europe with the completion of transatlantic cables. The United States Coast Survey, renamed the United States Coast and Geodetic Survey in 1878, was particularly active in this development, and not just in the United States. The Survey established chains of mapped locations through Central and South America, and

9506-565: Was supported and rewarded with thousands of pounds from the Board of Longitude, but he fought to receive money up to the top reward of £20,000, finally receiving an additional payment in 1773 after the intervention of Parliament. It was some while before either method became widely used in navigation. In the early years, chronometers were very expensive, and the calculations required for lunar distances were still complex and time-consuming. Lunar distances came into general use after 1790. Chronometers had

9604-476: Was the need of a navigator for immediate results. The second was the marine environment. Making accurate observations in an ocean swell is much harder than on land, and pendulum clocks do not work well in these conditions. In response to the problems of navigation, a number of European maritime powers offered prizes for a method to determine longitude at sea. The best-known of these is the Longitude Act passed by

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