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history:h_time1

History of Measuring Time

Astronomical measures:

  • lunar month
  • solar year = 365 days 5 hours 48min 46secs

 

Calendars:

Overview:

  • all early calendars except the Egyptian were based on the phases of the moon hence the word calendar which is derived from the latin kalendae which was the 1st day of each month on which the appearance of the new moon was proclaimed.
  • Egyptian calendar (3000-3500BC):
  • Egyptians devised a surprisingly accurate calendar based on astronomical calculations and the 3 seasons of the Nile river (flooding, sowing & harvest)
  • in the 3rd millenium, a year of 365 days was introduced which had 12 months of 30 days each, with the remaining 5 days set aside as a special period at the start of each year.

Babylonian star calendar (7thC BC):

  • astronomers noted that the rising of stars had a cycle of 360 days not the 365 days of the seasonal year
  • the star constellations along the ecliptic were named to form the zodiac signs with their astrological significances & which were translated into Greek then into Latin to give us our current names of these constellations.
  • Babylonian ideas reached India by the 6thC BC

Sumerians (2500BC):

  • developed a numbering system based on 60 giving rise to time and angle units of seconds and minutes.
  • Babylonian calendar (earlier than 1500BC):
    • 12 lunar months of 29 or 30 days, each beginning on the evening when the crescent of the new moon was first visible. The day started in the evening at sunset (this practice still survives in Judaism & Islam).
    • the names of the months depended on the region, but those from Nippur (southeast of Baghdad in Iraq) came to dominate, first in Babylonia, then in Assyria. These names are reflected in the Jewish calendar, because of the Babylonian annexation of Jerusalem in 586BC.
    • the series began at the Spring equinox with an extra month being added (“intercalated”) at irregular intervals to bring the lunar calendar into line with the seasons. From 2000BC, only the 6th & 12th months were repeated to produce this intercalation. The decision to repeat a month came from King Hammurabi of Babylon (1848-06 BC).
    • from ~500BC, a consistent cycle of seven intercalations of an extra month over 19 yrs (ie. 7 of the years had 13mths) was used as 19 solar years of 6939.60 days = 235 lunar months = 6939.69 days = “Metonic cycle”, presumably named after the Greek astronomer in the 5thC BC, although the cycle was known by the Babylonians before him.

early Roman calendar:

  • 12 months of alternating 29 & 30 days forming an “empty” year of 354days, thus every 3 years, an extra month (Mercedinus) was inserted to give a “full” year to try to realign the calendar with the sun.
  • year 0 was from the founding of Rome in 753BC, with subsequent years denoted A.U.C. (“ab urbe condita” - from the foundation of the city) thus 753AUC = 1 BC.
  • new year: 1 March until 153BC when it became 1 Jan
  • months: Ianuarius, Februarius, Martius, Aprilis, Maius, Iunius, Quintilis, Sextilis, September, October, November, December;
  • the months were divided into parts to represent phases of the moon: day one: kalendae; day 5 or 7 depending on month: nonae; day 13 or 15: idus (idus is the Greek for full moon - hence March 15 is the ides of March) 
  • days were named relative to the divisions of the month ie. Jan 2 = ante diem IV non. Jan = the 4th day before the Nones of Jan.

early Greek calendar:

  • year 0 was from the Olympic Register which listed the victors of the games since beginning in 776BC.
  • adopted the zodiac and star calendars from the Babylonians
  • the Greeks used star calendars such as Hesiod's calendar in 750-700BC and the parapegma in the late 5thC BC
  • a political calendar in Athens in 420BC suggests a solar basis with 366 days in a year of 6 months of 37 days and 4 months of 36 days

Julian calendar:

  • Julius Caesar revised the early Roman calendar & adopted a solar year of 365days and 6 hours, thereby creating a year of 365 days consisting of alternating months of 31 and 30 days excepting Feb which had 29 days in ordinary years. Every 4th year (including 45BC) would be a “leap” year with Feb. having 30 days.
  • This was implemented on 1 Jan 45 BC, with the year 46BC being a year of confusion having 445 days to bring the vernal equinox back to 25 March
  • Caesar died in 44BC so the 7th month (Quintilis) was named after him as this was his month of birth.
  • Unfortunately, Feb having 30 days in leap years created confusion, so Emperor Augustus changed Feb to 28days and 29 in a leap year, and making the 8th month (Sextilis) having 31days, which he named after himself.
  • Augustus also changed the 8 day market-oriented week to a 7 day week which reflected a variety of influences, including Mesopotamian & Greek planetary astronomy, and the Judaic concept of the Sabbath;
  • each hour of the 24 hour day was ruled by a Roman God in sequence of Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon.
  • each day took its name from the ruler of the 1st hour of that day, thus we have Saturn, Sun, Moon, Mars, Mercury, Jupiter, Venus
  • Christian calendar events:
    • Easter:
      • celebrates the death & resurrection (rebirth) of Jesus, which according to the main Roman calendar, he was crucified on 25 Mar, hence it falls at the start of the Nth Hemisphere Spring (1st Sunday after the full moon which occurs after the vernal equinox - 21 Mar) - this was fixed by the the Council of Nicaea in AD 325 to create uniformity.
      • Dionysius Exiguus in the 6thC AD developed Easter tables based on the 532yr cycle (19 Metonic years x 7 days in a week x 4 year cycle of Julian calendar which brought into alignment a Sunday, a full moon, and a solar date - March21st) developed 75yrs earlier by Victorius, Bishop of Acquataine, and in so doing, set the year of Christ's birth to 753AUC, although recent evidence suggests it probably was 3yrs earlier (ie. 4BC not 1BC).
    • Christmas:
      • derived from the Old English word Cristes maesse meaning Christ's mass, 1st documented usage dates back to 1123AD
      • celebrates the birth of Jesus, but as no-one knows when this was, the date actually derives from Pagan pre-Christian festivities, hence:
        • the Roman emperor Aurelius introduced the saturnalis and natalis invicti solis celebrating the renewed power of the sun at the Winter solstice on 25 Dec in 274AD
        • early Christianity:
          • variably celebrated in Dec, Jan or Mar
          • in Rome in 336 AD made 25 Dec one of its most important holy days - presumably believing Jesus was conceived on 25 Mar and being perfect, was born exactly 9months later ie. 25 Dec
          • observed in Constaninople by 380 AD
          • observed in Jerusalem in 5thC AD
        • England: 25 Dec
        • Armenia: 6 Jan
    • AD (anno domino - “in the year of our Lord”) was not in regular use until the English monk, the Venerable Bede adopted it in the 8thC AD
    • BC (“before Christ”) was not in regular use until the 17thC AD
  • New Years Day:
    • In England, new year started on 25 Dec until 14thC when it changed to start on 25 Mar, until 1752 when it changed again to start on 1 Jan.
    • In Scotland, new year started on 1 Jan since 1600, but for some tax purposes it is still said to start on 25 Mar or 5 April
    • NB. for the Celts 2000yrs ago, New Year began on 1 Nov when they celebrated the previous evening, the festival of Samhain, the Celtic lord of death & marked the beginning of the season of cold, darkness & decay. The Celts believed that Samhain allowed the souls of the dead to return to their homes for this evening. The Druids built a huge bonfire of oak branches which they considered sacred and burned animals & crops as sacrifices. Thence developed Halloween which is celebrated on 31 Oct! Pope Boniface IV in an effort to supplant this festival, changed “All Saints Day” from 13 May to 1 Nov in the 800's. In 998, Odile, the Abbot of Cluny established 2 Nov as “All Souls Day” in an effort to purify by prayer the souls of the dead not yet sufficeiently purified.

Gregorian calendar:

  • unfortunately, the Julian year was too long by 11min and 14secs, thus by the 16th century, it was out of synchrony with the solar year by 10days, hence Pope Gregory XIII in 1582 inaugurated his Gregorian or New Style calendar to correct this error
  • ie. the Spring Equinox according to Julian calendar: Mar 20, 300AD; Mar 17 700AD; Mar 14 1100AD; Mar 11 1500AD. 
  • he dropped 10 days from the calendar that year and decreed that there would not be a leap year 3 times in every 400yrs (ie. the years ending in 2 noughts, unless they were divisible by 400)
  • thus this calendar and the sun will remain in sync until the year 5000 when there will be a difference of one day.
  • unfortunately for historians & genealogists, not everyone adopted this calendar at the same time:
    • Roman catholic countries (1582)
    • Protestant American colonies & Britain (1752)
    • Russia (1918) - Julian 31 Jan became Gregorian 14 Feb
    • other Orthodox countries (1923)
  • it is now the standard calendar used throughout most of the world
  • Perpetual calendar based on Gregorian calendar:
    • day of week = mod7 {D+[26(N+1)/10]+Z+[Z/4]+[J/4]-2J-1}
      • where:
        • day of week: Sunday = 0,7 Monday = 1, etc
        • mod7 is modulus to base 7 ie. fraction of 7 left after dividing by 7
          • eg. {113}mod7 = 113/7 = 16 and 1/7th thus = 1 ie. Monday
        • [ ] = greatest integer function (ie. truncate decimal fraction NOT round)
        • D = day of month
        • N = month of year where Mar = 3 but Jan, Feb are 13 & 14 of previous year
        • Z = last 2 digits of the year
        • J = [year/100] ie. if April 1978 then J = 19, Z = 78
      • example:
        • 14/12/1981 ⇒ D = 14, N = 12, J = 19, Z = 81
        • DOW = mod7 {14+[26×13/10]+81+[81/4]+[19/4]-38-1
        • DOW = mod7 {14+[33.8]+81+[20.25]+[4.75]-38-1
        • DOW = mod7 {14 + 33 + 81 + 20 + 4 - 38 - 1}
        • DOW = mod7 {113} = 113/7 = 16 and 1/7th = 1 ⇒ DOW = Monday.

Jewish calendar:

  • old calendar:
    • start of each month must be determined by observation of the lunar crescent
  • new calendar:
    • started in 4thC AD, based on Metonic cycle of 19 solar years (~235 lunar months)
    • the day begins at sunset on the evening before the calendar day or at 6pm
    • each day has 24hours with each hour having 1080 halaquim
    • New Year (Rosh Hashanah or 1 Tishri) falls between 5 Sep & 5 Oct and is followed on 10 Tishri by Yom Kippur, the Day of Attonement

Islamic calendar:

  • lunar calendar based on a “common” year of 354 days with 12months of alternating 29 & 30days (a lunar month is ~29.5 days), and a kabishah year with an extra day on the last month.
  • unlike the Jewish calendar, there are no intercalated months to keep it in cycle with seasons, hence the Islamic New Year which falls on the 1st day of the Islamic month of Muharram, will occur on a different part of the seasonal year as a lunar year of 12 months is only 354 days, while 13 months would be 18 days longer than a solar year.
  • there are 19 common years and 11 kabishah years in each cycle of 30yrs
  • year 0 is from Hejira or Hegira, the day Mohammed fled Mecca in 622AD
  • the day begins at sunset on the evening before the calendar day
  • start of each month must be determined by observation of the lunar crescent
  • the 9th month is Ramadan in which Muslims fast
  • Islamic months:
    • Muharram (30days); Safar (29days); Rabi I; Rabi II; Jumada I; Jumada II; Rajab; Sha'ban; Ramadan; Shawwal; Dhu al-Qa'da; Dhu al-Hijja;

Chinese calendar:

  • Although the People's Republic of China uses the Gregorian calendar for civil purposes, a special Chinese calendar is used for determining festivals. 
  • The beginnings of the Chinese calendar can be traced back to the 14th century BC. Legend has it that the Emperor Huangdi invented the calendar in 2637 BC.
  • The Chinese calendar is based on exact astronomical observations of the longitude of the sun and the phases of the moon. This means that principles of modern science have had an impact on the Chinese calendar.
  • The Chinese calendar - like the Hebrew - is a combined solar/lunar calendar in that it strives to have its years coincide with the tropical year and its months coincide with the synodic months. It is not surprising that a few similarities exist between the Chinese and the Hebrew calendar:
    • An ordinary year has 12 months, a leap year has 13 months.
    • An ordinary year has 353, 354, or 355 days, a leap year has 383, 384, or 385 days.
  • When determining what a Chinese year looks like, one must make a number of astronomical calculations:
    • First, determine the dates for the new moons. Here, a new moon is the completely ``black'' moon (that is, when the moon is in conjunction with the sun), not the first visible crescent used in the Islamic and Hebrew calendars. The date of a new moon is the first day of a new month.
    • Secondly, determine the dates when the sun's longitude is a multiple of 30 degrees. (The sun's longitude is 0 at Vernal Equinox, 90 at Summer Solstice, 180 at Autumnal Equinox, and 270 at Winter Solstice.) These dates are called the Principal Terms and are used to determine the number of each month:
      • Principal Term 1 occurs when the sun's longitude is 330 degrees.
      • Principal Term 2 occurs when the sun's longitude is 0 degrees.
      • Principal Term 3 occurs when the sun's longitude is 30 degrees, etc
      • Principal Term 11 occurs when the sun's longitude is 270 degrees.
      • Principal Term 12 occurs when the sun's longitude is 300 degrees.
    • Each month carries the number of the Principal Term that occurs in that month.
    • In rare cases, a month may contain two Principal Terms; in this case the months numbers may have to be shifted. Principal Term 11 (Winter Solstice) must always fall in the 11th month.
    • All the astronomical calculations are carried out for the meridian 120 degrees east of Greenwich. This roughly corresponds to the east coast of China.
    • Some variations in these rules are seen in various Chinese communities.
    • Unlike most other calendars, the Chinese calendar does not count years in an infinite sequence. Instead years have names that are repeated every 60 years. Historically, years used to be counted since the accession of an emperor, but this was abolished after the 1911 revolution. This way of naming years within a 60-year cycle goes back approximately 2000 years. A similar naming of days and months has fallen into disuse, but the date name is still listed in calendars. It is customary to number the 60-year cycles since 2637 BC, when the calendar was supposedly invented. In that year the first 60-year cycle started.
    • Within each 60-year cycle, each year is assigned name consisting of two components:
      • The first component is a Celestial Stem:
        • 1: jia 2: yi 3: bing 4: ding 5: wu 6: ji 7: geng 8: xin 9: ren 10: gui
      • The second component is a Terrestrial Branch:
        • 1: rat; 2: ox; 3: tiger; 4: rabbit; 5. dragon; 6: snake 7: horse 8: sheep 9: monkey 10: rooster 11:dog 12: pig
    • The current 60-year cycle started on 2 Feb 1984. That date bears the name bing-yin in the 60-day cycle, and the first month of that first year bears the name gui-chou in the 60-month cycle.
    • This means that the year yi-you (rooster), the 22nd year in the 78th cycle, will start on 9 Feb 2005.
    • see Helmer Aslaksen's web site at http://www.chinesecalendar.net for more information.
    •  

Ethiopian calendar:

  • New Year starts in Spring on Sept 11th ?
  • Enkutatash is an important festival in the lives of Ethiopians. After three months of heavy rains the sun comes out creating a beautiful clear fresh atmosphere. The highland fields turn to gold as the Meskal daisies burst into flower. When Makeda the Queen of Sheba, returned to Ethiopia after her famous visit to King Solomon, her chiefs welcomed her forward by giving her “enku” or jewels. Enkutatash which means “gift of jewels”' has been celebrated ever since in spring. Meskerem is seen as a month of transition from the old year to the new. It is a time to express hopes and dreams for the future.

Days of the week:

  • Day Latin origin French Old English Pagan origin Sunday the Sun dimanche sunnan dæg Sun Monday Lunar lunedi mondæg Moon Tuesday Mars mardi tiwesgæg Tiw the ancient Teutonic deity Wednesday Mercury mercredi Wodnesdæg Woden (Odin the chief Norse god) Thursday Jove (Jupiter) jeudi fusion of Thunres dæg (Thunor) and Thorsdagr (Thor) Friday Venus vendredi Frigedæg Frigg (wife of Odin & goddess of marriage) Saturday Saturn samedi Sæterdæg, Sæternesdæg

Clocks:

  • Sundials:
    • Babylonians in 3500BC used a vertical stick called a gnomon which cast a shadow
    • this was eventually refined into a sundial & used by Egyptians in 1500BC who divided the daylight time into 12 segments which was probably originated from the Babylonians who regarded 12 as a mystical number because it had the great value of being divisable by 1,2,3,4 and 6.
    • the Greeks used their mathematical skills to give sundials great seasonal precision
  • Water clocks:
    • these were created to measure time at night and again were usually divided into 12 divisions
    • generally allowed a water container to empty at a constant rate
    • an alabaster water clock held in the Cairo museum dates back to 1380BC
    • the Romans gave the water clock far more accuracy
    • the Chinese in 8thCAD built water clocks with miniature waterwheels & devices that sounded gongs every hour
  • Other early clocks:
    • sandglasses
    • candle timers
    • oil lamp timers
  • Mechanical clocks:
    • replaced all other clocks when developed in the early 1300s
    • initially were large and driven by weights & used in monastries to warn a time-keeping monk to ring his bells
    • by 1335 there was a clock tolling the hours in Milan
    • based on escapement principle known as verge & foliot
    • by 1400, this technology had been scaled down enabling domestic clocks
    • by 1450, spring-driven clocks were developed but these were very inaccurate due to slowing down as the spring expanded, until in 1500 Henlein devised a compensation device known as a stackfeed, he created the 1st portable clock or watch, however, the clocks still lost as much as a 30min each day.
    • in 1641, Galileo proposed a pendulum clock but died before it was produced, leaving Huygens to produce the 1st commercially viable model in 1656, this heralded a boom in clock innovation & manufacture resulting in wooden wall clocks, grandfather clocks, the inclusion of the minute hand, the use of brass instead of iron, and the invention by Huygens of the balance or spiral spring.
    • in 1660, Robert Hooke devised the recoil or anchor escapement that allowed a reduction in arc of the pendulum
    • in 1715, George Graham devised the deadbeat escapement that ensured accuracy up to a few seconds every day
    • in 1721, George Graham attached a bowl of mercury to the pendulum to overcome the error caused by rise in temperature causing brass pendulum rod to expand (5 degree rise resulted in losing 5secs/day)
    • in 1765, Thomas Mudge devised the detached lever escapement which eventually became the standard escapement device & is still used in clocks & watches.
    • in 1840's, the harnessing of electricity resulted in the electric clock where electricity was used to replace the spring or pendulum & could also be used to relay the time from one clock to another in a series which was useful in factories where synchronisation was important.
    • in 1895, Charles Guillaume solved the temperature problem by using an alloy of nickel & steel called invar
    • in 1918, synchronous electric clocks became popular with home models made, but accuracy depended on the frequency of the power supply which was a problem
  • Quartz crystal clocks:
    • in 1929, quartz crystal which oscillates at 100,000Hz was used as timing device & in theory would be accurate to 1sec every 10yrs
  • Atomic clocks
    • clocks that were timed by the oscillations of an atom were developed in 1950s with the most sophisticated one being a cesium clock, invented in 1952 which would lose 1 sec every 1000years. This re-defined how long a second was as an atomic second (time required for 9,192,631,770 cycles of the Cesium atom at zero magnetic field) being equal to an average second of earth rotation time in 1900. 
    • this required us to introduce leap seconds to ensure the atomic second kept in line with the changing rotation period of the earth (see Science World or US Navy or below under UT) which is partly contributed to by tidal friction.
  • Digital clocks
    • although invented in the 1950s, these clocks did not become popular until the 1970s when its impact virtually destroyed the traditional clock & watch repair industry & made all timepieces cheap & accurate.

Global time:

Universal Time (UT):

  • since 1880, the legal definition of time in Britain has been set according to the time in Greenwich, England where the Royal Observatory stood (it has now been moved 20km away), this is called Greenwich Mean Time (GMT) or Universal Time (UT)
  • Universal time (UT) is simply the number of hours, minutes, and seconds which have elapsed since midnight (when the Sun is at a longitude of 180°) in the Greenwich time zone.
  • Since the Earth's rotation is irregular at the 0.1 second level, a local approximation to universal time not corrected for polar motion is often used. This is called UT0, and also referred to as Greenwich mean time, abbreviated GMT. In UT0, 24 universal hours are defined to be a mean solar day.
  • The actual universal time (denoted UT or UT1) is tied to the rotation of the Earth. Because the Earth's rotation rate is rather irregular and unpredictable at the 0.1 s level, Universal Time can only be deduced from observations of star transits. Once known, UT can be compared with known ephemeris time, and the difference can be derived (the value for this in 2003 is 68secs and increments by ~1sec per year). UT is always kept within 0.9 seconds of coordinated universal time by the insertion or deletion of leap seconds, usually at 23:59:59 UTC on either June 30 or December 31

coordinated universal time (UTC):

  • differs from international atomic time by an integral number of seconds and is the basis of most radio time systems and legal time systems. The step adjustments (leap seconds) are usually inserted after the 60th second of the last minute of December 31 or June 30. UTC is the time standard provided by WWV and other time broadcast services.

local time zones:

  • other places in the world have their own time zones which are related to GMT according to their longitude, with each degree of longitude equivalent to 111.32km and every 15 degrees creating a new time zone 1 hour different to the previous (15deg x 24hours = 360 degrees = 1 day rotation of the earth) eg. AEST is UT + 10hrs
  • these time zones are not adhered to strictly due to geographical & political boundaries, and also Summer Daylight times in various places
  • at 180deg to Greenwich is the International Date Line where crossing this meridian from east to west results in traveller jumping to the next day. This line was designated such in 1894 and is regarded as the beginning of each day.
  • NB. at the poles where all the meridians & thus time zones meet, the time is taken as being GMT.
  • NB. astronauts in orbit are technically subject to the International Date Line system, but in practice keep to the same time as their ground control station.

sidereal time:

  • Time measured relative to the “fixed stars” instead of the sun.
  • Sidereal time is useful because the offset of sidereal time from the right ascension of a celestial object, known as the hour angle, gives the time before or after transit of the star across the meridian line of the sky. 
    • hour angle:
      • The angle between an observer's meridian  and the hour circle on which some celestial body lies.
      • This angle is conventionally expressed in units of time (hours, minutes, and seconds), which gives the time elapsed since the celestial body's last transit at the observer's meridian (for a positive hour angle), or the time unit the next transit (for a negative hour angle).
    • meridian:
      • an imaginary circle in the local sky passing between the celestial poles and the zenith
    • zenith:
      • the highest point in the local sky
    • celestial poles:
      • the north and south points in the sky around which the celestial bodies appear to rotate
    • hour circle:
      • an imaginary circle passing through the celestial poles in a similar way to the longitude line on the map of earth.
      • each hour circle is 15degrees of arc away from the next one (ie. there are 24 hour circles in the celestial globe)
    • right ascension:
      • similar to the hour circle, but having a zero point of right ascension is the first point in Aries, just as the zero point for longitude on the Earth is the prime meridian at Greenwich.
      • thus any given star except the sun has an almost constant right ascension value but a constantly changing hour angle depending on the time of day and time of year and local position of the observer (ie. the local sidereal time) as the earth rotates on its axis and also revolves around the sun.
      • Right ascension is usually measured in units of time (hours, minutes, and seconds), with one hour of time approximately equal to 15° of arc (360°/24 hours=15°/hour).
    • declination:
      • the “latitude” of a celestial object from the celestial equator & is independent of the observer's location on earth
    • culmination:
      • the time at which a celestial body transits the meridian.

local sidereal time (LST):

  • The most useful form of sidereal time, it gives the right ascension of a transiting celestial object at a given location.
  • = sidereal time + (1.0027379093 x UT) - (longitude degrees West / 15)

ephemeris time (ET):

  • A uniform time measure now kept by atomic clocks. Ephemeris Time (ET) was used in the Astronomical Almanac from 1960-1983, but was replaced by barycentric dynamical time when the IAU 1976 System of Astronomical Constants was implemented in the Astronomical Almanac in 1984.

barycentric dynamical time (TDB):

  • Dynamical time for barycentric phenomena (TDB) which replaced ephemeris time when the IAU 1976 System of Astronomical Constants was implemented in the Astronomical Almanac in 1984. The difference between terrestrial dynamical time and TDB is due to variations in the gravitational potential around the Earth's orbit combined with velocity terms, and is always less than 2 milliseconds. TDB is used as a time scale of ephemerides referred to the barycenter of the solar system.

international atomic time (TAI):

  • is measured in the SI second, defined in terms of vibrations of a cesium atom. It is therefore not explicitly tied to the earth's rotation, although that was of course the motivation for the original definition of the second.

terrestrial dynamical time (TDT):

  •  is dynamical time for geocentric phenomena which replaced Ephemeris Time when the IAU 1976 System of Astronomical Constants was implemented in the Astronomical Almanac in 1984. TDT is independent of the variable rotation of the Earth, and the lengths of the tropical year and synodic month are generally defined in days of 86,400 seconds of international atomic time. TDT is used as the time scale of ephemerides for observations from earth's surface, and differs from international atomic time (TAI) by an offset TDT = TAI + 32.184 s,  which varies slowly with time
  •  

=====Getting accurate time in Australia & NZ:=====  

  • Astronomical and Geophysical observations are typically reported with reference to   Coordinated Universal Time  (UTC)  - see this linked page.
  • For astronomical observers in New Zealand and Australia, please also refer to the web site of the Royal Astronomical Society of New Zealand Occultation Section.
  • SOURCES OF TIME  -  Direct Links to a  Primary UTC  Time Standard:
  • CONVENIENT PORTABLE TRANSFER CLOCKS  -  For temporary use: To carry 'time' between a primary source and a remote field site, or a group of multiple observing stations, we may also use a  (secondary)  Local Clock  like a stopwatch or bleeper, which is first synchronised with a primary time source, and checked again after the observing run.
  • WHAT TIME FORMAT AND ACCURACY DO YOU NEED ?
    CONSIDER YOUR REQUIREMENTS:
    • How will an observed "Event" be related to a "Standard Time" system?
    • Do you need a time reference for a  “Single Spot Check”?  Such as 'setting' a stopwatch or observing one single well defined event?
    • Or do you need a recordable  “Continuous Time Stream for ~ 30 Minutes”?   For example for an asteroidal occultation or a grazing lunar occultation?
    • For Video recording one option is a  Video Time Inserter
    • Accuracy:  for visual observations a time reference accurate to 0.1 second (100 ms) is generally OK.  For instrumental timing (video or photometer), we really need 10 ms or 1 ms.
history/h_time1.txt · Last modified: 2013/01/14 21:31 by gary1