Measuring Time
Time
 Measure a day (apparent solar day) from apparent local noon to apparent
local noon
o Sun at zenith at any given location (longitude) on the earth's surface =
apparent local noon
o A.M. (Ante Meridian "before the meridian") before the sun has
reached the zenith at your meridian
o P.M. (Post Meridian "after the meridian") after sun passes the zenith at
your meridian
 Clock time = Mean Solar Time (averaged apparent solar time)
 We use Universal Time (UT)
o Replaced Greenwich Mean Time (GMT)
o Astronomical and Military uses UT1 - kept by the earth's movements
o Worldwide civil time
 Universal Coordinated Time (UTC)
 Calculated by atomic transition of Cesium
 Kept by laboratories around the world
o UT1 and UTC are not allowed to be more than 0.9 seconds apart - add
a leap second about once every year and a half to compensate
Time Zones
History
 Sir Sandford Fleming, a Canadian railway planner and engineer, outlined a
plan for worldwide standard time in the late 1870s
 Development of rapid railway systems and the consequent confusion of
schedules that used scores of different local times
 In 1884, delegates from 27 nations met in Washington, D.C., for the
Meridian Conference and agreed on the current system.
Current System
 Divided the 3600 around the earth by 24 hours in a day = 150 for each 1hour time zone, beginning at the Prime Meridian, designated 00 longitude
 International Date Line
o Placed in the Pacific ocean, far away from populated areas to avoid
confusion
o Place where the date changes when crossed
o Add a day when traveling west across the line
Calendars
Solar Year (Tropical Year)
 Interval between 2 consecutive vernal equinoxes
 365 days, 5 hours, 48 minutes, 46 seconds
 365.24220 mean solar days
Sidereal Year
 Earth completes an orbit (sun is back to same position with respect to the
stars)
 Affected slightly by precession
 365.25636 mean solar days (slightly longer)
Roman Lunar Calendar
 Original calendar contained 10 months of length 29 or 30 days
 Later modified to a 12 month calendar - average length 29.5 days = 354
days in the year
 11 days out of step with the seasons; At the end of 3 years - a month out
of step
Julian Calendar
 46 B.C., Julius Caesar = year to be 365 days in length and to contain 12
months - no longer a Lunar calendar (out of phase with the moon)
 Still ~1/4 day difference between the true year length - given 1 day every 4
years (leap years)
Gregorian Calendar
 Julian year still differs from the true year of 365.242199 days (11 minutes
and 14 seconds each year)
 By 1582 this error had accumulated to 10 days
 Pope Gregory XIII ordered another reform
o 10 days were dropped from the year 1582- October 4, 1582, was
followed by October 15, 1582
o 1 year = 365.24220 days
o 3322 years for the error to accumulate to 1 day
o Years equally divisible by 4 are leap years
o Century years not divisible by 400 were not to be considered leap
years
 1600 = leap year; 1700, 1800, 1900 = not leap years
What is Universal Time?
Universal Time (UT) which is sometimes referred to Greenwich Mean Time (GMT).
Times given in UT are almost always given in terms of a 24-hour clock.
Thus, 14:42 (often written simply 1442) is 2:42 p.m
Sometimes a Z is appended to a time to indicate UT, as in 0935Z.
In astronomical and navigational usage, UT often refers to a specific time called UT1, which is a measure of the
rotation angle of the Earth as observed astronomically
It is affected by small variations in the rotation of the Earth, and can differ slightly from the civil time on the
Greenwich meridian
the most common civil usage, UT refers to a time scale called "Coordinated Universal Time" (UTC) = basis for
the worldwide system of civil time
kept by time laboratories around the world
The International Bureau of Weights and provides the international standard UTC which is accurate to
approximately a nanosecond (billionth of a second) per day
The length of a UTC second is defined in terms of an atomic transition of the element cesium under specific
conditions - not directly related to any astronomical phenomena.
UTC is the time distributed by standard radio stations that broadcast time
can also be obtained readily from the Global Positioning System (GPS) satellites. The difference between UTC and
UT1 is made available electronically and broadcast so that navigators can obtain UT1
UTC is the basis for civil standard time in the U.S. and its territories
One can think of UT1 as being a time determined by the rotation of the Earth, over which we have no control,
whereas UTC is a human invention (the only "clock" keeping UT1 precisely is the Earth itself)
by international agreement, UTC is not permitted to differ from UT1 by more than 0.9 second.
When it appears that the difference between the two kinds of time may approach this limit, a one-second change
called a "leap second" is introduced into UTC (occurs on average about once every year to a year and a half)
The History of Time Zones
standard time was adopted in the late 19th century
development of rapid railway systems and the consequent confusion of schedules that used scores of different local times
Sir Sandford Fleming, a Canadian railway planner and engineer, outlined a plan for worldwide standard time in the late 1870s.
1884 delegates from 27 nations met in Washington, D.C., for the Meridian Conference and agreed on current system.
The present system employs 24 standard meridians of longitude (lines running from the North Pole to the
South, at right angles to the Equator) 15º apart, starting with the prime meridian through Greenwich,
England.
China - single time zone
Calendars
the monthly motion of the Moon (Lunar calendars) traditional Jewish and Chinese calendars
Lunar calendars require elaborate adjustments
yearly motion of the Sun (Solar Calendars) - cultures keeping track of the seasons
The Roman Lunar Calendar
Our Gregorian Calendar) is a basically solar calendar that grew from what was originally a Lunar calendar used by the
Romans.
The original calendar contained 10 months of length 29 or 30 days
This was later modified to a 12 month calendar - average length 29.5 days = 354 days in the year,
orbital period of the Earth is 365.242199 days - 11 days out of step with the seasons - at the end of 3 years it was almost a
month out of step.
The Julian Calendar
In 46 B.C., Julius Caesar = year to be 365 days in length and to contain 12 months
days added to months to bring the total from 354 up to 365 days months out of phase with the cycles of the Moon - no longer a Lunar calendar
still about a quarter day difference between the true length of the year and the 365 days
February was given an additional day every 4 years (leap years) and the average length of the Julian year with leap years
added was 365.25 days.
The Gregorian Calendar
Julian year still differs from the true year of 365.242199 days ( 11 minutes and 14 seconds each year)
128 years even the Julian Calendar was in error by one day with respect to the seasons
By 1582 this error had accumulated to 10 days
Pope Gregory XIII ordered another reform: 10 days were dropped from the year 1582- October 4, 1582, was followed by
October 15, 1582
Gregorian Calendar it was decreed that century years not divisible by 400 were not to be considered leap years. ( 1600
was a leap year but 1700 was not)
3322 years for the error to accumulate to 1 day.
A further modification to the Gregorian Calendar has been suggested: years evenly divisible by 4000 are not leap years
error between the Gregorian Calendar Year and the true year to 1 day in 20,000 years