Transcript Chapter 15

Chapter 15
Place and Time
Sections 15.1-15.6
Place & Time
• In Physical Science events occur at
different places and at different times.
• Another way to say it – events are
separated by space and time.
• Our five senses make it possible to
know about objects and their positions
relative to one another.
• Time is a bit more evasive – we relate
time to changes we observe in our
environment.
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Intro
15 | 2
One Dimensional Location
• Location requires a reference system with
one or more dimensions.
• A one-dimensional system is shown below.
• A straight line  (+)infinity to (-)infinity
• Origin and units of length must be indicated.
• Examples include temperature scales,
left/right, above SL/below SL, profit/loss.
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Section 15.1
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Cartesian Coordinates
• A two-dimensional system is one in which two
lines are drawn perpendicular with an origin
assigned at the point of intersection.
Horizontal line = x-axis
Vertical line = y-axis
• The system we commonly use is the
Cartesian coordinate system, named after the
French philosopher/mathematician René
Descartes (1596-1650).
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Section 15.1
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Cartesian Coordinates – Two Dimensional
• x number gives the distance from the y-axis.
• y number gives the distance from the x-axis.
• Many cities are laid out in a Cartesian pattern
with streets running N-S & E-W.
We want to be able to
determine locations on
earth and in space.
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Section 15.1
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Latitude and Longitude
• Location on earth is established by means of a
coordinate system – latitude & longitude
• Since the earth turns on axis, we can use the
geographic poles as north-south reference points.
• Geographic poles – the imaginary points on the
surface of the earth where the earth’s axis projects
from the sphere
• Equator – an imaginary line circling the earth’s
surface half way between the N & S poles
– The equator is a “great circle” – a circle on the surface of
earth in a plane that passes through the center.
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Section 15.2
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The Equator
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Section 15.2
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Latitude
• Latitude - the angular
measurement in
degrees north and
south of the equator
• The latitude angle is
measured from the
center of the earth
relative to the equator.
• Lines of equal latitude
are circles drawn on
the surface and parallel
to the equator.
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Section 15.2
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Latitude
• Lines of latitude are also called parallels –
– There are an infinite number of parallels between
0o and 90o N or S (of equator)
• Going from the Equator  poles these
parallels represent a series of complete
circles of which the equator is the largest and
they become progressively smaller going
North and South
• Only the equator is a “great circle.” All of the
other parallels are “small circles,” with the
North & South poles being points .
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Section 15.2
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Longitude
• Longitude - imaginary lines drawn on
the surface of the earth running from N
to S poles and perpendicular to the
equator
• Lines of longitude are also called
meridians.
• Meridians are half circles that are
portions of “great circles.”
• An infinite number of lines can be drawn
as meridians.
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Section 15.2
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Longitude
• Longitude is the
angular measurement,
in degrees, east or
west of the reference
meridian, the Prime
Meridian (0o) at
Greenwich, England.
– A large optical
telescope was located
there.
• Maximum value of
180o E or W
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Section 15.2
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Greenwich Observatory
• © Ron Hann All Rights Reserved
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Diagram
Showing
Latitude and
Longitude of
Washington,
D.C.
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Section 15.2
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Great Circle Distance
• The shortest surface distance between
any two points on earth is the great
circle distance. The Earth is not flat!!!
• A great circle is any circle on the
surface of a sphere whose center is the
center of the sphere.
• Nautical mile (n mi) – one minute of arc
of a great circle
• 1n mi = 1.15 mi
• 60 nautical miles = 1o
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Section 15.2
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Determining the Distance Between Two
Places - Example
• Determine the number of nautical miles
between place A (10oS, 90oW) and place B
(70oN, 90oE) Notice that we are going over
the North Pole to get there.
• How many degrees are between
points A & B?
• 10o + 90o + 30o = 130o
• 60 n mi = 1o
• 130o x 60 n mi/1o =
• 7800 n mi
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Section 15.2
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Maps
• Generally maps are designed for some type
of “navigation.”
• Since the earth is nearly a sphere (3-D) and
most maps are flat (2-D) they are necessarily
‘projections.’
• The places on a map are shown relative to
each other, and the fundamental frame of
reference is the lines of latitude and
longitude.
• Most with “north” at the top
• Scales are provided to determine distance.
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Section 15.2
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Time
• Time - the continuous forward flowing of
events
• The continuous measurement of time
requires the periodic movement of some
object as a reference.
• The second has been adopted as the
international unit of time.
• Vibration of the cesium-133 atom now
provides the reference of a second –
9,192,631,770 cycles per second
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Section 15.3
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Days
• Apparent Solar Day – the elapsed time
between two successive crossings of the
same meridian (line of longitude) by the
sun (361o)
• Sidereal Day – the elapsed time between
two successive crossings of the same
meridian by a star other than the sun
(360o)
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Section 15.3
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Solar Day vs. Sidereal Day
• The earth must rotate through 360o plus 0.985o to
complete one rotation w/ respect to the sun. The
Solar Day is 4 min. longer than the Sidereal Day.
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Section 15.3
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Days
• During one complete revolution (orbit)
around the sun, the earth rotates (spins)
365.25 times but one complete
revolution is only 360o.
• Therefore during each full rotation the
earth moves slightly less than 1o of
angular distance.
• 360o/365.25 days = 0.985o/day
• 360o/24hr  15o/hr  0.985o/4 minutes
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Section 15.3
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Time Measurement
• A 24-hour day begins at midnight and ends
24 hours later at midnight.
• Noon local solar time – when the sun is on
the observer’s meridian
• Ante meridiem (A.M.) – the hours before noon
• Post meridiem (P.M.) – the hours after noon
• 12 o’clock should be stated as “12 noon” or
“12 midnight.”
• In addition 12 midnight should have the dates
“12 midnight, July 26-27.”
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Section 15.3
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Standard Time Zones
• The earth is divided into 24 time zones,
each containing approx. 15o of longitude
or 1 hour. (Remember that the earth
rotates 15o/hour!)
• The first time zone begins at the prime
meridian and extends approximately
7.5o both east and west.
• The centers of each time zone are
multiples of 15o.
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Section 15.3
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Time Zones of the Conterminous
United States
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Section 15.3
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Losing and Gaining Time
• Traveling west you will “gain” time.
• As you cross into a new time zone, your
watch will be 1 hour ahead of the new time
zone.
• Example: Driving from eastern Kansas (at
noon) into Colorado (now it is only 11 A.M.)
• Driving east you “lose” an hour.
• Therefore if you travel all the way around the
earth going west, you will “gain” 24 hours.
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Section 15.3
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International Date Line
• The International Date Line is located at
the 180o meridian – exactly opposite the
Prime Meridian.
• When one crosses the IDL traveling
west, the date is advanced into the next
day.
• When one crosses the IDL traveling
east, one day is subtracted from the
present date.
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Section 15.3
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Apparent North-South Movement of the Sun
• During a year the sun appears to change its
overhead position from 23.5o N to 23.5o S.
– 23.5o N is the farthest north and 23.5o S is the
farthest south that the vertical noon sun reaches.
• Tropic of Cancer – the parallel at 23.5o N
• Tropic of Capricorn – the parallel at 23.5o S
• As the Earth revolves around the sun, the
noon sun is directly over different latitudes
during the year because of the constant 23.5o
tilt of the Earth to the sun.
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Section 15.4
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Diagrams of
Sun's Position
(Degrees
Latitude) at
Four Different
Times of the
Year
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Section 15.4
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Daylight
• Due to the great distance from the sun,
the light rays incident on earth’s surface
are parallel.
• Therefore, one half of the earth’s
surface will be illuminated (daylight) all
the time and one half will be in darkness
all the time.
• But the number of daylight hours at any
place on earth depends on the latitude
and the day of the year.
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Section 15.5
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Earth’s Tilt
• As the earth revolves around the sun, its axis
remains tilted 23.5o from the vertical.
• This constant tilt of the earth with respect to
the sun causes the earth’s seasons.
• As the earth revolves around the sun we also
designate 4 particular days – Winter solstice,
Vernal equinox, Summer solstice, and
Autumnal equinox .
• Light/dark hours are always the same at the
equator.
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Section 15.5
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Vertical Noon Position of the Sun
• Winter Solstice – 23.5oS = Tropic of
Capricorn
• Vernal Equinox – 0o = Equator
• Summer Solstice – 23.5oN = Tropic of
Cancer
• Autumnal Equinox – 0o = Equator
Solstice – “the Sun stands still”
equinox – “equal night”
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Section 15.5
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Approximate Duration of Daylight Hours
for June 21 & December 21
• Noon is the approximate midpoint of
daylight hours.
• Midnight is the approximate midpoint of
dark hours.
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Section 15.5
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The Year
• When the earth makes one complete
orbit around the sun, we call the
elapsed time is one year.
• More precisely, we can actually define
two different years.
• The Tropical Year & the Sidereal Year.
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Section 15.5
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Two Different Years
• Tropical Year – the time interval from one
vernal equinox to the next vernal equinox –
365.2422 mean solar days
– The elapsed time between 1 northward crossing of
the sun above the equator to the next northward
crossing.
• Sidereal year – the time interval for earth to
make one complete revolution around the
Sun with respect to any particular star other
than the sun – 365.2536 mean solar days
– 20 minutes longer than the tropical year
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Section 15.5
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The Sun’s Overhead Position
• Never greater than 23.5o latitude
• The sun’s position is always due south at 12
noon local solar time, for an observer in the
conterminous U.S., which includes Kansas!
• Solstice – farthest point of the sun from the
equator (“the sun stands still”)
• Summer Solstice – most northern position
– Vertical noon sun at 23.5o N
• Winter Solstice – most southern position
– Vertical noon sun at 23.5o S
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Section 15.5
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The Sun’s Overhead Position
• Therefore the sun’s position overhead
varies from 23.5o north to 23.5o south of
the equator
• When it is directly over the equator,
both the days and nights have 12 hours
all over the world.
• Equinox – sun is directly over the
equator
• Vernal Equinox – March 21
• Autumnal Equinox – September 22
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Section 15.5
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Earth's
Positions,
Relative to
the Sun
and the
Four
Seasons
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Section 15.5
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Seasons
• Seasons affect almost everyone.
• Many of our holidays were originally
celebrated as a commemoration of a certain
season of the year.
–
–
–
–
Easter – coming of spring
Halloween – beginning of winter
Thanksgiving – harvest
Christmas – sun beginning its “journey” north
• Original dates more-or-less set by the
movement of the earth around the sun.
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Section 15.5
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The Calendar
• The measurement of time requires the
periodic movement of some object as a
reference.
• Probably the first unit of measurement was
the “day.”
• The periodic movement of the moon (29.5
solar days) was likely the next time reference.
• Today’s month is based on the moon.
• The Sumerians (3000 B.C.) divided the year
into 12 lunar months of 30 days each.
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Section 15.5
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The Zodiac
• Zodiac – the central, circular section of the celestial
sphere that is divided into 12 sections
• Each section of the zodiac is identified by a
prominent group of stars called a constellation.
– Ancient civilizations name constellations for the the figure
the stars seemed to form.
• Due to the Earth’s annual revolution around the sun,
the appearance of the 12 constellations change
during the course of a year.
– A particular time of the year is marked by the appearance of
a particular constellation.
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Section 15.5
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Signs of
the Zodiac
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Section 15.5
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The Roman Calendar
• The early Roman calendar consisted of only
10 months.
• January and February did not exist but were
the period of waiting for spring to arrive.
• Later January and February were added.
• The Julian Calendar was adopted in 45 B.C.
during the reign of Julius Caesar.
• Augustus Caesar took over the throne after
his adopted father Julius died.
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Section 15.5
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The Roman Calendar
Pre-700 B.C.
~700 B.C.
425 B.C
March
April
May
June
Quintilis
Sextilis
September
October
November
December
January
March
April
May
June
Quintilis
Sextilis
September
October
November
December
February
January
February
March
April
May
June
Quintilis
Sextilis
September
October
November
December
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The Roman Calendar
• The names “July” and “August” were put into
use when Augustus Caesar ruled the empire
in honor of Julius and Augustus.
• In addition one day was added to August so
that it would be as long as July (taken away
from February.)
• Julian calendar had 365 days, and during
every year divisible by 4, an extra day was
added, since it takes approx. 365.25 days for
the earth to orbit the sun.
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Section 15.5
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Gregorian Calendar
• The Julian calendar was fairly accurate and
was used for over 1500 years.
• In 1582 Pope Gregory XIII realized that the
Julian calendar was slightly inaccurate.
– The Vernal Equinox was not falling on March 21.
• A discrepancy was found. To correct this the
Pope decreed that 10 days would be skipped.
• 365.2422 not 365.25 = discrepancy
• Every 400 years 3 leap years would be
skipped.
• This is the calendar we use today.
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Section 15.5
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Gregorian Calendar
• The leap years to be skipped were the
century years not evenly divisible by
400.
For example, the year 1900 was not a
leap year, but 2000 was.
• The corrections make the calendar
accurate to 1 day in 3300 years.
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Seven-Day Week
• Origin unknown
• Perhaps, ¼ of the lunar period,
coinciding with the moon’s change in
phase
• More likely due to the presence of
seven visible celestial objects in the sky
– sun, moon, Mars, Mercury, Jupiter,
Venus, and Saturn
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Section 15.5
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The Days of the Week
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Section 15.5
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Our Calendric Designation
• Notice that we say “Sunday, April 7,
2003.”
• The “Sunday” position falls within a
seven-day count that cycles endlessly.
• The “April 7” position falls within a 365day cycle that also repeats endlessly.
• The “2003” position is one that does not
repeat, but is unique.
– Year is measured from an agreed-upon
starting point – the birth of Christ.
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Section 15.5
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Precession of Earth’s Axis
• When we spin a toy top, it starts to
wobble after a few seconds
• Physicists call this wobble precession.
• Earth slowly precesses in a clockwise
direction.
• The period of precession is 25,800
years. In other words, it takes 25,800
years for the axis to precess through
360o.
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Section 15.6
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Precession of a Top & Earth
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Section 15.6
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Precession of Earth’s Axis
• As the earth precesses, Polaris will not longer
be the “north star.” It will be Vega.
• Precession of earth’s axis does not have an
influence on the seasons, because the
inclination of the earth (with respect to the
sun) will remain constant.
• However the earth’s precession will slowly
change the stars that can be seen in each
hemisphere and season.
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Section 15.6
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