Chapter 15 Place and Time Sections 15.1-15.6. Copyright © Houghton Mifflin Company. All rights reserved.15 | 2 Place & Time In Physical Science events

Chapter 15

Place and Time

Sections 15.1-15.6

Copyright © Houghton Mifflin Company. All rights reserved. 15 | 2

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.

Intro


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.

Section 15.1


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 (1596-1550).

Section 15.1


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.

Section 15.1


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.

Section 15.2


The Equator

Copyright © Bobby H. Bammel. All rights reserved.

Section 15.2


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.

Section 15.2


Latitude

• Latitude is also called parallels –– There is an infinite # -- 0o – 90o N or S

• 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 N & S

• Only the equator is a “great circle.” All of the other parallels are “small circles,” with the N & S poles being only points .

Section 15.2


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.

Section 15.2


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

Section 15.2


Diagram Showing Latitude and Longitude of Washington, D.C.

Section 15.2


Great Circle Distance

• The shortest surface distance between any two points on earth is the great circle distance.

• 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

Section 15.2


Determining the Distance Between Two Places - Example

• Determine the number of nautical miles between place A (10oS, 90oW) and place B (70oN, 90oE)

• How many degrees between points A & B• 10o + 90o + 30o = 130o

• 60 n mi = 1o

• 130o x 60 n mi/1o =• 7800 n mi

Section 15.2


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.

Section 15.2


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

Section 15.3


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)

Section 15.3


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.

Section 15.3


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

Section 15.3


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

• Post meridian (A.M.) – the hours before noon• Post meridian (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.”

Section 15.3


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.

Section 15.3


Time Zones of the Conterminous United States

Section 15.3


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 Texas (at noon) into New Mexico (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.

Section 15.3


Diagram Showing Times and Dates on Earth for Any Tuesday at 10 a.m. EST

Section 15.3


“Time Travel”

• Washington, D.C. is located at 39oN latitude.• At the 39oN parallel the earth is rotating at a

speed of approximately 800 mph.• Therefore if we flew in a plane at 800 mph

(along the 39th parallel) we would stay in the same position relative to the sun for as long as we flew!

• Therefore from our position in the plane it would appear that the sun “stood still.”

• In 24 hr we would arrive back in Washington, D.C. at the same time – but a day later!

Section 15.3


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.

Section 15.3


Finding the Standard Time and Date at Another Location – An Example

• It is 6 A.M. on March 21 in Los Angeles (34oN, 118oW). What are the time and date in Perth, Australia (32oS, 115oE)?

• Construct a circle with 24 even divisions on it.• These divisions represent 15o increments of a

full circle and each hour of the day.• Los Angeles falls in the time zone with the

120oW central meridian.• Perth falls in the time zone with the 120oE

central meridian.

Section 15.3


Finding the Standard Time and Date at Another Location – An Example (cont.)

• We know there are 24 hours in any day. We also are given that it is 6 A.M. in Los Angeles, therefore it is 10 P.M. in Perth.

Section 15.3


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.

Section 15.4


Diagrams of Sun's Position (Degrees Latitude) at Four Different Times of the Year

Section 15.4


Altitude and Zenith

• At 12 noon, the sun is on the observer’s meridian and appears at its maximum altitude about the southern horizon.– (for all observers north of the sun)

• Zenith – position directly overhead, therefore always 90o from the horizon

• Altitude – the angle measured from the horizon to the sun at noon

• Zenith Angle – angle from the zenith to the sun at noon

Section 15.4


Finding the Approximate Altitude of the Sun as Observed from Washington D.C. on June 21

• The angle between the sun and the observer is 39o – 23.5o = 15.5o.

• The altitude of the sun is 90o – 15.5o = 74.5o

Section 15.4


Finding the Approximate Altitude of the Sun as Observed from

Washington D.C. on December 21

Section 15.4


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.

Section 15.5


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.

Section 15.5


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

Section 15.5


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.

Section 15.5


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.

Section 15.5


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

Section 15.5


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.

• 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

Section 15.5


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

Section 15.5


Earth's Positions, Relative to the Sun and the Four Seasons

Section 15.5


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.

Section 15.5


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.

Section 15.5


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.

Section 15.5


Signs of the Zodiac

Section 15.5


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.

Section 15.5


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.

Section 15.5


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.

Section 15.5


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

Section 15.5


The Days of the Week

Section 15.5


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 365-day 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.

Section 15.5


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.

Section 15.6


Precession of a Top & Earth

Section 15.6


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.

Section 15.6

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Chapter 15 Place and Time Sections 15.1-15.6. Copyright © Houghton Mifflin Company. All rights reserved.15 | 2 Place & Time In Physical Science events