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Ralph S. (Ralph Stockman) Tarr.

A laboratory manual for physical and commercial geography online

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27



VII. — DETERMINATION OF LATITUDE AND LONGITUDE

Material. For Each Student. — Desk globe. Pencil. Ruler.

For General Class Use. — Rod and cardboard apparatus used in preceding exercise.
Purpose. A study of methods by which latitude and longitude may be determined for any given place.

Determina- On Sept. 23 and March 21 the sun is vertical at the equator. If you were at the equator,

tion of the sun would then be directly over your head at noon. Where would it appear if you were

latitude

JLic. e „„A„ at either pole?

suns noon r

altitude. What is the angular distance, in degrees, from a point in the sky directly overhead to a point

on the horizon? What is the angular difference,

then, between the position of the sun on those two days to observers, one at the equator and

the other at one of the poles ? How are the

degrees of latitude numbered as one goes from the equator toward the pole?

. How do the number

of degrees of the sun's height above the horizon vary as one goes from the equator to the pole ?



Complete this sentence: The numbering of degrees of latitude grows

as one goes from the equator toward the pole ; and the altitude of the sun above the horizon

grows as one goes from the equator toward the pole. Therefore, if

one observes at any place the altitude of the sun in degrees, and subtracts that number from

90°, the result will be the number equal to the degree of of the place.

The sun's apparent course is north of the equator after March 21, increasing in altitude
by a certain amount every day (see table on preceding page) up to June 21. To get the latitude
on any day between these two dates one must add a correction (from the table) to the latitude

as determined by the above method. Why do we add the correction ?



What correction must be applied between Sept. 23 and Dec. 21, when the sun is decreasing in

altitude?

A Figure 6 is a diagram of the apparatus used in this exercise.

29



B'



/C



/a



C B

Fig. 6. — Diagram to illustrate a Method of finding Latitude.
A-B, the length of the rod ; C'-B, the length of the noon shadow ; BCA, angle of the sun's altitude.

Reading To read the angle of the sun's altitude, as observed, we need first to draw to scale our read-

the angle ing (as secured in the preceding exercise) of the length of the shadow and the length of the
of the sun's r0( j

If you have studied geometry, prove that the angle B'AC is the angle of the latitude.
To obtain the exact latitude, a correction must be applied to this angle, as you have learned
above.



30




or Th _ ■*



Suggested
home work
for
students.



Next we need to use the protractor printed on the cardboard accompanying this exercise
(Fig. 7). This is simply a half circle whose circumference is marked off in degrees and half
degrees. Use the shears to cut it out carefully, as directed on the figure. Place the marked
center of the protractor accurately on the extremity of the line of the shadow (by means of a
pin stuck through the cardboard). Let the base line of the protractor coincide with the line
representing the shadow. Then read off the angle made by a line from the center of the pro-
tractor to the top of the line representing the height of the rod ; that is, the angle BCA.

The angle is degrees.

The correction is degrees.

The latitude as determined is degrees north latitude.

How does this compare with the latitude as mapped on your globe, or any other map of

your locality?

The angle of the altitude of the north star is equal to the latitude of the place where the observation of
the north star's position is made. The same reasoning that was applied to the position of the sun at the
equator and the poles applies also to the north star, except that the positions are reversed ; i.e. the north star
is in the zenith (directly overhead) at the poles and on the horizon at the equator. Therefore the altitude of
the north star gives the degree of latitude directly. Moreover, the north star does not change in position from
day to day as does the sun ; therefore, the observation can be made on any clear night and no corrections
need be applied. (It is not deemed worth while to bring in here the variation between culmination points.)

Fasten the protractor, by tacks, to a smooth board, as illustrated in Figure 8. From the center point
suspend a split shot by a thread for a plumb bob, as shown in Figure 8.



r -zs—- ■"■- -rrr







CENTER


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9






9


_,& TACK






TACK


V






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vy






V?




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s



BOARD



V



Fig. 8. — The Protractor used as a Quadrant Instrument.

Determina- We are now ready to make the reading. Go to some point where the horizon line is uninterrupted,

tion of the Point one end of the board (as indicated by the arrow) toward the north star, allowing the plumb bob to

latitude by swing freely. When you have pointed the board accurately at the north star, hold the thread of the plumb

observation bob firmly at the position to which it has swung. Now read the angle between the present position of the

tf the plumb bob and that which it had when the board was held horizontally. Make the observation a number of

WiOitb. star, times and set down the result as indicated on page 53.



31



Reading No. 1. — Angle

Reading No. 2. — Angle

Reading No. 3. — Angle

Reading No. 4. — Angle '

Average

How does this result compare in accuracy with that obtained by your sun observation ?



It is by application of these methods, with accurate instruments and refined corrections, that sailors and
explorers determine their latitude. Find out what a sextant is.

Determina- (Use your desk globe in finding answers to these questions.) How many degrees are

tion of

longitude, there in the circumference of a circle ? How many hours does it

take the sun to seemingly go around the earth ?

Therefore, over how many degrees of longitude does it pass in one hour ?



Is there any difference in the number of degrees of longitude the

sun passes over at the equator and at other latitudes, such, for example, as 45° N". latitude ?

When it is noon at Washington on a certain day, what time is it on the same

meridian to the north and south of Washington ?

Where does the sun rise? Where does it set?

Which places have noon first on a certain day, those to the east or to the west? '

If at the same actual time it is noon at Greenwich, Eng-
land (consult your globe), and seven o'clock in the morning at Philadelphia, U.S., what is the

longitude of Philadelphia as compared to that of Greenwich?



If now you set a very accurate watch (chronometer) exactly at noon

for Greenwich, or any meridian, and then start from there on an exploring expedition to
South America, how, by consulting this chronometer, can you tell the longitude of different

points in South America?



What advantage would there be in carrying several chronometers, all accurately set for Wash-
ington time, or that of some other meridian?



32



If there is a telegraph line extending from a place,

of which you know the position, to one whose longitude you wish to determine, how could the

telegraph be utilized to great advantage in the determination ? A



33




Materials.



Purpose.



Day and
night.



VIII.— ROTATION AND ITS EFFECTS

For Each Student. — Desk Globe. Dividers.

For General Class Use. — Cardboard and Rod Apparatus.

To understand how rotation governs daily change in time ; the significance of standard time;
and of date line.

Place your globe in a strong light. How much of it is illuminated?

How much of it is in the shadow? What do these areas determine

on the earth, when illuminated by the sun?

Between the lighted and unlighted parts is a

belt all around the globe which is partly lighted and partly in shadow. To what times in

the day would this belt correspond?



Sunrise Which way does the sun seem to move?

and sunset.



rotating, in which direction must it turn?



If the sun is standing still and the earth is



Slowly rotate the globe in the same direction.



Is it moving clockwise or counter-clockwise?

Stop the rotation when the conditions are such that the sun is just rising over your home.

Where is it then setting?

Where is the twilight zone?



Difference How long does it take the earth to make one complete rotation ?

in time.



Look on the globe to see how many degrees of longitude, east and

west, are marked on the equator. What is the sum of these?

How many degrees, then, must the sunrise pass over in one hour?

In two hours? In six hours? Rotate

35



the globe to see that the sunrise extends farther and farther westward on the equator as the

globe rotates. Is the same thing true at latitude 45°? At latitude 60°?

Does an object move faster or slower at the equator than at latitude 45°?

Why does the sunrise pass over the same number of degrees of longi-
tude in an hour in the two places?

Now, state why there is an hour's difference in sunrise

at every fifteenth degree.

Is there the same difference in noon? In sunset?

To the north of the equator? To the south of the equator ?

How much difference in time is there in one degree?

Is the time of day earlier or later to the east? ..

To the west? What about the time north and south along a given

meridian? .

Problems. When it is sunrise at 70° west longitude, is it before or after sunrise at 100° west longi-
tude? How much is the difference?

What is the difference in time of sunrise be-
tween 70° west longitude and 10° west longitude ?

Between 70° west longitude and 10° east longitude?

When.it is midday at your home, where is it midnight?

When the people in London (at 0° Long.) are rising, say at 7 o'clock, what

is the time at your home? By study of your

globe find the difference in time between (a) Chicago and San Francisco;

(b) New York and Peking ; (c) New York and Chicago ;

(d) New York and Rome. In each case state which city has the earlier time.



Local time. (Note. — Use the cardboard and rod apparatus, see Exercise VI.)

36



On a board placed in the sunlight set up a rod about six inches high. At half-past
eleven mark with a pencil where the end of the shadow of the rod falls on the board ; also
marking the time on the board. Be sure that your watch is exactly right. Every five min-
utes after that, mark the position (and time) of the end of the shadow until half-past twelve.
Then connect the points. Carefully measure (with dividers) the distance between the points
marked and the base of the rod. Which point is the shortest distance from the rod ? That

point marks the time of local noon. Why is that so?



Was it also noon by your watch?

If not, how much difference was there ?



Standard What objection is there to the use of local time?

time.



How have the difficulties been met?



Why should Greenwich be chosen as the place to start in reckoning standard time?



On what meridian is the standard time one hour later than at Greenwich?
Two hours?



Five hours? On the outline map of the United

States (Fig. 9) locate the meridians 75°, 90°. 105°, and 120° West Longitude. Why should these

meridians be chosen as the central meridians for our time belts?



37




Fig. 9. — Outline Map of the United States.



On the map (Fig. 9) sketch in the four standard time belts of the United States. (See Fig.
556, p. 404 of Text-book.) Why are the actual boundaries irregular and not on the meridians?



In which of the standard time belts do you



live?



Where and how much would you change your watch in going from New York to San Francisco?



From Chicago to Boston ?



The date If a person should travel completely around the earth, going from east to west, how many times (and

line. in which direction) would he have to change his watch if he made the change every fifteen degrees ?



On his return how would his time agree with that of his starting place ?



Would he have gained or lost a day ?



What would be the case in traveling around the earth in an easterly direction ?



38



If it is sunrise Monday where you live, would it also be Monday when the suu next rises at the Hawaiian

Islands? In China ? InLondon? InNew

York? What difficulty does this bring out ?



Where does Tuesday begin ?



Where would you begin it ?



What objections would there be to choosing a meridian running through a

country such as England, or Germany, or the United States ?




Fig. 10. — Map of World for plotting the International Date Line.

39



Such a line would be called a date line. Find

the date line on your globe. What reason can you suggest for locating the date line where it is ?



Does the date line follow a meridian exactly ? Draw in the date line on the map of the

world (Fig. 10). If you crossed that line Monday, June 20, coming from the east, what would be the

day of the week and month on the other side of the line ?

What if you were coming from the west?



Indicate this on the map, showing both routes by arrows, and on each side of the date line writing the days
and dates going each way.



4







40



Materials.



Purpose.



Local ob-
servations.



IX. —THE SEASONS

For Each Student. — Desk globe. Cotton string. Colored pencils. (Method of using string :
Tie tightly around globe at equator, then slip over globe in the positions indicated below.)

To understand how the earth's revolution around the sun, together with the inclination of the
earth's axis, determines seasons, zones, and length of day and night.

At what season is the sun highest in the heavens at noon where you live?

At what season lowest?

At what two opposite seasons is the sun halfway between the highest and lowest?



% At what seasons does the sun rise exactly in the east and set in the west ?



At what season does



it rise and set farthest south of true east and west ?



Farthest north of true east and west?

In a few sentences write a description of the changes in the sun's position for a year.



What is the cause of these changes ?



What is revolution?



How much is the earth's axis inclined ?



Winter Set your globe on the desk in front of you with its axis inclined 23^-°, and with the north-

season, pole end pointed away from you. (Your desk globe is probably set at this inclination.) Look at

the globe with your eyes on the level of the Tropic of Capricorn. Can you see the south pole ?

41



The north pole ? If you held a lighted candle where your

eyes were, how much of the south polar region would be lighted?

Of the north polar region?

Where, on the surface of the globe, would the light fall most directly?

Place the string around the globe to show the limits of the lighted

part in this position. To what latitude does it extend in the north?

How far does it go beyond the south pole?

Now rotate the globe. Under these conditions is there any night in the

Antarctic region ? Is there any day in the Arctic region ?

Is there day and night in the equatorial region ? In which hemisphere is the

period of daylight longest as the globe rotates?

Letting the globe represent the earth, and the light the sun, in which hemisphere is the sun*

most nearly vertical? Is this period summer

or winter at your home ? What season is it in the southern hemisphere ?

Why is it warmer at the equator than in the Antarctic region ?

Why warmer in the Antarctic than in the Arctic?



Briefly describe the changes in the sun's posi-

tion, and length of day, that one would observe in passing from the south to the north pole at this
season.



Summer Swing the globe around to the desk behind you, as if your body were the sun, keeping the

season. ax | s always inclined in the same direction. Now the north pole points toward you. With your

42



eyes at the same level as when you looked at the globe in the winter position, what part of
the globe is directly in front of them?

Again, imagine a lighted candle placed where your eyes are ; the light would fall most directly
on the Tropic of Cancer. Place the string around the globe to show the limits of the part that
would be illuminated if a strong light fell upon the globe when in this position. How much

of the north polar region would be lighted?

Of the south polar region ? Rotate the globe.

Is there any night in the Arctic region ? Is there any day in the Antarctic ?

What parts of the earth have day and night?

* In which hemisphere is the

period of daylight longest as the globe rotates ?

Letting the globe represent the earth, and the light the sun, in which hemisphere is the sun

most nearly vertical at this season ? What

season does this represent in the northern hemisphere ? In the

southern? . Why is it warm at the equator ?

Why is there no winter

there?

In which direction will a shadow fall at the equator at noon?

At the Tropic of Cancer?

At the Arctic circle ? _' At the Tropic of Capri-
corn? Where will the shadow be longest?

Explain why the sun rises and sets farther north at this season than in the winter.



Briefly describe the changes in the sun's position and the length of day that one would
observe in passing from the north to the south pole at this season.



43



Spring and Swing the globe around to the desk on your left, with the axis still pointing as before.

autumn. This will represent March 21. With your eye at the same level as before, you now look

directly at the equator. Place the string on the globe to show the area lighted in this

position. What part of the surface must then receive vertical sun's rays ?

Does the north pole receive sunlight in this position ?

The south pole ? In what proportions does the string divide

the equator?

Kotate the globe. How many hours of sunlight would a place on the equator have ?

Therefore, how many hours of night?

Does the string divide other parallels equally? At this period, how long is

the day and night in all parts of the earth? Why is this called the

spring equinox ? .

Does the sun also rise and set in the Arctic and Antarctic regions at this

season ? How does the sun appear at the poles themselves ?

Swing the globe around to the desk on your right, with the axis still pointing as before.
Answer the same questions as for the last position.



Why should this period (Sept. 23) be
44



called the autumnal equinox ?



By placing the globe in other positions, show that there is

no other season besides autumn and spring when the days and nights are equal all over the

earth. Where would they be equal on Dec. 21 ?

On June 21? :



Make a drawing (see Fig. 553, p. 401, in Text-book) to show the revolution of the earth about the sun,
fixing the position for June 21, Sept. 23, Dec. 21, and March 21, and shade the half of the globe to represent
night at each of these dates.



The zones. What reason can you give for placing the Tropic of Cancer, Tropic of Capricorn,
Arctic Circle, and Antarctic Circle where they are?

What is the latitude of each?



45



What reasons can you give for the fact that the torrid zone is warm at all seasons?



For the fact that the temperate zones have warm summers and cold winters ?



For the fact that the frigid zones are cold even in summer?



On the outline map (Fig. 11) sketch in the five zones with different colored pencils, and write
in their names. Why would you not expect to find an abrupt change in going from one zone



to the next?



To find Place the string on the globe in the position of the equinox. What is the number of degrees of longi-

the length
of day and
night.



tude on the lighted side at the equator ? „__. At 45° ?

At 60° ? Remembering that 15° of longitude represents one hour of time, how

many hours long is the day at the equinox ?

The night? Shift the string to the position of midwinter

(Dec. 21). How many degrees are lighted at the equator ? At the Tropic of

Capricorn? At 45° N. latitude ? At 45° S. latitude ?

At your own home ?

What is the length of daylight at each of these parts of the earth en Dec. 21 ? ,



summer position (June 21) make the same observations for that date.

46



.. With the string in the mid-







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SB}
w H m



47







48



Materials.



Purpose.



Introduc-
tory.



The rock
outcrop.



AUTUMN FIELD WORK
X. — FIELD EXCURSION FOR COLLECTION OF SPECIMENS

For Each Student. — A bag, or small basket, for carrying specimens.

For General Class Use. — One or more hammers.

To collect specimens (such as minerals, rocks, and fossils),' for later study in the laboratory ;
to gain a knowledge of the nature and characteristics of the bed rock ; and to study the soil.

For this excursion select the nearest rock outcrop within easy reach ; and, if possible, on
the way to it examine one or more bowlders. If different rock outcrops are accessible on this
excursion, examine several.

Describe the appearance of the outcrop



Was it revealed by the work of man, or of

water, or is it a hillside ledge with no soil on it?

. . What is the color of the rock?

Is it hard or soft ? (Try the effect of a hammer or

knife on it.) Is its texture coarse or fine-
grained? Can you see individual minerals in it?

If so, describe their appearance.



Is the rock in layers or not ? If in layers, what differences are there

between them?







Are the layers horizontal or inclined ?



49



Are there any fossils? If so, collect specimens. After the teacher

has told you what a sedimentary rock is, tell whether this is a sedimentary or a crystalline

rock. How is a sedimentary rock distin-
guished from a crystalline rock?



There are many kinds of sedimentary and crystalline

rocks. The teacher will tell you the name of this one. Write a brief description of it.



Collect a specimen for study in the laboratory when you are studying minerals and rocks.
When a rock is subjected to strain, as is often the case in the earth's crust, it breaks,

forming a parting called a joint plane. Are there joint planes in the outcrop ?

Write down any other observations you make, or any facts that the teacher tells you about
this outcrop.



Soils. Is there soil resting on the rock outcrop? Examine and describe it,

stating whether it is coarse or fine ; whether it is different at the

top and bottom

50



; whether it has pebbles in it or not ;

whether it grades down into the rock or is separated sharply from it

; and any other observations

you may make.



Other On your way from the outcrop watch carefully for bowlders and pebbles. Do you find

minerals

and rocks. an y? Are they like the rock in the outcrop ? .__: If not, collect

specimens for study in the laboratory.



51



Materials.



Purpose.



Introduc-
tory.



Effect of
expansion
and con-
traction.



XI. —EXCURSION FOR THE STUDY OF WEATHERING

For Each Student. — A small bag or basket for the collection of specimens.
For General Class Use. — One or more hammers.

To study in the field some of the processes by which rocks are disintegrated or broken down on
exposure to air.

The class should be taken to some rock outcrop, or failing that, to a bowlder. If several
outcrops are available, select the one at which it is possible to answer the largest number of
the following questions. Almost any outcrop will furnish opportunity to make some observa-
tions on weathering.

Is the rock of this outcrop composed of one or of several kinds of minerals?

Heat causes minerals to expand, and they

contract when cooled. Different minerals expand and contract at different rates. As this
outcrop is warmed in the day and cooled at night, what must be the changes in the rock as a

whole ?

What must happen as a result of the strains at the junction of minerals of different kinds ?



State briefly how changes in temperature aid in the disruption of rocks where they outcrop.



Effect of
frost.



Are there any visible cracks in the rocks? When might water enter the

rocks?

Does water expand or contract on freezing?

When the cracks are filled with water and the temperature falls below freezing point, what is



the effect of the freezing of the water in the cracks ?



Are there other smaller cracks in the



53



rock, — for instance, between the mineral grains? What influence do

these have in aiding in the breaking up of the rock?

Write a brief paragraph stating the

effect of frost in rock disintegration.



Effect of Are there any lichens on the outcrop ? ■... How do they cling to the rock?


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Online LibraryRalph S. (Ralph Stockman) TarrA laboratory manual for physical and commercial geography → online text (page 3 of 16)