Handbook of nature-study for teachers and parents

Chapter XLIX: Part IV: Earth and Sky (4)

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It now remains to consider the cause of the unexpected low pressure found at the poles, and the reason for the belts of high pressure at the tropics. If we refer to Fig. 2, it is evident that not all the air that ascends at the equator descends at the tropics, else there would be an absence of air at the higher latitudes, which is manifestly not the case. On the other hand, it is equally impossible that all the air ascending at the equator should move to the poles, because the space it could occupy decreases rapidly from a maximum at the equator to zero at the poles. Only a part of the air that ascends at the equator is, therefore, involved in the trade wind circulation and a part passes over the tropics and moves on toward the low pressure at the poles. Furthermore, some of the air that descends at the tropics moves along the surface toward the poles, obeying the law that impels air to move from high pressure to low pressure. Now every particle of air that passes over the tropics, every particle that moves northward along the surface, turns to the right in the northern and to the left in the southern hemisphere. All, therefore, miss the poles--on the right side in the northern and on the left side in the southern hemisphere. The result is that two great whirlpools develop in the atmosphere; one whirling about the north and the other whirling about the south pole. The outer margins of these whirlpools coincide with the tropical belts of high pressure.

As an example of a whirlpool we may take a basin having a vent at the center of the bottom. If the basin is filled with water, the plug withdrawn and the water given a slight rotary motion, its velocity will increase as it approaches the center and the rapid whirling will develop sufficient centrifugal force to open an empty core. Those who have visited the great whirlpool at Niagara, undoubtedly noticed that the whirling waters are held away from the center and piled up around the margins by the centrifugal force developed. Let us suppose that air starting from the equator, moves without friction or other resistances toward the pole. Its velocity must increase as its radius shortens, because the law of the conservation of areas requires that the radius must always sweep over equal areas in a given unit of time. (See law of conservation of areas.) At the equator, the air has an easterly motion equal to the eastward motion of the earth, which is 1,000 miles per hour. At latitude 60° the radius will have decreased one-half and the velocity, therefore, doubled; but at latitude 60° the eastward motion of the earth is only 500 miles per hour, so the air would be moving 1,500 miles per hour faster than the earth. At a distance of 40 miles from the pole the wind would attain an easterly velocity of 100,000 miles per hour, and moving on so short a radius would develop sufficient centrifugal force to hold all the air away from the pole and thus form a vacuum. That the supposed case of no friction is far from the truth is evidenced by the fact that the pressure at the north pole is but little less than at the equator, but the centrifugal force developed by the gyration winds, in thus withdrawing the air from the poles and piling it up at the tropics, may be fairly taken as sufficient cause for the low pressure found at the poles and the belts of high pressure at the tropics.

The questions that remain to be considered are: (1) the low pressure at the south pole as compared with the pressure at the north pole and (2) the unequal distance of the tropical belts of high pressure from the equator. These questions may be considered together.

It is to be remembered that the southern hemisphere is the water hemisphere, and that the prevailing westerlies, in gliding over the smooth water surface, are but little retarded by friction and, therefore, attain a higher velocity than the corresponding winds of the northern hemisphere, where the rougher surface materially retards their movement. As a consequence, the circumpolar whirl of the southern hemisphere is stronger, and develops a greater centrifugal force, thus holding a larger quantity of air away from the south pole and reducing the pressure to a greater degree than is brought about by the weaker winds of the northern hemisphere.

Since the circumpolar whirl of the southern hemisphere is the stronger of the two, it withdraws the air to a greater distance from the pole than does its weaker counterpart of the northern hemisphere, and piles it up in the tropical belt of high pressure about five degrees nearer the equator than does the weaker forces of the northern hemisphere.

STORMS

Photomicrograph by W. A. Bentley.]

Having gained a comprehensive view of the general, planetary wind system, we may now undertake the study of local disturbances that arise within the general circulation and are known as “storms.”

Storms are simply eddies in the atmosphere. They may be compared to the eddies that are often seen floating along with the current of a river or creek. In these eddies the water is seen to move rapidly around a central vertex, developing sufficient centrifugal force to hold some of the water away from the center, thus forming a well marked depression, frequently of considerable depth. The whole circulation of the eddy is quite independent of the current of the stream which carries it along its course, and while its general direction and velocity of movement coincide with that of the current, there are times when it will be seen to move quickly from side to side and again when it will remain nearly stationary for a time or take on a rapid movement.

The eddies or storms in the atmosphere act in much the same way. They are carried along by the general currents of the river of air in which they exist. Their general direction coincides with the direction of the current in which they are floating, and their rate of movement conforms in a general way to its velocity; but like the eddies in the river, they do not always move in straight lines nor at a uniform rate of speed.

There is one important respect in which the eddies in the air differ from eddies in water. The water eddy may revolve in either direction, depending upon the direction in which the initial force was applied, but the storm eddies in the atmosphere always revolve counter-clockwise in the northern hemisphere, and clockwise in the southern.

This is due to the deflecting force of the earth’s rotation, which is fully explained on page 872.

WEATHER MAPS

Photomicrograph by W. A. Bentley.]

A weather map is a sort of flashlight photograph of a section of the bottom of one or more of these great rivers of air. It brings into view the whole meteorological situation over a large territory at a given instant of time; and, while a single map conveys no indication of the movements continually taking place in the atmosphere, a series of maps, like a moving picture, shows not only the whirling eddies, the hurrying clouds and the fast-moving winds, but the ceaseless on-flow of the great river of air in which they float. Our present knowledge of the movements of the atmosphere has been gained chiefly from a study of weather maps; they form the basis of the modern system of weather forecasting, and their careful study is essential to any adequate understanding of the problems presented by the atmosphere. (See pages 884–885.)

_The Principles of Weather Forecasting_

Photomicrograph by W. A. Bentley.]

The forecasting of the weather has been made possible by the electric telegraph. It is based upon a perfectly simple, rational process constantly employed in everyday affairs. We go to a railway station and ask the operator about a certain train. He tells us that it will arrive in an hour. We accept his statement without question, because we are confident that he knows the speed at which the train is approaching, a few clicks of his telegraph instrument has told him just where it is and the time it will arrive, barring accidents, is a simple calculation. Information of coming weather changes are obtained in a similar manner. Although storms do not run on steel rails like a train, nevertheless their movements may be foreseen with a reasonable degree of accuracy, depending chiefly upon the size of the territory from which telegraphic reports are received and the experience and skill of the forecaster. As a rule, the larger the territory brought under observation, especially in its longitudinal extent (the general currents carry storms of the middle latitudes eastward around the world and those or the tropics westward), the earlier advancing changes may be recognized and the more accurately their movements foreseen.

_Forecasts Based on Weather Maps_

The forecasts issued by the United States Weather Bureau are based on weather maps, prepared from observations taken at 8 a. m. and 8 p. m. at about 200 observatories. In addition to the reports received by telegraph by the Central Office at Washington, the several forecast centers and other designated stations from observatories or stations in the United States, a system of interchange with Canada, Mexico, the West Indies and other island outposts in the Atlantic and Pacific gives to the forecaster two daily photographs of the weather conditions over a territory embracing nearly the whole of the inhabited part of the western hemisphere north of the equator. Any sort of disturbance within this vast region is photographed at once upon the weather map. If it be a West Indies hurricane or other destructive storm, its character is recognized instantly, its rate and direction determined and information of the probable time of its arrival sent to those places that lie in its path. The method is perfectly simple. Anyone with a weather map and a little experience can forecast the weather with some degree of accuracy, or, at least, gain an intelligent understanding of the conditions upon which the forecasts that accompany the map are based.

_Maps, Where Published and How Obtained_

Weather maps are published in many daily papers, and in somewhat larger form and more in detail, at many Weather Bureau stations. They may usually be obtained for school use by applying to the nearest Weather Bureau station or to the Chief of the Weather Bureau at Washington, D. C.

The forecasts that accompany the maps are simply an expression on the part of the official forecaster as to the weather changes he expects to occur in various parts of the country within the time specified, usually within 36 to 48 hours. His opinion is based upon the conditions shown by the map. He has no secret source of information. You may accept his conclusions, or, if in your opinion they are not justified, you have all the information necessary to make a forecast for yourself. Weather maps are published so extensively with a view to thus stimulating an intelligent interest in the problem of weather forecasting, and also that one may see at a glance what the temperature, rainfall, wind and weather is in any part of the country in which he may be interested. The friends of the weather service are those who best understand its work.

THE VALUE OF THE WEATHER SERVICE

Photomicrograph by W. A. Bentley.]

No one knows so well as the forecaster that the changes that appear most certain to come sometimes fail, or come too late; but taking all in all, about 85 out of 100 forecasts are correct. Of those that fail, probably not more than three or four per cent. fail because the changes come unannounced. Most forecasters predict too much, and their forecasts fail because the expected changes come after the time specified or not at all. It is fortunate that this is so; for it is better to be prepared for the change though it be late in coming than to have it come without warning.

The value of the weather service to the agriculture and commerce of the United States cannot be questioned seriously. That the appropriations for its support have been increased year by year from $1,500 in 1871 to nearly $1,500,000 in 1910 is evidence of its value and efficiency. A conservative estimate places the value of property saved by the warnings issued by the Weather Bureau at $30,000,000 annually.

LESSON CCXXII

EXPERIMENTS TO SHOW AIR PRESSURE

_Leading thought_--The air presses equally in all directions.

_Experiment 1--To show that air presses upward_--Fill a tumbler which has an unbroken edge as full of water as possible. Take a piece of writing paper and cover the tumbler, pressing the paper down firmly upon the edge of the glass. Turn the glass bottom side up and ask why the water does not flow out. Allow a little air to enter; what happens? Why? Turn the glass filled with water and covered with paper sidewise; does the water flow out? If not, why?

Photomicrograph by W. A. Bentley.]

_Experiment 2--To show that air passes downward_--Ask some of the boys of the class to make what they call a sucker. This is a piece of leather a few inches across. Through its center a string is drawn which fits very closely into the leather and is held in place by a very flat knot on the lower side. Dampen the leather and press it against any flat surface, and try to pull it off. If possible, place the sucker on a flat stone and see how heavy a stone can be lifted by the sucker. Ask why a sucker clings so to the flat surface. If a little air is allowed to get between the sucker and the stone, what happens? Why?

_Hints to the teacher regarding the Experiments_--The water is kept in the tumbler in Experiment 1 by the pressure of the atmosphere against the paper. If the tumbler is tipped to one side the water still remains in the glass, which shows that the air is pressing against the paper from the side with sufficient force to restrain the water, and if the tumbler is tipped bottom side up it shows the air is pressing upward with sufficient force to keep the water within the glass.

In the case of Experiment 2, we know that the leather pressing upon the floor or on the stone is not in itself adhesive, but it is made wet simply so that it shall press against the smooth surface more closely. The reason why we cannot pull it off is that the air is pressing down upon it with the force of about fifteen pounds to the square inch. If the experiment is performed at sea level, we should be able to lift by the string of the sucker a stone weighing fifteen pounds. The reason why the water falls out of the tumbler after a little air is let beneath the paper, is that then the air is pressing on both sides of the paper; and the reason why the sucker will not hold if there is any air between it and the stone, is because the air is pressing in both directions upon it.

_Supplementary reading_--The Wonderbook of the Atmosphere, Houston, Chapters III, IV, V.

LESSON CCXXIII

THE BAROMETER

_Leading thought_--The weight of our atmosphere balances a column of mercury about thirty inches high, and is equal to about fifteen pounds to the square inch. This pressure varies from day to day, and becomes less as the height of the place increases. The barometer is an instrument for measuring the atmospheric pressure. It is used in finding the height of mountains, and, to a certain extent, it indicates changes of the weather.

Photo by W. A. Bentley.]

_Method_--A glass tube about 36 inches long, closed at one end; a little glass funnel about an inch in diameter at the top; a small cup--a bird’s bathtub is a good size since it allows plenty of room for the fingers; mercury enough to fill the tube and have the mercury an inch or more deep in the cup. Be careful not to spill the mercury in the following process, or you will be as badly off as old Sisyphus with his rolling stone.

Set the closed end of the tube in the cup so that any spilled mercury will not be lost; with the help of the funnel slowly and carefully fill the tube clear to the top with the mercury; empty the rest of the mercury into the cup; place the end of one of the fingers of the left hand tightly over the open end of the tube and keep it there; with the right hand invert the tube, keeping the end closed with the finger, and place the hand, finger and all, beneath the mercury in the cup then remove the finger, keeping the open end of the tube all the time below the surface of the mercury. When the mercury has ceased to fall measure the distance from the surface in the cup to the top of the mercury in the tube.

_Observations_--1. How high is the column of mercury in the tube?

2. What keeps the mercury in the tube? Place the cup and the tube on a table in the corner of the room, place behind the tube a yardstick, and note whether the column of mercury is the same height day after day. If it varies, why?

3. Would the mercury column be as high in the tube if it were placed on top of a mountain as it would at the foot? Why?

_Supplementary reading_--Chap. II in The Wonderbook of the Atmosphere, Houston.

HOW TO READ WEATHER MAPS

_Teacher’s Story_

Weather maps may be obtained by writing to the nearest Weather Station, or by writing to the Chief of the Weather Bureau, Dr. Willis L. Moore, Washington, D. C., stating that you wish to post the maps in a public place. A supply of maps for three successive days for use in these lessons may be obtained at 20 cents per hundred. Sometimes they are sent free, if it is stated that they are to be used for school purposes.

Photo by W. A. Bentley.]

The words isobar and isotherm have been bogies which have frightened many a teacher from undertaking to teach about weather maps, and yet how simple are the meaning of these two words. Isobar is made up of two Greek words, _isos_ meaning equal and _baros_ meaning weight. Therefore, an isobar means equal weight, and on a map one of these continuous lines means that, wherever it passes, the atmosphere there has equal weight and the barometer stands at equal height. The isobar of 30 means that the mercury in the barometer stands 30 inches in height in all the regions where that line passes.

“Isotherm” comes from the two Greek words, _isos_ meaning equal and _therme_ meaning heat. Therefore, on the map the dotted lines show the region where the temperature is the same. If at the end of the dotted line you find 60 it means that, wherever that line passes, the thermometer stands at 60 degrees.

Many of the “highs” and “lows” enter the United States from the Pacific Ocean about the latitude of Washington State or southwest British Columbia. They follow one another alternately, crossing the continent in the general direction of west to east in a path which curves somewhat to the north, and they leave the United States in the latitude of Maine or New Brunswick. If they enter by way of lower California, they pass over to the Atlantic Ocean farther south. The time for the passage of a high or low across the continent averages about three and one-half days, sometimes a little more. These areas are usually more marked in winter, and wind storms are more marked and more regular.

A low area is called a cyclone and a high area an anti-cyclone. The destructive winds, popularly called cyclones, which occur in certain regions, should be called tornadoes instead, although in fact they are simply small and violent cyclones. But a cyclone, when used in a meteorological sense, extends over thousands of square miles and is not violent; while a tornado may be only a few rods in diameter and be very destructive. The little whirlwinds which lift the dust in the roads are rotary winds also, but merely the eddies of a gentle wind.

In a cyclone or “low,” and also in a tornado, the air blows from _all_ sides spirally inward _toward_ the center where there is a column of _ascending_ air.

In an anti-cyclone or “high” the air blows outward in every direction in curved lines _from_ a column of _descending_ air.

In the above map, the curved lines are isobars; the line of crosses from A to B indicates the course of the storm; the arrows indicate the direction of the wind, note that it is moving counter-clockwise around the area of low pressure; the shaded area indicates the region where it is raining or snowing; note that this is the area where the warm, moist air from the Gulf and the Ocean meets the colder air of the North.

The weather conditions during the passage of a cyclone are briefly as follows: Small, changing wisps of cirrus clouds appear about 24 hours before rain; these gradually become larger and cover the whole sky, making a nimbus cloud. The wind changes from northeast to east or southeast to south. The barometer falls; the thermometer rises, that is, air pressure is less to the square inch, and the temperature of the atmosphere is warmer. Rain begins and falls for a time, varying from an hour to a day or more. After the rain there appear breaks in the great nimbus clouds and finally the blue sky conquers until there are only a few or no clouds. The wind changes to southwest and west; the barometer rises, the temperature falls. The rain ceases, the sun shines out brightly. The low has passed and the high is approaching to last about three days.

U. S. DEPARTMENT OF AGRICULTURE, WEATHER BUREAU

EXPLANATION OF WEATHER SIGNALS

No. 1 Fair Weather

No. 2 Rain or Snow

No. 3 Local Rain or Snow

No. 4 Temperature

No. 5 Cold Wave

INTERPRETATION OF DISPLAYS

No. 1, alone, indicates fair weather, stationary temperature.
No. 2, alone, indicates rain or snow, stationary temperature.
No. 3, alone, indicates local rain or snow, stationary temperature.
No. 1, with No. 4 above it, indicates fair weather, warmer.
No. 1, with No. 4 below it, indicates fair weather, colder.
No. 2, with No. 4 above it, indicates rain or snow, warmer.
No. 2, with No. 4 below it, indicates rain or snow, colder.
No. 3, with No. 4 above it, indicates local rain or snow, warmer.
No. 3, with No. 4 below it, indicates local rain or snow colder.

WILLIS L. MOORE,
_Chief U. S. Weather Bureau_.
]

EXPLANATION OF WHISTLE SIGNALS

A warning blast of fifteen to twenty seconds duration is sounded to attract attention. After this warning the longer blasts (of four to six seconds duration) refer to weather, and shorter blasts (of one to three seconds duration) refer to temperature; those for weather are sounded first.

Blasts Indicate.

One long Fair weather.
Two long Rain or snow.
Three long Local rain or snow.
One short Lower temperature.
Two short Higher temperature.
Three short Cold wave.

By repeating each combination a few times, with intervals of ten seconds, liability to error in reading the signals may be avoided.

STORM AND HURRICANE WARNINGS

Storm warnings. Hurricane
warning.

NE. winds. SE. winds. NW. winds. SW. winds.

EXPLANATION OF STORM AND HURRICANE SIGNALS

_Storm warning_--A red flag with a black center indicates that
a storm of marked violence is expected.

The pennants displayed with the flags indicate the direction
of the wind; red, easterly (from northeast to south); white
(westerly from southwest to north). The pennant above the flag
indicates that the wind is expected to blow from the northerly
quadrants; below from the southerly quadrants.

By night a red light indicates easterly winds and a white
light below a red light westerly winds.

_Hurricane warning_--Two red flags with black centers
displayed one above the other indicates the expected approach
of a tropical hurricane or one of those extremely severe and
dangerous storms which occasionally move across the Lakes and
northern Atlantic coast.

No night hurricane warnings are displayed.
]

Note the course of the low that was on the Pacific Coast Dec. 24; this is indicated by the line of dots and dashes on the later maps.]

LESSON CCXXIV

HOW TO READ WEATHER MAPS

_Leading thought_--Weather maps are made with great care by the Weather Bureau experts. Each map is the result of many telegraphic communications from all parts of the country. Every intelligent person should be able to understand the weather maps.

_Method_--Get several weather maps of the nearest Weather Bureau Station. They should be maps for successive days, and there should be enough so that each pupil can have three maps, showing the weather conditions for three successive days.

_Observations_--1. Take the map of the earliest date of the three. Where was your map used? What is its date? How many kinds of lines are there on your map? Are there explanatory notes on the lower left-hand corner of your map? Explain what the continuous lines mean. Find an isobar of 30; to what does this figure refer? Find all the towns on your map where the barometer stands at 30 inches. Is there more than one isobar on your map where the barometer stands at 30?

2. Where is the greatest air pressure on your map? How high does the barometer stand there? How are the isobars arranged with reference to this region? What word is printed in the center of this series of isobars?

3. What do the arrows indicate? What do the circles attached to the arrows indicate?

4. In general, what is the direction of the winds with reference to this high center?

5. Is the air rising or sinking at the center of this area? If the wind is blowing in all directions from a center marked high, what sort of weather must the places just east of the high be having? Do the arrows with their circles indicate this?

6. Find a center marked low. How high does the barometer stand there? Does the air pressure increase or diminish away from the center marked low, as indicated by the isobars? Do the winds blow toward this center or away from it?

7. What must the weather in the region just east of the low be? Why? Do the arrows and circles indicate this?

8. Is there a shaded area on your map? If so, what does this show?

9. Compare the map of the next date with the one you have just studied. Are the highs and lows in just the same position that they were the day before? Where are the centers high and low now? In what directions have they moved?

10. Look at the third map and compare the three maps. Where do the high and low centers seem to have originated? How long does it take a high or low to cross the United States? How far north and south does a high or low, with all its isobars, extend?

11. What do the dotted lines on your map mean? Do they follow exactly the isobars?

12. What is the greatest isotherm on your map? Through or near what towns does it pass?

13. Do the regions of high air pressure have the highest temperature or the lowest? Do high temperatures accompany low pressures? Why?

14. What is the condition of the sky just east of a low center? What is its condition just west of low?

15. If the isobars are near together in a low, it means that the wind is moving rather fast and that there will be a well marked storm. Look at the column giving wind velocity. Was the wind blowing toward the center of the low on the map? If so, does that mean it is coming fast or slow? How does this fact correspond with the indications shown by the distance between the isobars?

16. Describe the weather accompanying the approach and passage of a low in the region where your town is situated? What sort of clouds would you have, what winds, what change of the barometer and thermometer?

_How to Find the General Direction and Average Rate of Motion of
Highs and Lows_

_Observations_--1. On the first map of the series of three given, put an X in red pencil or crayon at the center of the high and a blue one at the center of the low; or if you do not have the colored pencils, use some other distinguishing marks for the two. If there are two highs and two lows use a different mark for each one.

2. Mark the position of each center on this map for the following day with the same mark that you first used for that area. Do this for each of the highs and lows until it leaves the map or until your maps have been used. All the marks of one kind can be joined by a line, using a red line for the red marks and a blue line for the blue marks.

3. What do you find to be the general direction of the movement of the highs and lows?

4. Examine the scale marked statute miles at the bottom of the map. How many miles are represented by one inch on the scale?

5. With your ruler find out how many miles one area of high or low has moved in twenty-four hours; in three days. Divide the distance which the area has moved in three days by three and this will give the average velocity for one day.

6. In the same way find the average velocity of each of the areas on your map for three days and write down all your answers. From all your results find the average weekly velocity; that is, how many miles per hour and the general direction which has characterized the movement of the high and low areas.

_Supplementary reading_--The Wonderbook of the Atmosphere, Houston, Chapters XIV-XXIII.

_How to Keep a Daily Weather Map_

The pupils should keep a daily weather map record for at least six months. The observations should be made twice each day and always at the same hours. While it would be better if these records could be made at 8 o’clock in the morning and again at 8 o’clock in the evening, this is hardly practicable and they should, therefore, be made at 9 o’clock and at 4. The accompanying chart may be drawn enlarged. Sheets of manila paper are often used, so that one chart may cover the observations for a month.

Few schools are able to have a working barometer, but observations of temperature and sky should be made in every school. Almost any boy can make a weather vane, which should be placed on a high building or tree where the wind will not be deflected from its true direction when striking it. A thermometer should be placed on the north side of a post and on a level with the eyes; it should not be hung to a building, as the temperature of the building might affect it.

The direction of the wind and the cloudiness of the day may be indicated on the chart, as it is on the weather maps, by a circle attached to an arrow which points in the direction in which the wind is blowing.

_References_--Elementary Meteorology, Waldo, American Book Co., $1.50; Elementary Meteorology, Davis, Ginn and Co., $2.50; Bulletins from the United States Weather Bureau, Washington, D. C.

CHART FOR SCHOOL WEATHER-RECORDS.

THE STORY OF THE STARS

_Teacher’s Story_

“_Why did not somebody teach me the constellations and make me
at home in the starry heavens, which are always overhead,
and which I don’t half know to this day._”
--THOMAS CARLYLE.

For many reasons aside from the mere knowledge acquired, children should be taught to know something of the stars. It is an investment for future years; the stars are a constant reminder to us of the thousands of worlds outside our own, and looking at them intelligently, lifts us out of ourselves in wonder and admiration for the infinity of the universe, and serves to make our own cares and trials seem trivial. The author has not a wide knowledge of the stars; a dozen constellations were taught to her as a little child by her mother, who loved the sky as well as the earth; but perhaps nothing she has ever learned has been to her such a constant source of satisfaction and pleasure as this ability to call a few stars by the names they have borne since the men of ancient times first mapped the heavens. It has given her a sense of friendliness with the night sky, that can only be understood by those who have had a similar experience.

There are three ways in which the mysteries of the skies are made plain to us: First, by the telescope; second, by geometry, trigonometry and calculations--a proof that mathematics is even more of a heavenly than an earthly science; and third, by the use of the spectroscope, which can only be understood after we study physics. It is an instrument which tells us, by analyzing the light of the stars, what chemical elements compose them; and also, by the means of the light, it estimates the rate at which the stars are moving and the direction of their motion.

Thus, we have learned many things about the stars; we know that every shining star is a great blazing sun, and there is no reason to doubt that many of these suns have worlds, like the earth, spinning around them although, of course, so far away as to be invisible to us; for our world could not be seen at all from even the nearest star. We also know that many of the stars which seem single to us are really double--made up of two vast suns swinging around a common center; and although they may be millions of miles apart, they are so far away that they seem to us as one star. The telescope reveals many of these double stars and shows that they circle around their orbits in various periods of time, the most rapid making the circle in five years, another in sixteen years, another in forty-six years; while there is at least one lazy pair which seems to require fully sixteen hundred years to complete their circle. And the spectroscope has revealed to us that many of the stars which seem single through the largest telescope are really double, and some of these great suns race around each other in the period of a few hours, which is a rate of speed we could hardly imagine.

Astronomers have been able to measure the distance from us to many of the stars, but when this distance is expressed in miles it is too much for us to grasp. Thus, they have come to measure heavenly distance in terms of the rate at which light travels, which is 186,400 miles per second or about six trillions of miles per year; this distance is called a light-year. Light reaches us from the sun in about eight minutes, but it takes more than four years for a ray to reach us from the nearest star. It adds new interest to the Pole-star to know that the light which reaches our eyes left that star almost half a century ago, and that the light we get from the Pleiades may have started on its journey before America was discovered. Most of the stars are so far away that we cannot measure the distance.

Although the stars seem always to be in the same places, they are all moving through space just as our sun and its family are doing; but the stars are so far away that, although one may move a million miles a day, it would require many years of observation for us to detect that it moved at all. We know the rate at which some of the stars are moving but have no idea of their goal; nor do we have any idea where our sun is dragging us at the rate of nearly 800 miles per minute. It is thought that our sun and the other suns are whirling around some greater center or centers; but if so, the orbits are so many trillions of miles across that the suns all seem to be going somewhere in a straight line, each attending strictly to its own business.

Through the spectroscope we know something of the life of stars; we know that when they are young they are composed of thin gases and shine white or blue; and as they grow older, they become more solid and shine yellow, like our sun; and when older still, they grow red and are yet more condensed, like Bethelguese in Orion, which is an aged sun and which will, in time, grow cold and dark and invisible to us. The spectroscope reveals many dark stars whirling through space--vast, dead suns with their fires extinguished, never to be lighted again unless, in its swift course, one of them should hurl itself against another star with a fearful force which shall shatter it into gaseous atoms, and these be thrown into fierce spiral whirlpools, from which it shall again be fashioned into a white-hot sun and become a star in our sky.

The scientists are coming to understand a little of how the stars are made; for scattered through the skies are masses of misty light, called nebulæ, which means clouds; nebulæ are vast gaseous bodies composed of the stuff of which stars are made. Each nebula keeps its own special place in the heavens--just like a star, and is moving through space--like a star. The spectroscope shows that many of these shining, misty masses are made up of glowing gases, largely hydrogen; and many are disk-shaped, twisted into a spiral. There are grounds for believing that these spiral nebulæ are stars in the process of forming. Nebulæ are mostly telescopic, although two or three may be detected by the keen, unaided eye as a little blur of light, like that surrounding the third star of Orion’s sword. There are eight thousand or more nebulæ which have been discovered and mapped. Some idea of their tremendous size is given by Ball when speaking of the ring nebula of Lyra, which we cannot see with the naked eye, and yet if a railroad train started to cross its diameter at the middle, and went at the rate of a mile a minute, one thousand years would not complete the journey.

The number of stars that may be seen with the unaided eye, if one were to travel from the southern to the northern polar region, would be between six and seven thousand; but it would require very keen eyes to see two thousand at one time. With the help of the telescope, about eight hundred thousand stars have been discovered, classified and catalogued, while photography of the skies reveals millions. It is thought that the new international photographic chart, which shall cover all the space seen from our globe may show thirty millions of stars. The Milky Way or Galaxy, that great, white band across the heavens, is made up of stars which are so far away that we cannot see them, but see only their diffused light. It is well called a “River of Stars” flowing in a circle around our whole solar system; and, except during the spring months, one-half of it may be seen directly above us while the other half is hidden below us. The place of the Milky Way in the heavens seems fixed and eternal; any star within its borders is always seen at the same point. When the Northern Cross lifts itself toward the zenith we are able to see that, near that constellation, the star river divides into three streams with long, blue islands between.

_Reference books_--There are a large number of excellent text-books and popular books on astronomy. The following are a few which I have used most often: Astronomy for Everybody, Newcomb; Todd’s New Astronomy; The Friendly Stars, Martin; Starland, Ball; The Stars Through an Opera Glass, Serviss; Other Suns than Ours, Proctor; Other Worlds than Ours, Proctor.

_For children_--Earth and Sky, Holden; Stories of Starland, Proctor; The Children’s Book of Stars, G. E. Mitton; Story land of the Stars, Pratt; Stars in Song and Legend, Porter; The Planisphere, Thos. Whittaker.

HOW TO BEGIN STAR STUDY

THE POLE-STAR AND THE DIPPERS

_Teacher’s Story_

The way to begin star study is to learn to know the Big Dipper, and through its pointers to distinguish the Pole-star; for whenever we try to find any star we have to find the Big Dipper and Pole-star first so as to have some fixed point to start from. There are four stars in the bowl of the Big Dipper and three in the curved handle. A line drawn through the outer two stars of the bowl, if extended, will touch the North Star, or Pole-star. It is very important for us to know the Pole-star, because the northern end of the earth’s axis is directed toward it, and it is therefore situated in the heavens almost directly above our North Pole. For those of us who live in the northern Hemisphere, the North Star never sets, but is always in our sky. Of course, the North Star has nothing to do with the axis of our earth any more than the figure on the blackboard has to do with the pointer; it simply happens to lie in the direction toward which the northern end of the earth’s axis points. In the southern skies, there is no convenient star which lies directly above the South Pole, so there is no South Pole-star. It is also a coincidence that the needle of the mariner’s compass points toward the North Star; the earth being a large magnet exercises its influence on all substances which can be magnetized, and since the poles of our great earth-magnet are nearly in line with the poles of the earth’s axis, the magnetic needle naturally points north and south, and the North Star chances to be nearly in the direction toward which the northern end of the compass needle points.

The Pole-star cannot be seen from the southern hemisphere; but if we should start from Florida, on a journey toward Baffin’s Bay, we should discover that each night this star would seem higher in the sky. And if we should succeed in reaching the North Pole, we would find the Pole-star directly over our heads, and what a wonderful sight the stars would be from this point! For none of the stars which we could see would rise or set, but would move around us in circles parallel to the horizon.

The Big Dipper points towards the Pole-star, and to us seems to revolve around it every twenty-four hours but, of course, this appearance is caused by the fact that we ourselves are revolving from west to east. Therefore, the stars seems to revolve from west to east under the Pole-star and from east to west above it, or in exactly the opposite direction in which the hands of a clock turn. Owing to the movement of the earth in its orbit, the Big Dipper and all the other stars arrive at a certain point in our sky four minutes earlier each day or about two hours earlier each month; thus, the Big Dipper is east of the Pole-star with handle down in the evenings of January, while at the same time of night in July, it is west of the Pole-star with the handle up. But the time of year that a certain star reaches a certain point is so invariable, that if we know star time, or sidereal time as it is called, we can tell just what hour of the night it is when a star passes this point. Thus, the Big Dipper and the other polar constellations are the night clock of the sailors of the northern hemisphere; for though this great polar clock has its hands moving around the wrong way, it gains time with such regularity that anyone who understands is able to compute exact time by it.

The Little Dipper lies much nearer the Pole-star than does the Big Dipper; in fact, the Pole-star itself is the end of the handle of the Little Dipper. Besides the Pole-star, there are two more stars in the handle of the Little Dipper, and of the four stars which make the bowl, the two that form the outer edge are much the brighter. The bowl of the Little Dipper is above or below the Pole-star according to the hour of the evening, or the night of the year, for it apparently revolves about the Pole-star as does the Big Dipper. The two Dippers open toward each other, and some one said “they pour into each other.”

The Big Dipper is a part of a constellation called _Ursa Major_, the Great Bear; and the Little Dipper is the Little Bear, the handle of the dipper being the bear’s tail.

There is an ancient myth telling the story of the Big and Little Bears: A beautiful mother called Callisto had a little son whom she named Arcas. Callisto was so beautiful that she awakened the anger of Juno, who changed her to a bear; and when her son grew up he became a hunter, and one day would have killed his transformed mother; but Jupiter seeing the danger of this crime caught the two up into the heavens, and set them there as shining stars. But Juno was still vindictive, so she wrought a spell which never allowed these stars to rise and set like other stars, but kept them always moving around and around.

_References_--The Friendly Stars by Martin is a most delightful book and at the same time gives explicit directions for finding the stars and much interesting information concerning them. The planisphere is a little chart with a mechanical device which enables us to find what stars are in sight every night of the year, or at any time of night. It is published by Thos. Whittaker, Bible House, New York, and costs seventy-five cents.

LESSON CCXXV

THE TWO DIPPERS

_Leading thought_--The North Star or Pole-star may always be found by the stars known as the pointers in the Big Dipper; the stars of the Big Dipper seem to revolve around the Pole-star once in twenty-four hours.

_Method_--The time to begin these observations is when the moon is in its last quarter, so that the moonlight will not make pale the stars in early evening. Draw upon the blackboard, from the chart shown on page 890, the Big Dipper and the Pole-star, with a line extending through the pointers. Say to the pupils that this Big Dipper is above or below or at one side of the Pole-star, and that you wish them to observe for themselves where it is and tell you about it the next day. After they surely know the Big Dipper, ask the following questions:

_Observations_--1. Can you find the Big Dipper among the stars?

2. Is it in the north, south, east or west?

3. Which stars are the “pointers” in the dipper, and why are they called so?

4. Make a drawing showing how you can always find the Pole-star, if you can see the Big Dipper.

5. How many stars make the bowl of the Dipper?

6. How many stars in the handle?

7. Is the handle straight or is it curved?

8. Does the Big Dipper open toward the Pole-star, or away from it?

9. Is it above or below the Pole-star at eight o’clock in the evening, or at the right or the left of it?

10. Does the Big Dipper remain in the same direction from the Pole-star all night? Look at it at seven o’clock and again at nine o’clock and see if it has changed position?

11. Do you think it moves around the Pole-star once every twenty-four hours? In which direction? How could you tell the time of night by the Big Dipper and the Pole-star?

12. Does the Big Dipper ever rise and set?

13. The Big Dipper is also called the Great Bear. Can you find the stars which make the bear’s head and front legs?

After the pupils surely know the Big Dipper and Pole-star draw the complete diagram upon the board to show the Little Dipper and where it may be found, and call attention to the fact that the end of the Little Dipper’s handle is the Pole-star itself and that its bowl is not flaring, like that of the Big Dipper and that the two pour into each other. Let the pupils find the Little Dipper in the sky for themselves and ask the following questions:

_Observations_--14. Is the Little Dipper nearer or farther from the Pole-star than the Big Dipper?

15. How many stars in the handle of the Little Dipper?

16. How many stars make the bowl of the Little Dipper? Which of these stars are the brightest? Is the bowl of the Little Dipper above or below the Pole-star?

17. Does the Little Dipper extend in the same direction in relation to the Pole-star all night?

18. Make observations on the relation to each other of the two dippers at eight o’clock in the evening of January, February, March and April.

After the above lessons are well learned, give the following questions, and try to have the pupils answer by thinking:

_Questions about Polaris_ (_the North Star_) _for the pupils to think about and answer_:

19. How many names has the Pole-star? Can the Pole-star be seen from the southern hemisphere? If not, why not?

20. If you should start from southern Florida and travel straight north, how would the Pole-star seem to change position each succeeding night?

21. If you could stand at the North Pole, where would the Pole-star seem to be?

22. If you were at the North Pole, would any of the stars rise and set? In what direction would the stars seem to move and why?

23. How does the North Star help the sailors to navigate the seas and why?

24. How do astronomers reckon distances between us and the stars? What is a light-year?

_Topics for English lesson_--(_a_) What a star is. (_b_) What a constellation is. (_c_) How the stars and constellations received their names in ancient times. In ancient times the Big and Little Dippers were named the Big and Little Bears, and that is their Latin name to this day. Write a story about what the ancient Greeks told about these Bears and how they came to be in the sky.

_Supplementary reading_--Stories of Starland, Proctor, pp. 117–121; Storyland of the Stars, Pratt, p. 75; Child’s Study of the Classics, p. 33.

CASSIOPEIA’S CHAIR, CEPHEUS, AND THE DRAGON

_Teacher’s Story_

There are other constellations besides the two Dippers, which never rise and set in this latitude, because they are so near to the Pole-star that, when revolving around it, they do not fall below the horizon. There is one very brilliant star, called Capella, which almost belongs to the polar constellations but not quite, for it is far enough away from Polaris to dip below the horizon for four hours of the twenty-four.

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Handbook of nature-study for teachers and parentsChapter XLIX: Part IV: Earth and Sky (4)

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