A Text-Book of Astronomy.

by George C. Comstock.

PREFACE

The present work is not a compendium of astronomy or an outline course of popular reading in that science. It has been prepared as a text-book, and the author has purposely omitted from it much matter interesting as well as important to a complete view of the science, and has endeavored to concentrate attention upon those parts of the subject that possess special educational value. From this point of view matter which permits of experimental treatment with simple apparatus is of peculiar value and is given a prominence in the text beyond its just due in a well-balanced exposition of the elements of astronomy, while topics, such as the results of spectrum a.n.a.lysis, which depend upon elaborate apparatus, are in the experimental part of the work accorded much less s.p.a.ce than their intrinsic importance would justify.

Teacher and student are alike urged to magnify the observational side of the subject and to strive to obtain in their work the maximum degree of precision of which their apparatus is capable. The instruments required are few and easily obtained. With exception of a watch and a protractor, all of the apparatus needed may be built by any one of fair mechanical talent who will follow the ill.u.s.trations and descriptions of the text.

In order that proper opportunity for observations may be had, the study should be pursued during the milder portion of the year, between April and November in northern lat.i.tudes, using clear weather for a direct study of the sky and cloudy days for book work.

The ill.u.s.trations contained in the present work are worthy of as careful study as is the text, and many of them are intended as an aid to experimental work and accurate measurement, e. g., the star maps, the diagrams of the planetary orbits, pictures of the moon, sun, etc. If the school possesses a projection lantern, a set of astronomical slides to be used in connection with it may be made of great advantage, if the pictures are studied as an auxiliary to Nature. Mere display and scenic effect are of little value.

A brief bibliography of popular literature upon astronomy may be found at the end of this book, and it will be well if at least a part of these works can be placed in the school library and systematically used for supplementary reading. An added interest may be given to the study if one or more of the popular periodicals which deal with astronomy are taken regularly by the school and kept within easy reach of the students. From time to time the teacher may well a.s.sign topics treated in these periodicals to be read by individual students and presented to the cla.s.s in the form of an essay.

The author is under obligations to many of his professional friends who have contributed ill.u.s.trative matter for his text, and his thanks are in an especial manner due to the editors of the Astrophysical Journal, Astronomy and Astrophysics, and Popular Astronomy for permission to reproduce here plates which have appeared in those periodicals, and to Dr. Charles Boynton, who has kindly read and criticised the proofs.

GEORGE C. COMSTOCK.

UNIVERSITY OF WISCONSIN, _February, 1901_.

ASTRONOMY

CHAPTER I

DIFFERENT KINDS OF MEASUREMENT

1. ACCURATE MEASUREMENT.--Accurate measurement is the foundation of exact science, and at the very beginning of his study in astronomy the student should learn something of the astronomer's kind of measurement.

He should practice measuring the stars with all possible care, and should seek to attain the most accurate results of which his instruments and apparatus are capable. The ordinary affairs of life furnish abundant ill.u.s.tration of some of these measurements, such as finding the length of a board in inches or the weight of a load of coal in pounds and measurements of both length and weight are of importance in astronomy, but of far greater astronomical importance than these are the measurement of angles and the measurement of time. A kitchen clock or a cheap watch is usually thought of as a machine to tell the "time of day," but it may be used to time a horse or a bicycler upon a race course, and then it becomes an instrument to measure the amount of time required for covering the length of the course. Astronomers use a clock in both of these ways--to tell the time at which something happens or is done, and to measure the amount of time required for something; and in using a clock for either purpose the student should learn to take the time from it to the nearest second or better, if it has a seconds hand, or to a small fraction of a minute, by estimating the position of the minute hand between the minute marks on the dial. Estimate the fraction in tenths of a minute, not in halves or quarters.

EXERCISE 1.--If several watches are available, let one person tap sharply upon a desk with a pencil and let each of the others note the time by the minute hand to the nearest tenth of a minute and record the observations as follows:

2h. 44.5m. First tap. 2h. 46.4m. 1.9m.

2h. 44.9m. Second tap. 2h. 46.7m. 1.8m.

2h. 46.6m. Third tap. 2h. 48.6m. 2.0m.

The letters h and m are used as abbreviations for hour and minute. The first and second columns of the table are the record made by one student, and second and third the record made by another. After all the observations have been made and recorded they should be brought together and compared by taking the differences between the times recorded for each tap, as is shown in the last column. This difference shows how much faster one watch is than the other, and the agreement or disagreement of these differences shows the degree of accuracy of the observations. Keep up this practice until tenths of a minute can be estimated with fair precision.

2. ANGLES AND THEIR USE.--An angle is the amount of opening or difference of direction between two lines that cross each other. At twelve o'clock the hour and minute hand of a watch point in the same direction and the angle between them is zero. At one o'clock the minute hand is again at XII, but the hour hand has moved to I, one twelfth part of the circ.u.mference of the dial, and the angle between the hands is one twelfth of a circ.u.mference. It is customary to imagine the circ.u.mference of a dial to be cut up into 360 equal parts--i. e., each minute s.p.a.ce of an ordinary dial to be subdivided into six equal parts, each of which is called a degree, and the measurement of an angle consists in finding how many of these degrees are included in the opening between its sides.

At one o'clock the angle between the hands of a watch is thirty degrees, which is usually written 30, at three o'clock it is 90, at six o'clock 180, etc.

A watch may be used to measure angles. How? But a more convenient instrument is the protractor, which is shown in Fig. 1, applied to the angle _A B C_ and showing that _A B C_ = 85 as nearly as the protractor scale can be read.

The student should have and use a protractor, such as is furnished with this book, for the numerous exercises which are to follow.

[Ill.u.s.tration: FIG. 1.--A protractor.]

EXERCISE 2.--Draw neatly a triangle with sides about 100 millimeters long, measure each of its angles and take their sum. No matter what may be the shape of the triangle, this sum should be very nearly 180--exactly 180 if the work were perfect--but perfection can seldom be attained and one of the first lessons to be learned in any science which deals with measurement is, that however careful we may be in our work some minute error will cling to it and our results can be only approximately correct. This, however, should not be taken as an excuse for careless work, but rather as a stimulus to extra effort in order that the unavoidable errors may be made as small as possible. In the present case the measured angles may be improved a little by adding (algebraically) to each of them one third of the amount by which their sum falls short of 180, as in the following example:

Measured angles. Correction. Corrected angles.

A 73.4 + 0.1 73.5 B 49.3 + 0.1 49.4 C 57.0 + 0.1 57.1 ----- ----- Sum 179.7 180.0 Defect + 0.3

This process is in very common use among astronomers, and is called "adjusting" the observations.

[Ill.u.s.tration: FIG. 2.--Triangulation.]

3. TRIANGLES.--The instruments used by astronomers for the measurement of angles are usually provided with a telescope, which may be pointed at different objects, and with a scale, like that of the protractor, to measure the angle through which the telescope is turned in pa.s.sing from one object to another. In this way it is possible to measure the angle between lines drawn from the instrument to two distant objects, such as two church steeples or the sun and moon, and this is usually called the angle between the objects. By measuring angles in this way it is possible to determine the distance to an inaccessible point, as shown in Fig. 2. A surveyor at _A_ desires to know the distance to _C_, on the opposite side of a river which he can not cross. He measures with a tape line along his own side of the stream the distance _A B_ = 100 yards and then, with a suitable instrument, measures the angle at _A_ between the points _C_ and _B_, and the angle at _B_ between _C_ and _A_, finding _B A C_ = 73.4, _A B C_ = 49.3. To determine the distance _A C_ he draws upon paper a line 100 millimeters long, and marks the ends _a_ and _b_; with a protractor he constructs at _a_ the angle _b a c_ = 73.4, and at _b_ the angle _a b c_ = 49.3, and marks by _c_ the point where the two lines thus drawn meet. With the millimeter scale he now measures the distance _a c_ = 90.2 millimeters, which determines the distance _A C_ across the river to be 90.2 yards, since the triangle on paper has been made similar to the one across the river, and millimeters on the one correspond to yards on the other. What is the proposition of geometry upon which this depends? The measured distance _A B_ in the surveyor's problem is called a base line.

EXERCISE 3.--With a foot rule and a protractor measure a base line and the angles necessary to determine the length of the schoolroom. After the length has been thus found, measure it directly with the foot rule and compare the measured length with the one found from the angles. If any part of the work has been carelessly done, the student need not expect the results to agree.

[Ill.u.s.tration: FIG. 3.--Finding the moon's distance from the earth.]

In the same manner, by sighting at the moon from widely different parts of the earth, as in Fig. 3, the moon's distance from us is found to be about a quarter of a million miles. What is the base line in this case?

4. THE HORIZON--ALt.i.tUDES.--In their observations astronomers and sailors make much use of the _plane of the horizon_, and practically any flat and level surface, such as that of a smooth pond, may be regarded as a part of this plane and used as such. A very common observation relating to the plane of the horizon is called "taking the sun's alt.i.tude," and consists in measuring the angle between the sun's rays and the plane of the horizon upon which they fall. This angle between a line and a plane appears slightly different from the angle between two lines, but is really the same thing, since it means the angle between the sun's rays and a line drawn in the plane of the horizon toward the point directly under the sun. Compare this with the definition given in the geographies, "The lat.i.tude of a point on the earth's surface is its angular distance north or south of the equator," and note that the lat.i.tude is the angle between the plane of the equator and a line drawn from the earth's center to the given point on its surface.

A convenient method of obtaining a part of the plane of the horizon for use in observation is as follows: Place a slate or a pane of gla.s.s upon a table in the sunshine. Slightly moisten its whole surface and then pour a little more water upon it near the center. If the water runs toward one side, thrust the edge of a thin wooden wedge under this side and block it up until the water shows no tendency to run one way rather than another; it is then level and a part of the plane of the horizon.

Get several wedges ready before commencing the experiment. After they have been properly placed, drive a pin or tack behind each one so that it may not slip.

5. TAKING THE SUN'S ALt.i.tUDE. EXERCISE 4.--Prepare a piece of board 20 centimeters, or more, square, planed smooth on one face and one edge.

Drive a pin perpendicularly into the face of the board, near the middle of the planed edge. Set the board on edge on the horizon plane and turn it edgewise toward the sun so that a shadow of the pin is cast on the plane. Stick another pin into the board, near its upper edge, so that its shadow shall fall exactly upon the shadow of the first pin, and with a watch or clock observe the time at which the two shadows coincide.

Without lifting the board from the plane, turn it around so that the opposite edge is directed toward the sun and set a third pin just as the second one was placed, and again take the time. Remove the pins and draw fine pencil lines, connecting the holes, as shown in Fig. 4, and with the protractor measure the angle thus marked. The student who has studied elementary geometry should be able to demonstrate that at the mean of the two recorded times the sun's alt.i.tude was equal to one half of the angle measured in the figure.

[Ill.u.s.tration: FIG. 4.--Taking the sun's alt.i.tude.]

When the board is turned edgewise toward the sun so that its shadow is as thin as possible, rule a pencil line alongside it on the horizon plane. The angle which this line makes with a line pointing due south is called the sun's _azimuth_. When the sun is south, its azimuth is zero; when west, it is 90; when east, 270, etc.

EXERCISE 5.--Let a number of different students take the sun's alt.i.tude during both the morning and afternoon session and note the time of each observation, to the nearest minute. Verify the setting of the plane of the horizon from time to time, to make sure that no change has occurred in it.

6. GRAPHICAL REPRESENTATIONS.--Make a graph (drawing) of all the observations, similar to Fig. 5, and find by bisecting a set of chords _g_ to _g_, _e_ to _e_, _d_ to _d_, drawn parallel to _B B_, the time at which the sun's alt.i.tude was greatest. In Fig. 5 we see from the intersection of _M M_ with _B B_ that this time was 11h. 50m.

The method of graphs which is here introduced is of great importance in physical science, and the student should carefully observe in Fig. 5 that the line _B B_ is a scale of times, which may be made long or short, provided only the intervals between consecutive hours 9 to 10, 10 to 11, 11 to 12, etc., are equal. The distance of each little circle from _B B_ is taken proportional to the sun's alt.i.tude, and may be upon any desired scale--e. g., a millimeter to a degree--provided the same scale is used for all observations. Each circle is placed accurately over that part of the base line which corresponds to the time at which the alt.i.tude was taken. Square ruled paper is very convenient, although not necessary, for such diagrams. It is especially to be noted that from the few observations which are represented in the figure a smooth curve has been drawn through the circles which represent the sun's alt.i.tude, and this curve shows the alt.i.tude of the sun at every moment between 9 A. M. and 3 P. M. In Fig. 5 the sun's alt.i.tude at noon was 57. What was it at half past two?

[Ill.u.s.tration: FIG. 5.--A graph of the sun's alt.i.tude.]

7. DIAMETER OF A DISTANT OBJECT.--By sighting over a protractor, measure the angle between imaginary lines drawn from it to the opposite sides of a window. Carry the protractor farther away from the window and repeat the experiment, to see how much the angle changes. The angle thus measured is called "the angle subtended" by the window at the place where the measurement was made. If this place was squarely in front of the window we may draw upon paper an angle equal to the measured one and lay off from the vertex along its sides a distance proportional to the distance of the window--e. g., a millimeter for each centimeter of real distance. If a cross line be now drawn connecting the points thus found, its length will be proportional to the width of the window, and the width may be read off to scale, a centimeter for every millimeter in the length of the cross line.

The astronomer who measures with an appropriate instrument the angle subtended by the moon may in an entirely similar manner find the moon's diameter and has, in fact, found it to be 2,163 miles. Can the same method be used to find the diameter of the sun? A planet? The earth?