Astronomy

Preface

A great many people regard Astronomy as an intensely interesting- study ; yet allow their years to pass by without endeavoring to go beyond the phase of helpless wonder at the glories of the heavens. They would like to know something about the stars ; but do not try, simply because they consider the subject to be beyond their reach, or at best, to be grasped only by dint of years of patient study under a master in the science.

It is true, there is much more than enough material in the heavens for the study of any one lifetime. The phenomena of star motion present problems which tax the most powerful intellect, and some that undoubtedly lie above and beyond the ordinary comprehension. But the broadest facts are within easy mental reach ; and the fundamental principles of the science are not difficult to understand, as the result of a few hours patient reading. A few more hours will suffice to gain a " speaking acquaintance " with the more prominent stars — to recognize them when we see them in the sky, and to follow their changes of apparent position from hour to hour, and from month to month. This immense addition to the mental furniture may be (5)

6 PREFACE.

gained with no greater effort of the mind, and little more physical exertion, than is required to read an ordinary novel. To proceed only so far will be to do well ; but most of those who have taken this first step are agreeably surprised to see how easily the rest opens out before them, and find genuine recreation in exploring what had previously seemed to be impenetrable mysteries.

A careful perusal of this little book will enable any person of ordinary intelligence to understand the basic facts of modern astronomy ; and it will form a good foundation on which to build up a more comprehensive knowledge of the science, if desired.

Chicago, July 1st, 1882. E. 0.

C0IS T TEIS T TS.

Chapter I. Elementary Ideas

Apparent Motions of the Stars, p. 9— Rotation of the Earth, 11 — Its Form. 12 — Latitude and Longitude, Time, 15 — The Atmosphere, 20— Refraction and Twilight, 21.

Chapter II. The Sun

Apparent Motion of the Sun among the Stars, p. 23 — Equinoxes, 25 — Right Ascension and Declination, 27 — Solar and Sidereal Time, 28 — Equation of Time, 30 — Real Movement of the Earth, 31— The Seasons, 33— Sun Spots, 38— Rotation of the Sun, 39 — The Prominences and Corona, 40.

Chapter III. The Moon

Motion of the Moon. p. 44 — Her Phases, 47 — Eclipses, 50 — Harvest Moon, 54 — Distance and Size of the Moon, 55 — Her Path about the Earth and the Sun, 58 — Rotation, 59 — Appearance of her Surface, Craters, Mountains, Plains, etc., 60 o

Chapter IV. The Planets

Apparent Motions with reference to the Sun of Inferior Planets, p. 62 — of Superior Planets, 66 — Apparent Motions with reference to the Stars, 69 — Transits of Mercury and Venus, 72 — Th^ir use for finding the Distance of the Sun, 73 — Other Methods, 77— Kepler's Laws, 80— Gravitation, 82— Tides. 84 — Masses of the Sun and Moon compared with that of the

3 CONTEXTS.

Earth, 86— Mass of the Earth. 88— Bode's Law of Distances, 89 — Appearance of the different Planets, 90 — Conditions to which they are severally exposed, 95.

CHAPTEK V.

COMETS.

Apparent and Real Motions, p. 96— Their Aspects, 99— Their Physical Constitution. 100— Meteors. 101 — Remarkable Comets, 105 — Periodical Comets. 108 — Origin of Comets, 111.

CHAPTEK VI.

THE S-TARS.

Their Number. Magnitudes, p. 115 — The Constellations, 118 — The Milky Way r 123 — Clusters and Nebulae, Ti-*'> — Laplace's Nebular Theory, 126 — Distances of some of the Stars. 128—- Proper Motions, 130 — Motion of the Solar System. 190 — Double Stars, 131 — Physical Constitution of Stars : 133— Temporary and Variable Stars, 134 — Precession and Nutation of the Equinoxes, 135 — Signs of the Zodiac, 136.

Elements of Sun and Moon, 137.

Elements of the Principal Planets. 138.

Elements of Periodical Comets, 139.

ELEMENTARY ASTRONOMY.

Chapter I

The most casual observation suffices to show us that the sun rises in the east, gets gradually higher till it reaches its greatest elevation in the south,* when it begins to sink, finally setting in the west only to rise again in the east and perform the same round next day.

If we turn to the stars at night we shall find something similar, but on examining more closely it will be seen that some of them are longer above the horizon than others, and that certain stars never set but remain constantly in view. If we watch these latter we shall find that there is one star which hardly changes its position, keeping constantly at nearly the same height, whilst the other stars appear to circulate round it at different distances. This star is called the pole-star, for a reason which will presently be apparent. Now amidst all this diversity of movement there is one feature which is common to all; it is that every star returns to the same position after the lapse of twenty-four hours, and in this interval it will have described a circle. This latter fact may be rudely verified as follows : — Joint two rods together like a pair of compasses, and rest one of them in forks so placed that it

*The reader is supposed to be in the United States, or other northern countries. For southern countries it will be sufficient to substitute south for north?

10 APPARENT MOTIONS OF THE STARS.

points to the pole-star; then if the other rod be pointed to any other star it may be made to follow the star in its course by simply turning the first rod round in its forks, without altering the opening of the joint.

The motion will be thus like that of a pair of compasses, one leg of which is kept in an upright position as it turns. The second leg will, under these circumstances, describe a circle, as may be seen by running the upright leg through a sheet of cardboard (or stiff paper )held horizontally at such a height that the other leg, as it sweeps round, just touches it.

If we look a little more closely we shall find that the fixed rod will not continue to point exactly to the pole-star all through its course, and that if we direct the rod in the forks to a point which is mid- way between the highest and lowest and the most easterly and westerly positions of the pole-star, we shall get a point about which this star ( as well as all others ) describes a circle, the other leg of our compasses following it exactly in its motion by simply turning about the fixed leg* This point is called the pole of the heavens, and the daily motion of the stars may be represented almost exactly by supposing them (as the Ancients did) to be fixed in a hollow globe which spins once in twenty-four hours round an axis passing through the pole, so that the stars would in that time describe circles of different sizes but all having their centres at the pole. But though this represents the movements of the stars, such a crystal sphere can have no real existence, for we shall presently see that the sun is more than ninety millions of miles off, and the nearest of the stars are hundreds of thousands of times that tremendous distance from us, and a material

* A telescope mounted on one of the leers of our compasses so as to follow any star in its daily motion by simply turning about the other leg ( which is then called the polar axis ) is termed an equatorial. The turning of the axis maybe effected by means of clock-work, and the telescope will then continue to point to the star for a leng:h of time, during which it can be steadily gazed at.

BOTATION OF THE EARTH. 11

sphere of that size spinning round once a day is of course quite out of the question. We are thus led to conclude that the diurnal motions of the stars cannot be real, but are only apparent, arising from our own movement in an opposite direction, just as when we spin rapidly round in one direction, the chairs, tables, and other objects in the room all appear to turn round at the same rate but the opposite way, the part of the ceiling directly overhead being the only point which appears stationary, just like the pole of the heavens. And this gives us some clue as to what our real movement and that of the earth, on which we stand, must be. The earth must be spinning round an imaginary axis, pointing nearly in the direction of the pole-star, and carrying us and all terrestrial objects round with it once in twenty-four hours. But if we accept this explanation, how will the shape of the earth accord with such amotion ? The portion of the earth which we can see at any one time appears a flat (or undulating) plain, bounded by a line — the horizon — where the sky seems to meet it; but we must not hastily conclude from this that the whole earth is a flat plain, for what we see is only a perspective view of a very small portion. If we look out over an expanse of sea (which is free from the irregularities of the land) it will be easy to assure ourselves that it is not really a plane,* for ships appear to sink lower and lower as they move away from us, so that first the hull and then the lower sails are cut off by the sea between us and the ship, whilst on ascending a cliff the lower sails and hull again come into sight. This shows that the surface of the sea is really curved (convex), for we know that objects disappear behind an undulation of the ground in exactly the same way, and that by ascending a hill they come into view again as

* A plane is a flat surface, like an extremely thin sheet of

Saper, or the surface of a table, but of unlimited extent. Thus le plane of a circle is the flat surface in which the circle lies, boundless in every direction.

12 FORM OF THE EARTH.

soon as the visual line from the object to our eye clears the top of the swell in the ground.

There is another circumstance which now helps us materially in Ending the shape of the earth. Wherever we are, whether on the top of a mountain or on board a ship at sea, the portion of the earth which we can see, always appears a circle ; though when we are at a considerable height, this circle seems much smaller, whilst we really see a larger portion of the earth's surface. Now the only figure with which we are acquainted that appears circular from all points of view and at all distances, is a globe; and further, we knowthat this seems to grow smaller as we remove the eye from near its surface, whilst the actual portion of it which we see is really larger. This may readily be verified by means of a terrestrial globe or a large ball, but it must be remembered that in the actual case the distance from the earth's surface to which we can get would be represented on an 18-inch globe by about the thickness of a sheet of paper. From all this we infer that the earth is a globe, which rotates about an axis nearly in the direction of the pole-star, and this conclusion is supported by what is observed of the motions of the stars in other countries. Any one who makes a voyage to southern latitudes will observe that the pole about which the stars revolve, while keeping the same position near the pole-star, appears to sink as he journeys south, in consequence of which those stars, which "are only just visible in these climes in the southern horizon, get higher and higher, and new constellations come into sight. This goes on till the pole-star sinks in the northern horizon, when it is seen that there are really two poles about which the heavens appear to turn, one on the northern and one on tl e southern horizon; from which it is clear that the direction of the axis of motion is in the horizontal plane. The point of the earth at which this is the case is said to be in the equator, because it is equidistant from the two poles of the earth. The celestial equator is a

ITS POLES. 13

great circle,* of which the plane is parallel to the earth's equator, and which, therefore, lies half-way between the poles of the heavens. It is advisable here to distinguish clearly between the pole of the heavens and the pole of the earth. The former is simply the direction of the line about which the stars appear to turn, or about which the earth really turns, whilst the latter is the extremity of the axis through the earth's centre about which it turns, and it has, therefore, a definite position on the earth as well as a definite direction in space. As there are two ends to a straight line, there are of course two poles, which are distinguished as north and south. To return to our traveller. After crossing the equator he will find that the southern pole (though it is not distinguished by any bright star near it like the pole-star) rises higher and higher till it reaches the same height above the southern horizon as the north pole had originally above the northern, when the traveller has traversed the same distance south of the equator as he had previously done before reaching it ; and he will fnrther have remarked that the north pole sinks, or the south pole rises, through an angle of one degreef for every sixty-nine miles he travels south.

All this is exactly what we should expect if the earth be a globe spinning about an axis, for in this case this axis, being in the direction of the pole of the heavens, would remain fixed among the stars, whilst the horizon (which is the direction of the surface where the spectator is) would change as the observer moved, so that instead of the pole-star approaching the horizon, it is really the horizon which approaches the

* A great circle of a sphere (or globe) is one of which the plane passes through the centre of the sphere, dividing it into two equal parts or hemispheres. A small circle divides the sphere into two unequal parts.

t A right angle, which is the angle of a square or that formed by two perpendicular lines, is divided into ninety equal parts, called degrees, ninety degrees being usually written thus 90°.

14 SIZE OF THE EARTH.

direction of the pole-star. At the equator the pole-star will be in the horizon, and will rise one degree for every 69 miles we travel north (owing to the tilting of the horizon), until at the pole, after having travelled over 6,200 miles, it would be 90° high, or in the zenith.* Hence the circumference of the earth, which is four times the distance from the equator to either pole, is 25,000 miles about, and the diameter 7,900 miles, the .circumference of a circle being about 3 1-7 times its diameter (more exactly 3*1416). Strictly speaking the earth is not a perfect globe, but more like an orange, bulging out slightly at the equator, a result of its spinning round so rapidly, which tends to throw the particles at the equator off, like mud from a rapidly moving carriage wheel. The amount of this bulging is however very small, being only 13^ miles. As a point on the earth's equator moves round in a circle of the above size in one day, it must be moving at the rate of over 1,000 miles an hour, or 1,500 feet a second, which is more than the speed of a cannon ball. This motion is so smooth, however, that we feel nothing of it, as all the objects round us on the earth (including the atmosphere) move together with ourselves. But if it were not for the attraction of the vast mass of the earth, which pulls all bodies towards its centre (making a stone thrown up in the air fall back to the surface), we should fly off just as a stone whirled round in a sling does when the pull of the string is let go. It may seem very difficult to believe that we can really be moving at such an enormous speed, but unless we admit this motion we must suppose the stars to be swung around us with a velocity many million times greater; and, besides this, there is direct proof that the earth is moving in the way we have explained. This is given by the gyroscope, which is nothing but a top with a heavy disc, suspended in gimbals, so that it is free to turn in any direction. When this is set spinning rapidly, it requires great force to twist it out

* The point directly overhead.

LATITUDE— TIME. 15

of its direction, as any one may readily experience with the ordinary toy gyroscopes. If left to itself, then, we know that its axis will continue to point in the same direction, and as it appears to follow a star, to which it is once pointed, in its daily course from rising to setting, we may conclude that the star is really fixed, and the motion which it appears to have in common with the gyroscope is really caused by the spinning of the earth about its axis once in every twenty-four hours.

From what precedes, we see that the elevation of the pole measures the distance of the observer from the equator; and this is called the latitude of the place, north or south, as the north or south pole of the heavens is elevated. This latitude is measured in degrees, the distance from either pole to the equator being ninety degrees, or in other words, the latitude of the pole is 90°. The equator being the line formed by all points which are equidistant from the two poles, will be a circle, and, further, all places which have the same latitude, being at the same distance from the pole, will lie on a circle parallel to the equator; this is called a circle of latitude. Thus when we say that a place has a certain latitude, we fix its position so far as to settle that it lies somewhere on a certain circle, which is determined by measuring the elevation of the pole. This is readily done; for the pole lies midway between the highest and lowest positions of the pole-star, being at the centre of the circle w T hich this star describes, and it is only necessary to measure the angular elevations above the horizon of this star at its highest and lowest points by means of a divided circle. The same method would answer equally well if applied to any other star tolerably near the pole. But to fix the position of any place on the globe it is necessary to know whereabouts on the circle of latitude it lies, and this is practically a more difficult matter, and one that requires for its determination, astronomically, the introduction of a new element — time. For the measurement of time uniform motion is required, and

16 TIME— THE MERIDIAN.

this condition is secured more or less perfectly in the motion of the hands of a clock under the control of a pendulum, or of a watch or chronometer regulated by the balance and its spring ; but the rotation of the earth supplies us with the only perfect timekeeper with which we are acquainted, and the apparent movement of the stars resulting from it, really serves to regulate all our clocks and watches. In fact the heavens may, for this purpose, be compared to one of those lamp clocks in which the globe turns round and brings the hours painted on it up to an index. The stars represent the hours and minutes, though not placed at regular intervals : but where are we to find the index which is to tell us the time? We must look for this in the movements of the stars themselves. We saw that they describe circles about the pole, so that their paths are tilted with reference to the horizon ; the highest point of the star's course will evidently be a convenient point to which to fix our index, and a little consideration will show that the highest point of each circle will be where it meets a vertical plane through the north and south points (or in other words, through, the zenith and pole): for the leg of the compasses, of which we have spoken before, and which we suppose to follow the course of any star, will evidently reach its highest point when the plane formed by the two legs (one of wdiich points to the pole) is vertical. This plane then, which is called the meridian, may be taken as our index. Though this is only an imaginary index, it can be used as readily as if it really existed in the heavens, by simply mounting a telescope on trunnions, hke a gun, and making it swing vertically, north and south, so as, at the proper elevation, to point exactly to the pole.* When such a telescope points exactly to any

* A Telescope so mounted is called a transit instrument. For greater accuracy it is provided with cross wires lor spider threads) fixed at the place where the image is formed, bo ii- t" mark the centre of the field. These cross wires being Been .it the same time as the star, enable us to judge when the telescope is pointed exactly to it.

LONGITUDE. 17

star, we know that the star has come up to the Index, and when it next comes up to this position we know that exactly twenty four hours have elapsed, and if our clock does not indicate the same time as before, we know at once that it has gained or lost so much in the day. It is thus easy to find how our clock is going ; but to tell the time at any instant it is necessary to know the intervals at which the stars are placed on our celestial clock, and this can best be done by noting with our clock the difference of times between the arrival of pairs of stars at the index (their meridian passage or transit as it is called) and correcting this for the proportional gain or loss of the clock in the interval, on the assumption that it has gone uniformly for twenty-four hours; by repeating such observations a great many times and taking the average, the irregularities of the clock will, on the whole, nearly balance, and an accurate measure of the interval of transit of each pair of stars be arrived at. Thus taking any particular star as our starting point for h , Ave can easily find the time at any other part of the twenty- four hours. But it is more convenient for purposes of ordinary life to take the sun as our starting point, though on account of his irregular apparent motion among the stars (which will be discussed more fully in the next chapter), he does not make quite such a good clock as the stars, gaining at certain parts of the year and losing at others, so that we must make allowance for his being so many minutes fast or slow, just as with an ordinary clock. The time at which the sun comes up to our index is called apparent noon, and if we make allowance for his being fast or slow, we readily get the instant of mean noon (as it is called) at which our clocks ought to point exactly to h .

We may now proceed with the question of longitude. For convenience of illustration we compared the heavens to a globe which turned round and brought the hours up to a fixed index, but really it is the celestial globe which is fixed and the index or meridian which moves. Now let us take two places with two different indexes,

18 WASHINGTON TIME.

or meridians, which, by the rotation of the earth, arc brought up successively to the sun, giving the instants of noon for the two places respectively. When it is noon for the second place the sun will be west of the meridian at the first, and the time that has elapsed since the sun was in the meridian of the first place, will be the same part of the time between successive returns of the sun to this meridian, as the angle turned through by the earth to bring the second meridian up to the sun (from the position for noon at the firsl station), is of the angle turned through from one noon to the next. The same would hold for the stars, moon, or planets, and the difference of longitude of the two places will be the same partof 24* if expressed in time, or of 360° if expressed ■ in angular measure. The longitude of a place, then, is nothing but the difference between its time and that of the place selected as our starting point (Washington for the United States), so that having the means of determining local time (e. <jr., noon) at any place (as explained above) what is wanted is a means of knowing Washington time. We may mention here two ways of obtaining this knowledge, reserving that which depends on the motion of the moon, and which is so extensively used at sea, for a future chapter. The first method to which we shall refer makes use of chronometers. Thanks to the improvements effected in its construction, under the stimulus of large rewards offered by the government of England, this refined form of watch may be relied on to give Washington time within two or three seconds (corresponding to about half a mile in the deduced longitude) after a voyage of a week or ten da; that the longitude may be found to that degree of accuracy after a short voyage. But there is another method of far greater accuracy which has been introduced of late years in England wherever the telegraph extends to. In this method standard time is obtained by a signal sent from the Royal Observatory at a known instant of Greenwich time ; such signals are sent out every morning at 10 h a. m., at noon, and at 1

CIRCLES OF LATITUDE AND LONGITUDE. 19 p.m. exactly, and are distributed all over the United Kingdom by the post-office telegraphs, their chief use being to regulate time on railways and in public buildings.

From what precedes it is clear that the position of any place on the earth is completely determined when we know the circle of latitude on which it lies, and also its position on that circle, i. e., its latitude and longitude ; but the reader has, perhaps, not got a very clear idea of what is represented by longitude on the earth itself. To assist him in this, we should explain what is meant by a meridian on the earth, having already explained what is the astronomical meridian of a place. If we start from any place and travel due north or due south we shall always remain in the same vertical plane through the pole, which will therefore continue to be our meridian, so that all the places we pass through will have their noon at the same

instant, and therefore have the same longitude. The line along which we travel is called a terrestrial meridian, and, since we are evidently going directly towards the pole, all meridians will pass through the poles. We may, therefore, conceive the earth as divided into a series of circles of latitude, all parallel to the equator, and into a series of circles of longitude, or meridians, all passing through the poles, which, by their crossing, define the positions of places on the earth. These are the lines which are drawn on maps, though for the sake of clearness only those at convenient intervals are put in; it is, however, necessary to imagine a circle of latitude and one of longitude passing at right angles to each other through every point of the map. The

20 THE A TMOSPHERE.

figure represents the circles of latitude drawn for every 10°, and the meridians for every hour; the numbers placed between the meridians of I h and IP represent the number of miles traversed by places on the corresponding circles of latitude in one hour.

We have not yet spoken of a very important part of the earth, and that which, in fact, renders our globe habitable — its atmosphere. At every point of the surface we find air, which makes its presence felt in various ways, and which exerts a pressure of 15 lbs. on every square inch of surface of a vessel from which the air has been exhausted. This pressure can be measured accurately by the mercurial barometer, an instrument in which the pressure of the air on the mouth of a vertical tube, closed at the upper end and exhausted of air, is balanced by the weight of mercury which it will hold suspended. It is in this way found that the pressure at the sea level is nearly the same at all parts of the earth, being equal to the weight of a column of mercury about 30 inches high, and having the same section as the surface on which the pressure is exerted, which corresponds to a column of air of the same density throughout as that at the surface, and 27,000 feet, or about five miles high. But on rising above the sea level, whether in ascending a mountain or in a balloon, we soon find that the pressure decreases less rapidly than would be the case if the atmosphere remained of the same density as at the surface : thus we have to ascend to 2,850 feet to get through the first tenth of the atmosphere ; through 3,200 feet more to get through the next tenth ; through 3,650 feet more for the third tenth; and 4,250 feet more for the fourth tenth, and so on. In this way it would require an ascent of about 18,000 feet to pass through the lower half of the atmosphere (as measured by its weight) and an ascent of about 30,000 feet to traverse the lower two-thirds, starting in each case from the sea level. Since the pressure of the atmosphere is due to the weight of the superincumbent air, it is easy to see that the air gets lighter, and therefore less dense, as we ascend, and this is in perfect accordance with