Astronomy for Everybody

Front matter

Science for Everybody

ASTRONOMY FOR EVERYBODY

A Popular Exposition of the Wonders of the Heavens

Part I

PART I

THE CELESTIAL MOTIONS

PART I. THE CELESTIAL MOTIONS

Part I, Chapter 1

I

A View of the Universe

Let us enter upon our subject by taking a general view of this universe in which we live, fancying ourselves looking at it from a point without its limits. Far away, indeed, is the point we must choose. To give a conception of the distance, let us measure it by the motion of light. This agent, darting through 186,000 miles in every second, would make the circuit of the earth several times between two ticks of a watch. The standpoint which we choose will probably be well situated if we take it at a distance through which light would travel in 100,000 years. So far as we know, we should at this point find ourselves in utter darkness, a black and starless sky surrounding us on all sides. But, in one direction, we should see a large patch of feeble light spreading over a considerable part of the heavens like a faint cloud or the first glimmer of a dawn. Possibly there might be other such patches in different directions, but of these we know nothing. The one which we have mentioned, and which we call the universe, is that which we are to inspect. We therefore fly toward it—how fast we need not say. To reach it in a month we should have to go a million times as fast as light. As we approach, it continually spreads out over more of the black sky, which it at length half covers, the region behind us being still entirely black.

Before reaching this stage we begin to see points of light glimmering here and there in the mass. Continuing our course, these points become more numerous, and seem to move past us and disappear behind us in the distance, while new ones continually come into view in front, as the passengers on a railway train see landscape and houses flit by them. These are stars, which, when we get well in among them, stud the whole heavens as we see them do at night. We might pass through the whole cloud at the enormous speed we have fancied, without seeing anything but stars and, perhaps, a few great nebulous masses of foggy light scattered here and there among them.

But instead of doing this, let us select one particular star and slacken our speed to make a closer inspection of it. This one is rather a small star; but as we approach it, it seems to our eyes to grow brighter. In time it shines like Venus. Then it casts a shadow; then we can read by its light; then it begins to dazzle our eyes. It looks like a little sun. It is the Sun!

Let us get into a position which, compared with the distances we have been travelling, is right alongside of the sun, though, expressed in our ordinary measure, it may be a thousand million miles away. Now, looking down and around us, we see eight star-like points scattered around the sun at different distances. If we watch them long enough we shall see them all in motion around the sun, completing their circuit in times ranging from three months to more than 160 years. They move at very different distances; the most distant is seventy times as far as the nearest.

These star-like bodies are the planets. By careful examination we see that they differ from the stars in being opaque bodies, shining only by light borrowed from the sun.

Let us pay one of them a visit. We select the third in order from the sun. Approaching it in a direction which we may call from above, that is to say from a direction at right angles to the line drawn from it to the sun, we see it grow larger and brighter as we get nearer. When we get very near, we see it looking like a half-moon — one hemisphere being in darkness and the other illuminated by the sun's rays. As we approach yet nearer, the illuminated part, always growing larger to our sight, assumes a mottled appearance. Still expanding, this appearance gradually resolves itself into oceans and continents, obscured over perhaps half their surface by clouds. The surface upon which we are looking continually spreads out before us, filling more and more of the sky, until we see it to be a world. We land upon it, and here we are upon the earth.

Thus, a point which was absolutely invisible while we were flying through the celestial spaces, which became a star when we got near the sun, and an opaque globe when yet nearer, now becomes the world on which we live.

This imaginary flight makes known to us a capital fact of astronomy: The great mass of stars which stud the heavens at night are suns. To express the idea in another way, the sun is merely one of the stars. Compared with its fellows it is rather a small one, for we know of stars that emit thousands or even tens of thousands of times the light and heat of the sun. Measuring things simply by their intrinsic importance, there is nothing special to distinguish our sun from the hundreds of millions of its companions. Its importance to us and its comparative greatness in our eyes arise simply from the accident of our relation to it.

The great universe of stars which we have described looks to us from the earth just as it looked to us during our imaginary flight through it. The stars which stud our sky are the same stars which we saw on our flight. The great difference between our view of the heavens and the view from a point in the starry distances is the prominent position occupied by the sun and planets. The former is so bright that during the daytime it completely obliterates the stars. If we could cut off the sun's rays from any very wide region, we should see the stars around the sun in the daytime as well as by night. These bodies surround us in all directions as if the earth were placed in the centre of the universe, as was supposed by the ancients.

What the Universe Is

We may connect what we have just learned about the the universe at large with what we see in the heavens. What we call the heavenly bodies are of two classes. One of these comprises the millions of stars the arrangement and appearance of which we have just described. The other comprises a single star, which is for us the most important of all, and the bodies connected with it. This collection of bodies, with the sun in its centre, forms a little colony all by itself, which we call the solar system. The feature of this system which I wish first to impress on the reader's mind is its very small dimensions when compared with the distances between the stars. All around it are spaces which, so far as we yet know, are quite void through enormous distances. If we could fly across the whole breadth of the system, we should not be able to see that we were any nearer the stars in front of us, nor would the constellations look in any way different from what they do from our earth. An astronomer armed with the finest instruments would be able to detect a change only by the most exact observations, and then only in the case of the nearer stars.

A conception of the respective magnitudes and distances of the heavenly bodies, which will help the reader in conceiving of the universe as it is, may be gained by supposing us to look at a little model of it. Let us imagine that, in this model of the universe, the earth on which we dwell is represented by a grain of mustard seed. The moon will then be a particle about one fourth the diameter of the grain, placed at a distance of an inch from the earth. The sun will be represented by a large apple, placed at a distance of forty feet. Other planets, ranging in size from an invisible particle to a pea, must be imagined at distances from the sun varying from ten feet to a quarter of a mile. We must then imagine all these little objects to be slowly moving around the sun at their respective distances, in times varying from three months to 160 years. As the mustard seed performs its revolution in the course of a year we must imagine the moon to accompany it, making a revolution around it every month.

On this scale a plan of the whole solar system can be laid down in a field half a mile square. Outside of this field we should find a tract broader than the whole continent of America without a visible object in it unless perhaps comets scattered around its border. Far beyond the limits of the American continent we should find the nearest star, which, like our sun, might be represented by a large apple. At still greater distances, in every direction, would be other stars, but, in the general average, they would be separated from each other as widely as the nearest star is from the sun. A region of the little model as large as the whole earth might contain only two or three stars.

We see from this how, in a flight through the universe, like the one we have imagined, we might overlook such an insignificant little body as our earth, even if we made a careful search for it. We should be like a person flying through the Mississippi Valley, looking for a grain of mustard seed which he knew was hidden somewhere on the American continent. Even the bright shining apple representing the sun might be overlooked unless we happened to pass quite near it.

Part I, Chapter 2

II

Aspects of the Heavens

The immensity of the distances which separate us from the heavenly bodies makes it impossible for us to form a distinct conception of the true scale of the universe, and very difficult to conceive of the heavenly bodies in their actual relations to us. If, on looking at a body in the sky, there were any way of estimating its distance, and if our eyes were so keen that we could see the minutest features on the surface of the planets and stars, the true structure of the universe would have been obvious from the time that men began to study the heavens. A little reflection will make it obvious that if we could mount above the earth to a distance of, say, ten thousand times its diameter, so that it would no longer have any perceptible size, it would look to us, in the light of the sun, like a star in the sky. The ancients had no conception of distances like this, and so supposed that the heavenly bodies were as they appeared, of a constitution totally different from that of the earth. We ourselves, looking at the heavens, are unable to conceive of the stars being millions of times farther than the planets. All look as if spread out on one sky at the same distance. We have to learn their actual arrangement and distances by reason.

It is from the impossibility of conceiving these enor- mous differences in the distances of objects on the earth and the heavens, that the real difficulty of forming a mental picture of them in their true relation arises. I shall ask the reader's careful attention in an attempt to present these relations in the simplest way, so as to connect things as they are with things as we see them.

Let us suppose the earth taken away from under our feet, leaving us hanging in mid space. We should then see the heavenly bodies —sun, moon, planets, and stars— surrounding us in every direction, up and down, east and west, north and south. The eye would rest on nothing else. As we have just explained, all these objects would seem to us to be at the same distance.

A great collection of points scattered in every direction at an equal distance from one central point, must all lie upon the inner surface of a hollow sphere. It follows that, in the case supposed, the heavenly bodies will appear to us as if set in a sphere in the centre of which we appear to be placed. Since one of the final objects of astronomy is to learn the directions of the heavenly bodies from us, this apparent sphere is talked about in astronomy as if it were a reality. It is called the celestial sphere. In the case we have supposed, with the earth out of the way, all the heavenly bodies on this sphere would at any moment seem at rest. The stars would remain apparently at rest day after day and week after week. It is true that, by watching the planets, we should in a few days or weeks, as the case might be, see their slow motion around the sun, but this would not be perceptible at once. Our first impression would be that the sphere was made of some solid, crystalline substance, and that the heavenly bodies were fastened to its inner surface. The ancients had this notion, which they brought yet nearer the truth by fancying a number of these spheres fitting inside of each other to represent the different distances of the heavenly bodies.

With this conception well in mind, let us bring the earth back under our feet. Now we have to make a draft upon the reader's power of conception. Considered in its relation to the magnitude of the heavens, the earth is a mere point; yet, when we bring it into place, its surface cuts off one half of the universe from our view, just as an apple would cut off the view of one side of a room from an insect crawling upon it. That half of the celestial sphere which, being above the horizon, remains visible is called the visible hemisphere; the half below, the view of which is cut off by the earth, is called the invisible hemisphere. Of course we could see the latter by travelling around the earth.

Having this state of things well in mind, we must make another draft on the reader's attention. We know that the earth is not at rest, but revolves unceasingly around an axis passing through its centre. The natural result of this is an apparent rotation of the celestial sphere in the opposite direction. The earth rotates from west toward east; hence the sphere seems to rotate from east toward west. This real revolution of the earth, with the apparent revolution of the stars which it causes, is called the diurnal motion, because it is completed in a day.

Apparent Daily Revolution of the Stars

Our next problem is to show the connection between the very simple conception of the rotation of the earth and the more complicated appearance presented by the apparent diurnal motion of the heavenly bodies which it brings about. The latter varies with the latitude of the observer upon the earth's surface. Let us begin with its appearance in our middle northern latitudes.

For this purpose we may in imagination build a hollow globe representing the celestial sphere. We may make it as large as a Ferris wheel, but one of thirty or forty feet in diameter would answer our purpose. Let Figure 1 be an inside view of this globe, mounted on two pivots, P and Q, so that it can turn round on them diagonally. In the middle, at O, we have a horizontal platform, NS, on which we sit. The constellations are marked on the inside of the globe, covering the whole surface, but those on the lower half are hidden from view by the platform. This platform, as is evident, represents the horizon.

The globe is now made to turn on its pivots. What will happen? We shall see the stars near the pivot P revolving around the latter as the globe turns. The stars on a certain circle KN will graze the edges of the platform, as they pass below P. Those yet farther from P will dip below the platform to a greater or less extent, according to their distance from P. Stars near the circle EF, halfway between P and Q, will perform half their course above, and half below the platform. Finally, stars within the circle ST will never rise above the level of the platform at all, and will remain invisible to us.

To our eyes the celestial sphere is such a globe as this, of infinite dimensions. It seems to us to be continually

Fig. 1.—The Celestial Sphere as it appears to us.

revolving round a certain point in the sky as a pivot, making one revolution in nearly a day, and carrying the sun, moon, and stars with it. The stars preserve their relative positions as if fastened to the revolving celestial sphere. That is to say, if we take a photograph of them at any hour of the night, the same photograph will show their appearance at any other hour, if we only hold it in the right position.

The pivot corresponding to P is called the north celestial pole. To dwellers in middle northern latitudes, where most of us live, it is in the northern sky, nearly midway between the zenith and the northern horizon. The farther south we live, the nearer it is to the horizon, its altitude above the latter being equal to the latitude of the place where the observer stands. Quite near it is the pole star, which we shall hereafter show how to locate. To ordinary observation, the pole star seems never to move from its position. In our time it is little more than a degree from the pole, a quantity with which we need not now concern ourselves.

Opposite the north celestial pole, and therefore as far below our horizon as the north one is above it, lies the south celestial pole.

An obvious fact is that the diurnal motion as we see it in our latitude is oblique. When the sun rises in the east it does not seem to go straight up from the horizon, but moves over toward the south at a more or less acute angle with the horizon. So when it sets, its motion relative to the horizon is again oblique.

Now, imagine that we take a pair of compasses long enough to reach the sky. We put one point on the sky at the north celestial pole, and the other point far enough from it to touch the horizon below the pole. Keeping the first point at the pole we draw a complete circle on the celestial sphere with the other point. This circle just touches the north horizon at its lowest point and, in our northern latitudes, extends to near the zenith at its highest point. The stars within this circle never set, but only seem to perform a daily course around the pole. For this reason this circle is called the circle of perpetual apparition.

The stars farther south rise and set, but perform less and less of their daily course above our horizon, till we reach the south point of it, where they barely show themselves.

Stars yet farther south never rise at all in our latitudes. They are contained within the circle of perpetual occultation, which surrounds and is centred on the south celestial pole, as the circle of perpetual apparition is centred on the north one.

Figure 2 shows the principal stars of the northern heavens within the circle of perpetual apparition for the Northern States. By holding it with the month on top we shall have a view of the constellations as they are seen about eight o'clock in the evening. It also shows how to find the pole star in the centre by the direction of the two outer stars or pointers in the Dipper, or Great Bear.

Now let us change our latitude and see what occurs. If we journey toward the equator, the direction of our horizon changes, and during our voyage we see the pole star constantly sinking lower and lower. As we approach the equator, it approaches the horizon, reaching it when we reach the equator. It is plain enough that the circle of perpetual apparition grows smaller until, at the equator, it ceases to exist, each pole being in our horizon. Now the diurnal motion seems to us quite different from what it is here. The sun, moon, and stars, when they rise, commence their motion directly upwards. If one of them rises exactly in the east, it will pass

Fig. 2.—The Northern Sky and the Pole Star.

through the zenith; one rising south of the east will pass south of the zenith; one rising north of the east, north of the zenith.

Continuing our course into the southern hemisphere, we find that the sun, while still rising in the east, generally passes the meridian to the north of the zenith. The main point of difference between the two hemispheres is that, as the sun now culminates in the north, its apparent motion is not in the direction of the hands of a watch, as with us, but in the opposite direction. In middle southern latitudes, the northern constellations, so familiar to us, are always below the horizon, but we see new ones in the south. Some of these are noted for their beauty, the Southern Cross, for example. Indeed, it has often been thought that the southern heavens were more brilliant and contained more stars than the northern ones. But this view is now found to be incorrect. Careful study and counts of the stars show the number to be about the same in one hemisphere as in the other. Probably the impression we have mentioned arose from the superior clearness of the sky in the southern regions. For some reason, perhaps because of the drier climate, the air is less filled with smoke and haze in the southern portions of the African and American continents than it is in our northern regions.

What we have said of the diurnal motion of the northern stars round and round the pole, applies to the stars in the southern heavens. But there is no southern pole star, and therefore nothing to distinguish the position of the southern celestial pole. The latter has a number of small stars around it, but they are no thicker than in any other region of the sky. Of course, the southern hemisphere has its circle of perpetual apparition, which is larger the farther south we travel. That is to say, the stars in a certain circle around the south celestial pole never set, but simply revolve around it, apparently in an opposite direction from what they do in the north. So, also, there is a circle of perpetual occultation containing those stars around the north pole which, in our latitudes, never set. After we go beyond 20° south latitude we can no longer see any part of the constellation Ursa Minor. Still farther south the Great Bear will only occasionally show itself to a greater or less extent above the horizon.

Could we continue our journey to the south pole we should no longer see any rising or setting of the stars. The latter would move around the sky in horizontal circles, the centre or pole being at the zenith. Of course, the same thing would be true at the north pole.

Part I, Chapter 3

III

Relation of Time and Longitude

We all know that a line running through any place on the earth in a north and south direction, is called the meridian of that particular place. More exactly, a meridian of the earth's surface is a semicircle passing from the north to the south pole. Such semicircles pass in every direction from the north pole, and one may be drawn so as to pass through any place. The meridian of the Royal Observatory at Greenwich is now adopted by most nations, our own included, as the one from which longitudes are measured, and by which in the United States and a considerable part of Europe the clocks are set.

Corresponding to the terrestrial meridian of a place is a celestial meridian which passes from the north celestial pole through the zenith, intersects the horizon at its south point, and continues to the south pole. As the earth revolves on its axis it carries the celestial as well as the terrestrial meridian with it, so that the former, in the course of a day sweeps over the whole celestial sphere. The appearance to us is that every point of the celestial sphere crosses the meridian in the course of a day.

Noon is the moment at which the sun passes the meridian. Before the introduction of railways, people used to set their clocks by the sun. But owing to the obliquity of the ecliptic and the eccentricity of the earth's orbit around the sun, the intervals between successive passages of the sun are not exactly equal. The consequence is that, if a clock keeps exact time, the sun will sometimes pass the meridian before and sometimes after twelve by the clock. When this was understood, a distinction was made between apparent and mean time. Apparent time was the unequal time determined by the sun; mean time was that given by a clock keeping perfect time month after month. The difference between these two is called the equation of time. Its greatest amounts are reached every year about the first of November and the middle of February. At the former time, the sun passes the meridian sixteen minutes before the clock shows twelve; in February, fourteen or fifteen minutes after twelve.

To define mean time astronomers imagine a mean sun which always moves along the celestial equator so as to pass the meridian at exactly equal intervals of time, and which is sometimes ahead of the real sun and sometimes behind it. This imaginary or mean sun determines the time of day. The subject will perhaps be a little easier if we describe things as they appear, imagining the earth to be at rest while the mean sun revolves around it, crossing the meridian of every place in succession. We thus imagine noon to be constantly travelling around the world. In our latitudes, its speed is not far from a thousand feet per second; that is to say, if it is noon at a certain place where we stand, it will one second afterward be noon about one thousand feet farther west, in another second a thousand feet yet farther west, and so on through the twenty-four hours, until noon will once more get back where we are. The obvious result of this is that it is never the same time of day at the same moment at two places east or west of each other. As we travel west, we shall continually find our watches to be too fast for the places which we reach, while in travelling east, they will be too slow. This varying time is called local or astronomical time. The latter term is used because it is the time determined by astronomical observations at any place.

Standard Time

Formerly the use of local time caused great inconvenience to travellers. Every railway had its own meridian which it ran its trains by; and the traveller was frequently liable to miss his train by not knowing the relation between his watch or a clock and the railway time. So in 1883, our present system of standard time was introduced. Under this system, standard meridians are adopted fifteen degrees apart, this being the space over which the sun passes in one hour. The time at which noon passes a standard meridian is then used throughout a zone extending seven or eight degrees on each side. This is called standard time. The longitudes which mark the zones are reckoned from Greenwich. It happens that Philadelphia is about seventy-five degrees in longitude, or five hours in time from Greenwich. More exactly, it is about one minute of time more than this. Thus the standard meridian which we use for the Middle States passes a little east of Philadelphia. When mean noon reaches this meridian, it is considered as twelve o'clock throughout all our Eastern and Middle States as far west as Ohio. An hour later, it is considered twelve o'clock in the Mississippi Valley. An hour later, it is twelve o'clock for the region of the Rocky Mountains. In yet another hour, it is twelve o'clock on the Pacific coast. Thus we use four different kinds of time, Eastern time, Central time, Mountain time, and Pacific time, differing from each other by entire hours. Using this time, the traveller only has to set his watch forward or back one hour at a time, as he travels between the Pacific and the Atlantic coast, and he will always find it correct for the region in which he is at the time.

It is by this difference of time that the longitudes of places are determined. Imagine that an observer in New York makes a tap with a telegraph-key at the exact moment when a certain star crosses his meridian, and that this moment is recorded at Chicago as well as New York. When the star reaches the meridian of Chicago, the observer taps the time of its crossing over his meridian in the same way. The interval between the two taps shows the difference of longitude between the two cities.

Another method of getting the same result is for each observer to telegraph his local time to the other. The difference of the two times gives the longitude.