Astronomy for Everybody/Part 1/Chapter 2

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.

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

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

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.