First Experiences Of A Starlight Night

( Originally Published Early 1900's )

LET us suppose a would-be observer of the stars to station himself on some fine evening soon after sunset in an open and if possible elevated position. A varied and striking, not to say picturesque, spectacle would soon unfold itself to his gaze. Stars invisible during the daytime, because their light was overcome by the superior light of the sun, would soon appear. They would become visible at first only one by one, as it were ; then several would seem to start into being, and finally their number would increase, until it might be supposed that many thousands were visible, though in point of actual fact no more than about 3000 stars at the outside can ever be seen by the naked eye at any one time or place.

An attentive scrutiny, prolonged in one case for an hour or two, and in another case for a day or two, will disclose a twofold fact : first, that all the objects assumed to be stars are moving in a body over the face of the sky from hour to hour, whilst two or three brighter ones are to be noticed which not only participate in the constant movement from hour to hour of the whole mass, but have an individual motion of their own in virtue of which either from day to day, or in other cases from week to week, they will be noticed to change their relative positions with respect to the twinkling stars around them. Pausing for a moment to distinguish between these two classes of celestial objects, it may be stated that the bodies which twinkle, and have (seemingly) no relative movement, are the " fixed stars," properly so called ; whilst the others, it may be only two or three in number on any given evening, and which do not twinkle, are objects of a totally distinct character, and known as " planets.

Taking the sky as a whole, with its 2000 or its 3000 naked-eye stars, the observer (if in a northern latitude) will notice, if he turns his back to the south, remembering where the sun was at midday, that after successive intervals, say of a of an hour, new stars are presenting them-selves on the right, rising above the horizon. If he will follow some one group in particular far into the night, he will find that it gradually rises in the heavens in the direction from east to west. After a certain interval it ceases to rise higher; then descends on his left, and finally disappears below the western horizon. This onward march is not an attribute of all the stars quite in the simple form thus mentioned, for of some of them it must be said that they do not rise above the horizon nor sink down below it, because they are always above it. Such are the stars which face our observer, who with his back to the south is looking towards the north. Of the stars thus circumstanced there are some which seem to describe a pathway which scrapes, as it were, the northern horizon ; whilst others seem to describe circular paths, which become more and more contracted towards a certain star in particular. That star seems almost motionless throughout the entire night, and is known as the " Pole Star." The stars which are, as above stated, always above the horizon, would always be visible during the whole 24 hours were it not for the sunlight. As a matter of fact, indeed, the larger of them can on any fine day be traced by means of a large telescope round and round during the whole 24 hours day after day throughout the year, weather permitting.

The movement of the heavens which has just been referred to is commonly called the " diurnal movement." A better conception of it perhaps may be had if we imagine (as indeed the ancients did) that we are in the centre of a literal sphere ; that the stars are attached to the interior surface of such a sphere ; and that it is endued from with-out with a rotatory motion once in every period of time which we designate a day of 24 hours. Regarding the universe thus, we must, by one more forward stretch of the imagination, consider the heavens to be always revolving around an invisible axis called the axis of the world, which passes through the place of observation and a particular point near to the Pole Star. The direction of motion will be from east to west ; and whilst for us in England the visible polar point of this imaginary axis will be the North Pole, the other end of the axis will be pointing in the opposite direction to another point called South Pole. For the reader to obtain a full and true realisation of these statements, which in the abstract no doubt have a visionary sound, he must take a voyage to the Southern hemisphere say, to the Cape of Good Hope or Australia. Doing this, he will come face to face with a condition of things which at first sight may be a little puzzling. He will have lost both the North Pole and the Pole Star, and also the constellation of the Great Bear and other constellations which we associate with the north, and will find himself called upon to study a very different situation. In arder to discover a polar point he will have to face the south instead of the north ; he will find no bright star at, or anywhere near, the South Pole ; and no Great Bear to recall the memories of childhood and the nursery.

The remarks in the preceding paragraph will have paved the way for the statement which must now be made, that the study of the stars as regards their location in the heavens is intimately mixed up with terrestrial questions of geography ; in other words, that the observer's opportunity of surveying the fields of view afforded by the heavens ever depends upon the latitude (not the longitude) of his place of observation on the earth. Wherever he may be, provided he be not immediately at the equator or pole, he will have to consider the heavens as comprising 3 distinct regions, each with its own particular peculiarities. The first, bounded by an imaginary circle called the " circle of perpetual apparition " ; the second, bounded by another imaginary circle called the "circle of perpetual occultation"; the third being all the area not embraced by either of the others. All the stars lying between the first circle and the visible pole will be perpetually visible to our observer throughout the year, barring of course accidents of sunshine or weather. All the stars lying between the second circle and the opposite (or invisible) pole will be perpetually invisible to our observer, because none of them rise above his horizon. This is the condition of things as regards an observer in the Northern hemisphere. Looked at on the other hand from a station, say in Australia, the converse of the foregoing will be the condition of things. The stars perpetually visible in England will be perpetually invisible in Australia, and the stars perpetually out of view in England will be perpetually in view in Australia.*

The reader will by this time quite understand that when we talk about the celestial sphere, or the vault of heaven, or the axis of the world, or the poles, we are re-sorting to pure abstractions which are only calculated to convey in a crude fashion ideas of apparent movements which it is difficult to describe in words, or to indicate by pictures, or to reproduce in model with mechanical appliances. It may, however, be said that a pair of globes intelligently studied may be of some service. Perhaps it is worth while to note in passing that ideas and expressions on this subject which we employ simply as figures of speech, were made use of by the astronomers of antiquity in a literal and material sense. Many of them fully believed in the existence of a solid celestial vault with a material axis provided with pivots turning in fixed sockets, the stars being fastened to the surface of the vault by nails or such like attachments. Vitruvius may be mentioned as one of the best known writers of antiquity who has recorded as facts ideas of this sort.

It would not be in accordance with the design of this little work to go very deeply into matters of the kind brought under the reader's notice in the pages immediately preceding. Suffice it then to add that whilst the longitude of an observer's position has nothing to do with the question of whether he sees some stars and not others on any given night, it has a good deal to do with the question of what stars are visible at any given moment of time to an American at New York, to an Englishman in London, or to a Hindoo at Calcutta. For instance, when a Londoner is going to bed at the hour of 11p. m., the New Yorker will be sitting clown to his dinner at 6 p. m., whilst the Calcutta Hindoo will be preparing for breakfast. The difference of 11hours of absolute time which exists between New York and Calcutta will result in each of those places having a totally different batch of constellations presented to its gaze; because London occupies an intermediate position, the Londoner will see certain stars over his head which to the Calcutta Hindoo will appear setting near the W. horizon, and which to the New Yorker will appear low down in the E. horizon, just rising.

Whilst it is intended as far as possible to exclude from this volume matters of mathematics and geometry, there are a few such matters which must be stated to and be comprehended by the reader if he would follow up, to any good purpose, the study of astronomy as a pleasant and profitable occupation.

We sometimes have to speak of a body being in a " vertical " position. This means " upright," and a heavenly body is in a vertical position when it is exactly over the observer's head. The vertical of a place, then, is the direction from which a body, set free to fall as it will, seems to come when it strikes the earth at the place. It is indicated by the direction of a string made fast at one extremity, whilst the other extremity supports a weight of some kind. Such a combination constitutes a plumb-line, and is used by masons and bricklayers for the express purpose of ensuring the uprightness or verticality of their work. Further, it may be stated that the vertical of a place is constantly perpendicular to the surface of water there which is at rest.

The imaginary point in the sky where the vertical pro-longed from the ground upwards meets the celestial vault is the " zenith " of the place of observation. It is of course the point exactly above the observer's head. If one could conceive the vertical prolonged downwards through the earth and coming out on the other side, and carried forwards till it met the celestial sphere at another point, it would do so at a point which is called the " nadir " of the observer on the upper side, so to speak, of the earth. An observer standing out on an open plain, or better still in a boat on the open sea, will notice that his view of the land in the one case, and of the sea in the other, is cut off from the sky by a circular boundary line, he himself being in the centre of the circle. This circle is called the " horizon." It really is a horizontal plane passing through the place of observation at right angles to the vertical.

The " plane of the meridian " of a place is an imaginary plane passing through what we have spoken of as the axis of the heavens and the vertical of the place. Suitable observation shows that the uppermost and lowermost points in the circles seemingly described by all the stars are situated in this plane. The intersection of this plane of the meridian with the horizon to the north and to the south constitutes what we call the "meridian line," or simply the " meridian " of the place of observation. What it is and what it means will perhaps best be grasped by a consideration of the original meaning of the word. It comes from z Latin words, through a single Latin word, the words of origin being medius middle, and dies, day meaning in effect the point of the horizon immediately be-low the place in the heavens where the sun is when it has run half its daily course from sunrise to sunset.

With the horizon and the meridian understood, the cardinal points, north, south, east, and west seem to come naturally. An observer placed in the direction mentioned at the beginning of this chapter, that is, facing the Pole Star, will (in England) be facing the North ; immediately behind him will be the South ; whilst on his right will be the East and on his left the West. These words in English convey very little to us, but in their Latin forms are much more expressive. The Latin, by the way, reappears in the French. For instance, the Latin for " North " is septentrio, which recalls the 7 (septem) stars near the North Pole ; in French it is septentrion. Then the South has already been mentioned and reaches us in French as midi. Then the East is oriens (Fr. l'orient), i. e., the place where the sun rises. And the West is occidens (Fr. l'occident), i. e , the place where the sun falls, i. e., sets.

It is sometimes necessary to consider the position of a star or the distance of one star from another by making a measurement or an estimate along the plane of the horizon, or along some other plane parallel thereto. This is spoken of as a measurement in " azimuth " ; or, to put it in another way, let us imagine a plane passing through the zenith and through any star whatever ; that would be at the moment of observation the azimuthal plane of the star ; and the angle between this plane and the plane of the meridian, or the star's distance from the meridian thus measured, would be the star's " azimuth " at the particular moment when the observation was made.

A few words respecting angular distances and their measurement seem now needed, but they must be very general because the study of angles is a matter which concerns geometry in the first instance and astronomy only in a secondary sense.

Every circle is considered to be divided into 36o degrees, every degree (°) being subdivided into 6o minutes, and every minute (') into 6o seconds. Formerly every second (") was divided into 6o thirds, but this method of counting has become quite obsolete, and when it is necessary, as it often is, to deal with fractions of a second, re-sort is had to decimals. Occasions, indeed, sometimes arise when it is convenient to go no further than whole minutes and to express as decimals of a minute the seconds which we wish to record. Indeed, on occasions, even the minutes and seconds taken together are set down as simple decimals of a degree. Thus, 45° 12' 20" might be expressed as 45° 12'.33 or 45°.205.

The whole circle being taken at 360°, a half-circle em-braces 18o ; a quarter circle, or " quadrant," is 90°, whilst the eighth, or "octant," represents 45°. An intermediate subdivision, a sixth, or "sextant," furnishes a word which has an astronomical application, but it is to an instrument, and not to the space which the word suggests. The words " octant " and "sextant " as portions of a circle are not in use, notwithstanding that the words themselves exist.

Applying to the circle thus divided the 4 cardinal points already mentioned we obtain the divisions which constitute the dial of the " mariner's compass," and an attentive consideration of the manner in which that is divided will pave the way for a due comprehension of the manner in which angles are measured for astronomical purposes.

It will be seen by the diagram that if a circle is divided into 4 quadrants we are furnished with the 4 principal points, N., E., S., and W. Each quadrant therefore embraces 90° of the 360° which constitute the entire circle. Dividing each quadrant into two halves gives us the sub-divisions known as N.E., S.E., S.W., and N.W. Each of these represents the half of 90°, or 45°. Then by subdividing each half-quadrant into half again we obtain what are quarter-quadrants, though no such phrase is in use. The quarter-quadrants give us the points known as N.N.E., E.N.E., E.S.E., S.S.E., S.S.W., W.S.W., W.N.W., and N.N.W.

We have now got our circle divided into i6 portions each of 22i°. The sailor, however, carries the matter 2 steps further, and by again subdividing into halves the intervals just mentioned he arrives at the 32 " points of the compass," as they are called ; then by another subdivision into halves he obtains 64 subdivision of the circle, though the final appellation is not a " point," but a "half-point."

Speaking generally, the subdivision of the circle for the purpose of steering a ship does not need (except in special cases, of course) any great refinement ; that is to say, an order to vary a ship's course by half a point, or about 51°, is precise enough on the open seas.* But the astronomer in measuring angular distances in the case of the sun and planets, and still more in the case of the stars, has to deal with arcs infinitely smaller than those which the " man at the wheel " is concerned with. Not only arcs as small as I", but even fractions of a second have to be taken into account by the use of instruments far larger in size and more finely graduated than the portable instruments, such as sextants and theodolites, used by sailors at sea, and by surveyors on land.