The Grouping Of The Stars Into Constellations

( Originally Published Early 1900's )

THE visible stars are commonly treated as arranged in groups which are called " constellations." The circumstances under which this grouping was brought about involve so many interesting historical points that the history of the constellations may well form a separate chapter. Let me then limit the present chapter to a few general hints and remarks on the finding of the constellations.

A reader who wishes to be able to do this with facility must enter upon the study of the stars methodically, and in accordance with a definite plan, and must be prepared to persevere with his work at regular and not very long intervals of time through an entire period of 12 months. In making this suggestion I lay a good deal of stress on the work being done systematically, and without any considerable gaps of time in the doing of it. The importance of this will be understood when it is borne in mind that a given star comes to the meridian every night 4 minutes sooner than it did on the preceding night. This has the effect in the course of a fortnight of displacing a star by 15° of arc, the time of observation remaining the same. In other words, if an observer wishes to see a given star on the meridian a fortnight after his first observation of it, he must take post at his telescope (supposing he is using one which only works up and down in the meridian) one hour earlier in the evening than the hour at which the first observation was made. It would soon be seen in practice why there was not only no advantage in thus altering one's times but a positive disadvantage. The ordinary object of a professed student would be, not to have a constant change in the hours of his occupation, but to have a constant change in the occupation itself ; that is, in the stars he is wishing to learn the names of, the hours of work remaining (for his personal convenience) probably much the same, say 9 p. M. to midnight.

There is another and also cogent reason for keeping at this work regularly and avoiding long breaks. Though the stars maintain, so far as they themselves are concerned, speaking generally, the same relative positions throughout the year, yet it makes a good deal of difference to the unpractised eye in the identification of particular stars whether they are looked at whilst they are in the neutral position (as it may be called) of the meridian, or when they are oblique to the meridian, eastwards of it and rising, or westwards of it and setting. These differences can only be properly appreciated by being considered experimentally on a starlight night in the open air, for no verbal account of them can be adequate.

A tourist who is travelling through a hilly country, unless he is a professional land surveyor employed on professional work, does not go through the formality of marking off his meridian line and of setting out by an instrument the bearings of particular peaks and towers for the purpose of getting a record of them on paper. As a mere traveller for pleasure he will probably have with him nothing more than a map and a pocket compass ; and his aim will be only to identify particular mountain-tops, church towers, villages, or other objects of special interest. There is only one way in which he will attempt to proceed. Assuring himself by means of his guide-book or map, or by the aid of local information, of some one, two or three prominent objects, the identity of which evidently admits of no doubt, he will then feel his way by eye first from one point and then from another point, constantly consulting his map and compass. By some such process as this, after starting with two or three objects recognised to a certainty, he will be able to learn the names of all the places within sight which he wishes to identify, to the number it may be of several dozen.

This mode of procedure may be commended to the would-be student of the Starry Heavens. Such an one should obtain practice for his work by making sure at starting of the names of two or three prominent stars. He should then feel his way in between them by fixing in his mind, one after another, minor triangles of stars, comparing every one with his map as he goes along, taking particular care not to proceed with the identification of a second triangle until he has quite satisfied himself that he has accurately identified the stars forming the first.

It has already been mentioned that the practice has long prevailed of designating the more conspicuous stars in every constellation by the letters of the Greek alphabet. An adequate knowledge of the small letters of this alphabet is therefore an indispensable accomplishment for every student of the starry heavens.

I will now endeavour to apply the foregoing ideas to the study of the stars, starting with the Great Bear as being the most conspicuous of those constellations which never set In the latitude of London. The tail and hind-quarters consist of 7 brilliant stars. Four of these (a, ß, y, a) have long been likened to a wain or waggon, the other 3 (e, t, ri) being fancifully called the horses ; the 7 taken together making " Charles's Vain " or the "Plough " to mention some old English designations.* The hind wheels or the 2 stars (ß, a) farthest from the horses are called the " Pointers," because they point towards the Pole Star (a Ursae Minoris) at the tip of the Little Bear's tail. A line carried from the Pointers beyond the Pole Star leads to Cepheus and Cassiopeia constellations abutting on the sIilky Way where it comes nearest to the Pole. Cassiopeia comprises several prominent stars which form a group resembling the letter W or the letter M, according to the time of year at which they are viewed. The 2 northern-most wheels of the waggon (8, a, Ursae Majoris) point to the bright star Capella in Auriga, which is also circumpolar in British latitudes, but not in the United States. The stars of the Great Bear may be advantageously employed by the student as an approximate scale of angular distances in making estimates of the distances between star and star. Thus :—The Pointer (a) nearest to the Pole is 281° from it ; from 1 to y is 8° , from C to 0 is 7° ; from Stoeis51°; from atoßis5°; from y to 8 and from to C is in both cases 4 1/2.

Descending diagonally along the Milky Way from Cassiopeia towards Capella (a Aurigae) we come to a Persei, and a little farther from the Pole we find Algol (ß Persei), a celebrated variable star in Medusa's head. If we carry our eyes across the Milky Way in the opposite direction we arrive at Deneb, the brightest star (a) of Cygnus (the Swan) ; and beyond Cygnus, a little out of the Milky Way, is Vega, the brightest star (a) in Lyra (the Lyre). Draco (the Dragon) consists of a long winding chain of stars running partly round Ursa Minor (the Little Bear). In the space bounded by Cassiopeia, Cygnus, and Draco, lies the constellation Cepheus.

Near Algenib (y Pegasi) and pointing directly towards it are 2 conspicuous stars of Andromeda (a, B), whilst a 3rd (y) lies a little beyond them. Andromeda will always be readily known by reason of the connection of the bright star (a) in her head with the large trapezium of Pegasus (a, )B, y), the 4 stars forming the wellknown " Square of Pegasus."

An imaginary line projected through the Great Bear and Capella passes to the " Pleiades," the celebrated group in Taurus (the Bull), of which we shall have more to say hereafter (in Chap. XIV. post), and then turning at a right angle reaches Aldebaran (a Tauri, alias the "Bull's-eye ") and the shoulders (a, y) of Orion. Orion is to the naked eye by far the most magnificent of all the constellations, whilst it is peculiarly rich in telescopic objects. Orion may always be identified by the 3 bright stars in its " Belt," which occupies the middle of a large quadrangle of still brighter stars. Aldebaran is a reddish star, the most prominent of the " Hyades," a cluster resembling the letter V, and not far from the Pleiades. Aldebaran, the Pleiades, and Algol (B Persei) make the upper, while Menkab (a Ceti), in the Whale's jaw, with Aries, make the lower points of a large W. The head of Aries (the Ram) is indicated by two principal stars (a, ß), the latter of which has a small attendant.

An imaginary line drawn from the Pole Star and carried midway between the Great Bear and Capella (a Aurigae) passes to Castor and Pollux (a, Geminorum), two well-known stars in the heads of Gemini the Twins ; whilst forwards to the S. of Gemini it will meet Procyon (a Canis Minoris) the brightest star of the Lesser Dog. From thence by bending the line across the Milky Way and carrying it as far again it will reach Sirius (a Canis Majoris) in the Greater Dog's mouth, and will then pass to a somewhat conspicuous star, which in England is quite in the southern horizon, a Columbae, 33deg S. of the middle star in Orion's belt.

Algol (B Persei) and Castor point to Regulus (a Leonis, alias Cor Leonis, the " Lion's heart ") which is situated at one end of an arc with Denebola (B Leonis), the tuft of the Lion's tail at the other end. S. of Regulus and pre-ceding it, i. e., coming to the meridian before it by about j hour, is Cor Hydra (a), the space between them being occupied by the modern and insignificant constellation of the Sextant.

The Pole Star and the middle horse of the waggon (C) direct us to Spica, the brightest star (a) of Virgo, considerably distant, whilst forwards, towards the horizon, we shall reach Centaurus. The Pole Star and the first horse (n Ursae Majoris) conduct us nearly upon Arcturus in Bootes (a), by which fine star, with Spica (a Virginis) and Regulus (a Leonis), a splendid triangle is formed. Following at a distance to the southward is Antares (a Scorpii), " the Rival of Mars," which with Arcturus and Spica constitute another large triangle, having within it the two bright stars, a and B Librae.

Corona Borealis, the Northern Crown, is nearly in a line between Vega (a Lyrae) and Arcturus (a Bootis) ; and the heads of Hercules and Ophiuchus lie between Lyra and Scorpio. In the Milky Way, below the part nearest to Lyra and on a line drawn from Arcturus through the head of Hercules, is the bright star Altair in the Eagle (a Aquilae), which makes with Vega and Deneb (a Cygni) a conspicuous triangle. Closely following Aquila is a remarkable group of stars forming the constellation Delphinus, the Dolphin.

The last and brightest (a) of the 3 principal stars in Andromeda makes with 3 stars of Pegasus (a, ß, y) the large " Square " or trapezium already mentioned, of which the side formed by $ and a points to Fomalhaut (a Piscis Australis), situated in the mouth of the Southern Fish, between the tails of Cetus and Capricornus.

The line of the ecliptic may without difficulty be traced by the observer when his eye becomes familiar with the stars now about to be enumerated. Not far from the Pleiades are the Hyades with Aldebaran (a Tauri), a little S. of the ecliptic. To the N. W. of Aldebaran at some distance is the chief star of Aries (a) ; while to the N. E. of that star are Castor and Pollux (a and (B Geminorum). Regulus (a Leonis) is on the line of the ecliptic ; and Spica (a Virginis) is but a very little to the S. of it. A start being thus made with the ecliptic, the zodiacal constellations will be easily distinguished in their order from W. to E. as follows : Aries lies immediately between Andromeda on the N. and Cetus on the S., the three asterisms reaching nearly from the horizon to the zenith ; Taurus will be recognised by the Pleiades, Aldebaran (a) and the Hyades ; Gemini, the highest of the signs as seen in the Northern hemisphere, by Castor and Pollux (a and ß) ; Cancer, by the historic group Praesepe, in the midst of a waste rather void of stars ; Leo by the stars Regulus (a) and Denebola (,d) ; Virgo, by Spica (a) to the S. of Coma Berenices; Libra in middistance between Virgo and the next constellation Scorpio ; Scorpio, by the red star Antares (a) and its 3 other very conspicuous stars (ß, S, ir,) ; Sagittarius as being the lowest (i. e., most southerly) of all the signs ; Capricornus S. of the Dolphin ; Aquarius under the neck of Pegasus ; and the Pisces between Pegasus, Andromeda, and Cetus. The following familiar lines, though they do not rise to a high standard of " poetry," are nevertheless very convenient as an aid to the memory

The Ram, the Bull, the heavenly Twins,
And next the Crab, the Lion, shines.
The Virgin, and the Scales;
The Scorpion, Archer, and Sea-goat,
The Man that holds the water-pot,
And Fish with glitt'ring tails.

The account just completed of what may be called a " personally conducted " tour of the heavens, is at the best a hasty and superficial performance, and I hope that the bulk of my readers who have accompanied me thus far will aspire to something higher and more exact, even though there may be involved some details, the mastery of which will require a certain amount of effort and application.

A full list of the several constellations arranged in the order in which they come to the meridian, that is to say in the order of their Right Ascensions, will be found in the Appendix ; but it is necessary to explain here what the term " Right Ascension " means, and also what an-other and allied term " Declination " means. Perhaps this will be easiest done by means of a terrestrial analogy.

Everybody, I suppose, knows that Khartoum, the scene of a grievous tragedy, is in Africa. But how many of my readers could open an atlas, turn to the map of Africa, and go straight with his finger-tip to the city of Khartoum ? But if he knew beforehand that Khartoum was situated in latitude 15° 35' N. and longitude 32° 30' E. of Greenwich, the finding of it would be an easy mat-ter, promptly accomplished by the aid of a network of lines' running up and down and across the face of the map. Now what latitude and longitude are for terrestrial geography, declination and right ascension are for celestial geography (so to speak), only just a little different.

It is not difficult to make clear what declination is, but an explanation of right ascension will not be taken in so readily. We have already seen that the whole visible sky is to be regarded as in some sense a sphere, with us, on the earth, apparently as its centre ; and that the aforesaid sphere turns on an imaginary axis directed to 2 poles. Midway between the 2 poles lies the equator, and as it is a semi-circle (or 18o°) from pole to pole the polar distance of the celestial equator (which is the earth's equator pro-longed to the heavens) will be 90°. For some purposes it is occasionally the practice of astronomers to count angular distances from the N. pole towards the equator, but the regular and ordinary practice is to count from the equator to the poles, N. or S., as the case may be. Hence we obtain the expressions " north declination " and " south declination," as applied to the places of the stars, and these expressions are, in a certain sense, the counterpart of the expressions " north latitude " and " south latitude " used with reference to places on the earth.

The term " right ascension " is not to be brought home to the mind quite so easily. In the case of terrestrial longitudes there is no difficulty in finding a definite and immovable terminus to start from. Many European nations are using the meridian of Greenwich for this purpose, though Frenchmen count from Paris, Germans from Berlin, and so on. But in the case of the stars a fixed zero is not so easy to find and still less easy to keep. How-ever, astronomers have long been agreed to make what is called the " First point of Aries," alias the " Vernal Equinox," their starting-point for right ascensions. This is the point where the sun, in the course of its annual journey through the signs of the zodiac, crosses the equator, going from south to north, in the month of March on the 20th day of that month. The phrase "vernal equinox " means the moment of equal day and equal night in the spring.* It is also at this moment that the clocks used by astronomers in their observatories read oh. om. os. Owing to the operation of disturbing causes, the nature and description of which do not belong to this chap-ter, or indeed to this volume, this point is incessantly shifting in the heavens. By virtue of a change called "the precession of the equinoxes," the actual place of the equinox goes backwards about 50" every year, and this is what I meant by saying above that the zero for celestial longitudes is not only not easy to find but when found cannot readily be kept. It must suffice, then, for my present purpose to remark that if we wish to fix the right ascension of a star we must imagine a meridian to pass through it ; then imagine a meridian to pass through the vernal equinox and note the angle which the former meridian makes with the latter measured in degrees of arc along the equator from W. to E. That angle will be the star's R. A. It may be expressed either in degrees, minutes, and seconds of arc (° ' "), or in hours, minutes, and seconds of time (h. m. s.). The latter method is now universally employed, the former having been discarded.

The relation of arc to time in connection with the measurement of angles of right ascension will be readily remembered by noting that a minute or second of time represents a space of 15 times the corresponding denomination in arc, while the hour is 15 times one degree, that is 15°. The minute and second of time are denoted by the initial letter of their names, whilst the minute and second of arc are denoted by special symbols. Thus we arrive at the following little table which the reader should get clearly fixed on his mind.

Perhaps this is as good a place as any at which to warn the reader against a trap which he is very apt to fall into. The " signs of the zodiac " are not the same as the "constellations of the zodiac " (more often spoken of as the zodiacal constellations). Twenty centuries or so ago the astronomers of antiquity, with the 12 zodiacal constellations within their knowledge, got into the natural and not inconvenient habit of talking of the sun in its apparent annual journey through the heavens along the ecliptic as passing successively into and out of the several signs of the zodiac. Each of these signs was regarded as occupied by a constellation from which it took its particular name. Commencing at the vernal equinox the first 3o° through which the sun passed, or the region of stars in which the sun was located during the month following, was called the sign Aries. The second 300 was called the sign Taurus, and so on through the 12 signs, which are identical in name and follow in the same order as the existing 12 zodiacal constellations. Although there are still 12 signs and 12 constellations, sign and constellation no longer correspond. Though the sun when it crosses the equator in the month of March enters the sign Aries, it does not reach the constellation Aries till nearly a month later. This discrepancy is due to the yearly accumulations of 5o" each which have been, going on during the 20 centuries mentioned and which are connected with the phenomenon of the precession of the equinoxes already briefly alluded to.

These preliminary explanations will suffice to enable the reader now to settle down seriously to a study of the constellations. This task must be carried out on starlight nights with the aid of a good star-atlas * and a bull's-eye lantern, assisted or not, as may be convenient, by an opera-glass. In the Appendix will be found a Table of the constellations, omitting a few insignificant modern ones not generally recognised by astronomers.