Astronomy for Everybody/Part 6/Chapter 3

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The present chapter is intended for those who wish to be able to recognise the principal constellations, and to know where to look for the several planets. The problem of pointing out the constellations is complicated by the effect of the twofold motion of the earth; on its axis and around the sun. In consequence of the former the constellations change their apparent position in the course of the night, and the result of the latter is that different constellations are seen at different seasons.

We explained in a former chapter how, in consequence of the motion of the earth in its orbit round the sun, the latter seems to us to perform an annual circuit among the constellations. Hence, if a star is east of the sun, we shall see it approach nearer to the sun every day. If we look out night after night at the same hour we shall find it farther and farther advanced toward the west. In consequence of this change it must rise and set earlier every day than it did the day before. More exactly, the time between two risings and settings of the same star is twenty-three hours fifty-six minutes four and a half seconds. While in the course of a year the sun rises three hundred and sixty-five times, a star rises three hundred and sixty-six times. The latter will therefore during the year have risen at every hour of the day and night.

​Astronomers avoid all confusion from this cause by the use of sidereal time, that is star-time, or time measured by the stars. As already explained, a sidereal day is the interval between two successive pasages of a star over the meridian, and is three minutes fifty-six seconds less than our ordinary day. It is divided into twenty-four sidereal hours, and each hour into sidereal minutes and seconds. A sidereal clock gains three minutes fifty-six seconds daily on an ordinary clock and thus shows the same time at the same position of the stars the year around.

One who wishes to keep the run of the stars will find it very convenient to have some idea of sidereal time. This may be had by the following rule: Double the number of the month; the product will be the sidereal time at six o'clock in the evening. At seven o'clock it will be one hour later, and at eight it will be two hours later, and so on.

Suppose, for example, that one looks at the sky in November at nine o'clock in the evening. This is the eleventh month; multiplying by two gives twenty-two, adding three gives twenty-five, from which we drop twenty-four, giving one hour as the sidereal time. The time thus obtained will not often be more than an hour in error, except during the first week or ten days of the month, when it may be an hour or more too great. It may then be diminished by one hour.

Applying the same rule in January we have five hours as the sidereal time at nine in the evening. But early in the month the sidereal time at nine in the evening will be four hours instead of five.

​At 0 hours sidereal time the equinoctial colure is on the meridian; at six hours, the solstitial colure, and so on.

With this preliminary explanation let us proceed to the study of the constellations. I assume the reader to be somewhere in the latitude of the United States. Then the principal northern constellations will never set, and will be visible in whole or in part every evening in the year. With them, therefore, we begin.

A figure, showing these constellations, is found in the first part of the present book (Fig. 2). To see how they will appear hold the cut with the month at top; we then have the position at eight o'clock in the evening. For a later hour turn it a little in the direction of the arrows. For example, in July, at ten o'clock, we hold it so as to have August at the top.
Fig. 50.—Ursa Major, or The Dipper. The Roman numerals on top give the sidereal time without the trouble of calculating it.

First find Ursa Major, the Great Bear, generally called the Dipper, an implement which the constellation resembles much more than it does a bear. This you can always do except perhaps in autumn when, if you are far south, it may be more or less below the northern horizon. Notice the pair of stars forming the outside ​of the bowl of the dipper. They are called the Pointers, because they point toward the pole star, as shown by the
Fig. 51.—Ursa Minor. dotted line. This is the central star of the map. It is called Polaris.

The pole star belongs to the constellation Ursa Minor, the Lesser Bear; the rest of the constellation you will see by following a curved line of stars from the pole toward XVI hours. You will thus fall on another star as bright as Polaris but a little redder in colour. This is Beta Ursæ Minoris.

If you cannot see the pointers you will still easily find Polaris if you know the exact north, because it is nearly midway between the zenith and the northern horizon—nearer the latter, however,
Fig. 52.—Cassiopeia. the farther south we are. It can be easily distinguished from its neighbour, Beta, by its whiter colour, Beta being slightly red or dingy in comparison.

On the opposite side of the pole, at the same distance as Ursa Major, is Cassiopeia, the Lady in the Chair. The chair has a very crooked back but could be made comfortable by a cushion in the hollow.

There are several other constellations in the region ​around the pole, but they have few bright stars and are of less interest than those we have mentioned. Among them is Draco, the Dragon, whose form coils itself up between the Bears, and whose head is represented by a triangle of stars in XVIII hours, near the August zenith.

The zenithal and southern constellations to be looked for will vary with the season. We begin with the position of the sphere at 0 hours sidereal time, which occurs at ten o'clock in October, eight in November, and six in December.

The equinoctial colure is first to be imagined. It passes from the pole upward near the westernmost bright star of Cassiopeia and can be traced south through the eastern side of the square of Pegasus. The latter easily recognised landmark of the sky is formed by four stars of the second or third magnitude. The square is fifteen degrees on a side.

Northeast from the northeast corner of the square is the Great Nebula of Andromeda. It is plainly visible to the naked eye as a whitish, ill-defined patch of light, and is a fine object when seen in a telescope.

The Milky Way now spans the heavens like a slightly inclined arch, resting on the east and west regions of the horizon, and having its keystone a little north of the zenith, in Cassiopeia. Tracing it from this constellation toward the east, we first have Perseus, which stands in the Milky Way itself. The brightest star in this constellation is Alpha Persei, of the second magnitude.

​East of Alpha is a white mass like a httle cloud. With a small telescope, even with a good field glass, we see this mass to be a collection or cluster of small stars. It is the Great Cluster of Perseus and, in the figure of the constellation, forms the hilt of the hero's sword.

In a sort of offshoot toward the south (or southeast as the constellation is now situated) lies a row of three stars. The middle and brightest of these is the wonderful variable star, Algol, whose changes will be described in a later chapter. It is also called Beta Persei.

Below Perseus, the first large constellation is Auriga, the Charioteer. It is marked by Capella, the Goat, a star of the first magnitude and one of the brightest now above the horizon—indeed, one of the four or five brightest in the sky. But it has no other striking stars.

In the southeast are Aldebaran and the Pleiades, which will be described later. Meanwhile let us follow the course of the Milky Way from the zenith toward the west.

The first collection of bright stars west of Cassiopeia is now Cygnus, the Swan, lying centrally in the Milky Way. Five stars are arranged somewhat in the form of a cross and mark the body, neck, and extended wings of the bird. The brightest of the group is Alpha Cygni, or Deneb, nearly, but not quite, of the first magnitude.

Low and to the right of Cygnus, and a little outside of the Milky Way, is the constellation Lyra, the Harp, marked by the beautiful and very bright bluish star, Vega. It has no other star of greater magnitude than the third, but what it has will repay careful study. ​In the figure given here, notice the star to the left of Vega; Epsilon Lyræ it is called. A keen eye will, on careful examination, see that this star is really composed of two, lying so close together that it is not easy to distinguish them. With an opera glass this will more easily be accomplished.
Fig. 53.—Lyra, the Harp. But the most curious fact is that if a telescope be pointed at the pair, each of the stars will be found to be double, so that Epsilon Lyræ is really composed of four stars.

Another star, about as near to Vega as Epsilon is, lies at one corner of a parallelogram or elongated diamond, which stretches south of Vega., At the farther blunt corner of the diamond lies Beta Lyræ, marked B in the figure, a remarkable variable star. To the left of it is Gamma. The law of variation will be described in a later chapter.

To the right of Lyra, and In the Milky Way, lies Aquila, the Eagle. It will be described later.

The other constellations low in the west will be described later. At present we shall pass rapidly over the constellations of the Zodiac.

If the ecliptic were painted on the sky we should now see it rising to the north of the east point of the horizon, passing in the south to mid-sky, where it would cross the ​equator at a small angle, and then, passing to the west, reach the western horizon twenty-three degrees south of west. At the time we suppose, Sagittarius, the Archer, is mostly below the western horizon. Capricornus, the Goat; Aquarius, the Water Bearer, and Pisces, the Fishes, fill up the space to the meridian. The stars of these constellations are mostly faint, few or none exceeding the third magnitude.

Reaching the meridian, we see the square of Pegasus above the Zodiac, not far south of the zenith. East of it is the constellation Aries, the Ram. Three of its principal stars, of the second, third, and fourth magnitudes, form an obtuse triangle. The brightest is Alpha Arietis.

Two thousand years ago this constellation marked the first sign of the zodiac, and the equinox was just below Alpha Arietis, as explained in speaking of the precession of the equinoxes.

Southeast from the square of Pegasus is a widely extended constellation, Cetus, the Whale. Its two brightest stars. Alpha and Beta, are of the second magnitude. The latter lies nearly below the southeast star of the square of Pegasus and is quite by itself. Alpha is some distance farther east. West of Alpha, and a little south, is a remarkable star, Mira Ceti, the wonderful star of Cetus, which is invisible to the naked eye except for a month or two in each year, when it attains the fourth, third, and often the second magnitude.

A little west of south, quite low down, is Fomalhaut, nearly of the first magnitude, in the constellation Pisces Australis, the Southern Fish.

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The next position of the stars we shall describe comes six hours after the preceding one; that is at two o'clock A. M. in November
Fig. 54.—The Hyades. and at eight o'clock P. M. in February. During this six-hour interval another section of the Milky Way has risen in the east and passed over toward the south. The Milky Way now passes nearly through the zenith, resting on the horizon near the north and south points.

Near its course and east of the meridian we see the constellation Taurus, the Bull, of which the brightest star
Fig. 55.—The Pleiades, as seen with the naked eye. is Aldebaran, forming the eye of the bull in the mythological figure. Aldebaran is easily recognised by its red colour. It lies on the end of one branch of a V-shaped cluster called Hyades. Notice the pretty pair of stars in the middle of one leg.

​Near by is the best known cluster in the sky, the Pleiades, or "seven stars." Only six stars are made out by ordinary unaided vision, but to a good eye five others

are visible, making eleven in all. The term "seven stars" is therefore a misnomer; as a reason for it, it was said in ancient times that the number was originally seven but that one faded away. This "lost Pleiad" is probably a myth, as we do not find stars fading away permanently. ​With a telescope we find the cluster to contain quite a number of yet smaller stars, as can be seen by the telescopic view which we give.

The central and brightest star of the group is called Alcyone, and was supposed by Maedler to be the central star of the universe. But this notion is quite baseless.

East of Taurus and near the zenith is Gemini, the Twins, marked by two stars nearly of the first magnitude, Castor and Pollux. The latter is the northernmost and a little the brighter of the two.

The next zodiacal constellation is Cancer, the Crab, but it contains no conspicuous stars. Its most noticeable feature is Prœsepe, a cluster of stars, which are singly invisible to the naked eye, and look collectively like a small patch of light. The smallest telescope will show a dozen stars in the patch.

Leo, the Lion, is also well up in the east. It may be recognised by Regulus, a star nearly of the first magnitude, and a curved row of stars in the form of a sickle, of which Regulus is the handle.

In the south we now have the most brilliant constellation in the heavens, the beautiful Orion. The three stars of the second magnitude in a row forming the belt of the warrior are familiar from childhood to all who watch the sky. Below them hangs another row of three stars, the upper one quite faint. The middle one of these has a hazy aspect, and is really not a star at all, but one of the most splendid objects in the sky, the Great Nebula of Orion. A mere spy-glass will show its character, but a ​large telescope is required to bring out the magnificence of its form.

The corners of the constellation are marked by four stars. The brighter of the two uppermost,
Fig. 57.—Orion. Alpha Orionis, or Betelguese, is reddish in colour. At the opposite corner is Rigel, blue in colour and also of the first magnitude. The two upper stars are in the shoulders of the figure. Midway and above them a triangle of small stars forms the head.

East of Orion is Canis Minor, the Little Dog, containing Procyon, of the first magnitude. Below it and southeast of Orion is another collection of bright stars forming the constellation Canis Major, the Great Dog, containing Sirius, the Dog Star, the brightest fixed star in the heavens.

The third position of the sphere, sidereal time twelve hours, occurs in February at two A. M.; in May at eight P. M. Lyra has now risen in the northeast and Capella is going downward in the northwest. The Milky ​Way may not be visible at all unless the air is very clear. It will then be seen skirting the northern and western horizon. Regulus has passed the meridian, and Orion and Canis Major have set, or are low down in the southwest.

In mid-heaven, southeast of the zenith, is Arcturus, of a dingy yellow colour,
Fig. 58.—The Northern Crown. but one of the brightest first magnitude stars.

East of Arcturus (now below it) is Corona Borealis, the Northern Crown, a beautiful semicircle of stars, of which the brightest is of the second magnitude.

Near the zenith is Coma Berenices, the Hair of Berenice, a collection of faint stars mostly of the fifth magnitude. East of south across the meridian from Leo is Virgo, the Virgin, conspicuous only by Spica, a white star of nearly the first magnitude. Libra, the Balance, east and southeast of Virgo, has no conspicuous stars.

The fourth position of the sphere, eighteen hours sidereal time, occurs in May at two A. M.; in August at eight P. M. Capella has now set, Lyra is near the zenith, ​Cassiopeia is in the northeast, and the most splendid portion of the Milky Way is near the meridian. We have
Fig. 59.—Aquila. described all the constellations that lie near its course north of Lyra; let us now trace it to the south.

One of the noticeable features of the Milky Way now to be seen is the great bifurcation, or separation into two branches. The split can be traced from Cygnus, where it begins, past Lyra and halfway to the southern horizon. Here we see Aquila, the Eagle, in the cleft, marked by Altair, of the first magnitude. It is in a line between two other stars of the third and fourth magnitudes.

At this point the westernmost branch of the Milky Way
Fig. 60.—Delphinus, the Dolphin. diverges yet farther and seems to terminate, but if the air is clear we shall see that it recommences near the horizon.

East of Aquila is a small but very pretty constellation of which the scientific name is Delphinus, the Dolphin, but which is popularly known as Job's Coffin.

Between Lyra and the beautiful Corona, now some distance west of the zenith, lies the widely extended ​

​constellation, Hercules. Alpha, its brightest star, is below the second magnitude and may be known by its reddish colour and by a white star, Alpha Ophiuchi, a little farther east. The most remarkable object in this constellation is the Great Cluster of Hercules which, to the naked eye, is a very faint patch, but which a great telescope resolves into a universe of stars.

Near the horizon, west of south, is the zodiacal constellation
Fig. 62.—Scorpius, the Scorpion. Scorpius, the Scorpion. Its western boundary is a curved row of stars forming the claws of the animal; east of them is Antares, or Alpha Scorpii, reddish in colour, and nearly of the first magnitude.

In the Milky Way, due south, and therefore east of Scorpius, is Sagittarius, the Archer, with quite a collection of stars of the second and third magnitudes. The bow and arrow of the archer are easily imagined.

Next toward the east are Capricornus and Aquarius, already mentioned. The brightest star in the former has a companion so close to it that it is a sign of not bad eyesight to be able to distinguish it.