Part 34
CHAP. XX.
_Of the fixed Stars._
[Sidenote: Why the fixed Stars appear bigger when viewed by the bare eye than when seen through a telescope.]
391. The Stars are said to be fixed, because they have been generally observed to keep at the same distance from each other: their apparent diurnal revolutions being caused solely by the Earth’s turning on its Axis. They appear of a sensible magnitude to the bare eye, because the retina is affected not only by the rays of light which are emitted directly from them, but by many thousands more, which falling upon our eye-lids, and upon the aerial particles about us, are reflected into our eyes so strongly as to excite vibrations not only in those points of the retina where the real images of the Stars are formed, but also in other points at some distance round about. This makes us imagine the Stars to be much bigger than they would appear, if we saw them only by the few rays which come directly from them, so as to enter our eyes without being intermixed with others. Any one may be sensible of this, by looking at a Star of the first Magnitude through a long narrow tube; which, though it takes in as much of the sky as would hold a thousand such Stars, yet scarce renders that one visible.
[Sidenote: A proof that they shine by their own light.]
The more a telescope magnifies, the less is the aperture through which the Star is seen; and consequently the fewer rays it admits into the eye. Now since the Stars appear less in a telescope which magnifies 200 times than they do to the bare eye, insomuch that they seem to be only indivisible points, it proves at once both that the Stars are at immense distances from us, and that they shine by their own proper light. If they shone by borrowed light they would be as invisible without telescopes as the Satellites of Jupiter are: for these Satellites appear bigger when viewed with a good telescope than the largest fixed Stars do.
[Sidenote: Their number much less than is generally imagined.]
392. The number of Stars discoverable, in either Hemisphere, by the naked eye, is not above a thousand. This at first may appear incredible, because they seem to be without number: But the deception arises from our looking confusedly upon them, without reducing them into any order. For look but stedfastly upon a pretty large portion of the sky, and count the number of Stars in it, you will be surprised to find them so few. Or, if one considers how seldom the Moon meets with any Stars in her way, although there are as many about her Path as in other parts of the Heavens (the _Milky way_ excepted) he will soon be convinced that the Stars are much thinner sown than he was aware of. The _British_ catalogue, which, besides the Stars visible to the bare eye, includes a great number which cannot be seen without the assistance of a telescope, contains no more than 3000, in both Hemispheres.
[Sidenote: The absurdity of supposing the Stars were made only to enlighten our nights.]
393. As we have incomparably more light from the Moon than from all the Stars together, it were the greatest absurdity to imagine that the Stars were made for no other purpose than to cast a faint light upon the Earth: especially since many more require the assistance of a good telescope to find them out, than are visible without that Instrument. Our Sun is surrounded by a system of Planets and Comets; all which would be invisible from the nearest fixed Star. And from what we already know of the immense distance of the Stars, the nearest may be computed at 32,000,000,000,000 of miles from us which is more than a cannon bullet would fly in 7,000,000 of years. Hence ’tis easy to prove, that the Sun seen from such a distance, would appear no bigger than a Star of the first magnitude. From all this it is highly probable that each Star is a Sun to a system of worlds moving round it though unseen by us; especially, as the doctrine of a plurality of worlds is rational, and greatly manifests the Power, Wisdom, and Goodness of the great Creator.
[Sidenote: Their different Magnitudes.]
394. The Stars, on account of their apparently various magnitudes, have been distributed into several classes or orders. Those which appear largest are called _Stars of the first magnitude_; the next to them in lustre, _Stars of the second magnitude_, and so on to the _sixth_, which are the smallest that are visible to the bare eye. This distribution having been made long before the invention of telescopes, the Stars which cannot be seen without the assistance of these instruments are distinguished by the name of _Telescopic Stars_.
[Sidenote: And division into Constellations.]
395. The antients divided the starry Sphere into particular Constellations, or Systems of Stars, according as they lay near one another, so as to occupy those spaces which the figures of different sorts of animals or things would take up, if they were there delineated. And those Stars which could not be brought into any particular Constellation were called _unformed Stars_.
[Sidenote: The use of this division.]
396. This division of the Stars into different Constellations or Asterisms, serves to distinguish them from one another, so that any particular Star may be readily found in the Heavens by means of a Celestial Globe; on which the Constellations are so delineated as to put the most remarkable Stars into such parts of the figures as are most easily distinguished. The number of the antient Constellations is 48, and upon our present Globes about 70. On _Senex_’s Globes are inserted _Bayer_’s Letters; the first in the Greek Alphabet being put to the biggest Star in each Constellation, the second to the next, and so on: by which means, every Star is as easily found as if a name were given to it. Thus, if the Star γ in the Constellation of the _Ram_ be mentioned, every Astronomer knows as well what Star is meant as if it were pointed out to him in the Heavens.
[Sidenote: The _Zodiac_.]
397. There is also a division of the Heavens into three parts. 1. The _Zodiac_, (ζωδιακὸς) from ζώδιον _Zodion_ an Animal, because most of the Constellations in it, which are twelve in number, are the figures of Animals: as _Aries_ the Ram, _Taurus_ the Bull, _Gemini_ the Twins, _Cancer_ the Crab, _Leo_ the Lion, _Virgo_ the Virgin, _Libra_ the Balance, _Scorpio_ the Scorpion, _Sagittarius_ the Archer, _Capricornus_ the Goat, _Aquarius_ the Water-bearer, and _Pisces_ the Fishes. The Zodiac goes quite round the Heavens: it is about 16 degrees broad, so that it takes in the Orbits of all the Planets, and likewise the Orbit of the Moon. Along the middle of this Zone or Belt is the Ecliptic, or Circle which the Earth describes annually as seen from the Sun; and which the Sun appears to describe as seen from the Earth. 2. All that Region of the Heavens, which is on the north side of the Zodiac, containing 21 Constellations. And 3. that on the south side, containing 15.
[Sidenote: The manner of dividing it by the antients.]
398. The antients divided the _Zodiac_ into the above 12 Constellations or Signs in the following manner. They took a vessel with a small hole in the bottom, and having filled it with water, suffered the same to distil drop by drop into another Vessel set beneath to receive it; beginning at the moment when some Star rose, and continuing until it rose the next following night. The water fallen down into the receiver they divided into twelve equal parts; and having two other small vessels in readiness, each of them fit to contain one part, they again poured all the water into the upper vessel, and strictly observing the rising of some Star in the _Zodiac_, they at the same time suffered the water to drop into one of the small vessels; and as soon as it was full, they shifted it and set an empty one in it’s place. By this means, when each vessel was full, they observed what Star of the _Zodiac_ rose; and though not possible in one night, yet in many, they observed the rising of twelve Stars, by which they divided the _Zodiac_ into twelve parts.
399. The names of the Constellations, and the number of Stars observed in each of them by different Astronomers, are as follows.
Key to table P = _Ptolemy._ T = _Tycho._ H = _Hevelius._ F = _Flamsteed._
The antient Constellations. P T H F Ursa minor The Little Bear 8 7 12 24 Ursa major The Great Bear 35 29 73 87 Draco The Dragon 31 32 40 80 Cepheus Cepheus 13 4 51 35 Bootes, _Arctophilax_ 23 18 52 54 Corona Borealis The northern Crown 8 8 8 21 Hercules, _Engonasin_ Hercules kneeling 29 28 45 113 Lyra The Harp 10 11 17 21 Cygnus, _Gallina_ The Swan 19 18 47 81 Cassiopea The Lady in her Chair 13 26 37 55 Perseus Perseus 29 29 46 59 Auriga The Waggoner 14 9 40 66 Serpentarius, _Ophiuchus_ Serpentarius 29 15 40 74 Serpens The Serpent 18 13 22 64 Sagitta The Arrow 5 5 5 18 Aquila, _Vultur_ The Eagle } 12 23 } 15 71 Antinous Antinous } 3 19 Delphinus The Dolphin 10 10 14 18 Equulus, _Equi sectio_ The Horse’s Head 4 4 6 10 Pegasus, _Equus_ The Flying Horse 20 19 38 89 Andromeda Andromeda 23 23 47 66 Triangulum The Triangle 4 4 12 16 Aries The Ram 18 21 27 66 Taurus The Bull 44 43 51 141 Gemini The Twins 25 25 38 85 Cancer The Crab 23 15 29 83 Leo The Lion } 30 49 95 } 35 Coma Berenices Berenice’s Hair } 14 21 43 Virgo The Virgin 32 33 50 110 Libra, _Chelæ_ The Scales 17 10 20 51 Scorpius The Scorpion 24 10 20 44 Sagittarius The Archer 31 14 22 69 Capricornus The Goat 28 28 29 51 Aquarius The Water-bearer 45 41 47 108 Pisces The Fishes 38 36 39 113 Cetus The Whale 22 21 45 97 Orion Orion 38 42 62 78 Eridanus, _Fluvius_ Eridanus, _the River_ 34 10 27 84 Lepus The Hare 12 13 16 19 Canis major The Great Dog 29 13 21 31 Canis minor The Little Dog 2 2 13 14 Argo Navis The Ship 45 3 4 64 Hydra The Hydra 27 19 31 60 Crater The Cup 7 3 10 31 Corvus The Crow 7 4 9 Centaurus The Centaur 37 35 Lupus The Wolf 19 24 Ara The Altar 7 9 Corona Australis The southern Crown 13 12 Pisces Australis The southern Fish 18 24
The New Southern Constellations.
Columba Noachi Noah’s Dove 10 Robur Carolinum The Royal Oak 12 Grus The Crane 13 Phœnix The Phenix 13 Indus The Indian 12 Pavo The Peacock 14 Apus, _Avis Indica_ The Bird of Paradise 11 Apis, _Musca_ The Bee or Fly 4 Chamæleon The Chameleon 10 Triangulum Australis The South Triangle 5 Piscis volans, _Passer_ The Flying Fish 8 Dorado, _Xiphias_ The Sword Fish 6 Toucan The American Goose 9 Hydrus The Water Snake 10
_Hevelius_’s Constellations made out of the unformed Stars.
_Hevelius._ _Flamsteed._ Lynx The Lynx 19 44 Leo minor The Little Lion 53 Asterion & Chara The Greyhounds 23 25 Cerberus Cerberus 4 Vulpecula & Anser The Fox and Goose 27 35 Scutum Sobieski Sobieski’s Shield 7 Lacerta The Lizard 10 16 Camelopardalus The Camelopard 32 58 Monoceros The Unicorn 19 31 Sextans The Sextant 11 41
[Sidenote: The _Milky Way_.]
400. There is a remarkable track round the Heavens, called the _Milky Way_ from its peculiar whiteness, which is owing to a great number of Stars scattered therein; none of which can be distinctly seen without telescopes. This track appears single in some parts, in others double.
[Sidenote: Lucid Spots.]
401. There are several little whitish spots in the Heavens, which appear magnified, and more luminous when seen through telescopes; yet without any Stars in them. One of these is in _Andromeda_’s girdle, first observed _A. D._ 1612, by _Simon Marius_; and which has some whitish rays near its middle: it is liable to several changes, and is sometimes invisible. Another is near the Ecliptic, between the head and bow of _Sagittarius_: it is small, but very luminous. A third is on the back of the _Centaur_, which is too far South to be seen in _Britain_. A fourth, of a smaller size, is before _Antinous_’s right foot; having a Star in it, which makes it appear more bright. A fifth is in the Constellation of _Hercules_, between the Stars ζ and η, which spot, though but small, is visible to the bare eye if the sky be clear and the Moon absent.
[Sidenote: Cloudy Stars.
Magellanic Clouds.]
402. _Cloudy Stars_ are so called from their misty appearance. They look like dim Stars to the naked eye; but through a telescope they appear broad illuminated parts of the sky; in some of which is one Star, in others more. Five of these are mentioned by _Ptolemy_. 1. One at the extremity of the right hand of _Perseus_. 2. One in the middle of the _Crab_. 3. One unformed, near the Sting of the _Scorpion_. 4. The eye of _Sagittarius_. 5. One in the head of _Orion_. In the first of these appear more Stars through the telescope than in any of the rest, although 21 have been counted in the head of _Orion_, and above 40 in that of the _Crab_. Two are visible in the eye of _Sagittarius_ without a telescope, and several more with it. _Flamsteed_ observed a cloudy Star in the bow of _Sagittarius_, containing many small Stars: and the Star _d_ above _Sagittary_’s right shoulder is encompassed with several more. Both _Cassini_ and _Flamsteed_ discovered one between the _Great_ and _Little Dog_, which is very full of Stars visible only by the telescope. The two whitish spots near the South Pole, called the _Magellanic Clouds_ by Sailors, which to the bare eye resemble part of the Milky-Way, appear through telescopes to be a mixture of small Clouds and Stars. But the most remarkable of all the cloudy Stars is that in the middle of _Orion’s Sword_, where seven Stars (of which three are very close together) seem to shine through a cloud, very lucid near the middle, but faint and ill defined about the edges. It looks like a gap in the sky, through which one may see (as it were) part of a much brighter region. Although most of these spaces are but a few minutes of a degree in breadth, yet, since they are among the fixed Stars, they must be spaces larger than what is occupied by our solar System; and in which there seems to be a perpetual uninterrupted day among numberless Worlds which no human art ever can discover.
[Sidenote: Changes in the Heavens.]
403. Several Stars are mentioned by antient Astronomers, which are not now to be found; and others are now visible to the bare eye which are not recorded in the antient catalogues. _Hipparchus_ observed a new Star about 120 years before CHRIST; but he has not mentioned in what part of the Heavens it was seen, although it occasioned his making a catalogue of the Stars; which is the most antient that we have.
[Sidenote: New Stars.]
The first _New Star_ that we have any good account of, was discovered by _Cornelius Gemma_ on the 8th of _November_ A. D. 1572, in the Chair of Cassiopea. It surpassed _Sirius_ in brightness and magnitude; and was seen for 16 months successively. At first it appeared bigger than _Jupiter_ to some eyes, by which it was seen even at mid-day: afterwards it decayed gradually both in magnitude and lustre, until _March_ 1573, when it became invisible.
On the 13th of _August_ 1596, _David Fabricius_ observed the _Stella Mira_, or wonderful Star, in the _Neck_ of the _Whale_; which has been since found to appear and disappear periodically, seven times in six years, continuing in its greatest lustre for 15 days together; and is never quite extinguished.
In the year 1600, _William Jansenius_ discovered a changeable Star in the _Neck_ of the _Swan_; which, in time became so small as to be thought to disappear entirely, till the years 1657, 1658, and 1659, when it recovered its former lustre and magnitude; but soon decayed, and is now of the smallest size.
In the year 1604 _Kepler_ and several of his friends saw a new Star near the heel of the right foot of _Serpentarius_, so bright and sparkling that it exceeded any thing they had ever seen before; and took notice that it was every moment changing into some of the colours of the rainbow, except when it was near the horizon, at which time it was generally white. It surpassed _Jupiter_ in magnitude, which was near it all the month of _October_, but easily distinguished from it by a steady light. It disappeared between _October_ 1605 and the _February_ following, and has not been seen since that time.
In the year 1670, _July_ 15, _Hevelius_ discovered a new Star, which in _October_ was so decayed as to be scarce perceptible. In _April_ following it regained its lustre, but wholly disappeared in _August_. In _March_ 1672 it was seen again, but very small; and has not been visible since.
In the year 1686 a new Star was discovered by _Kirch_, which returns periodically in 404 days.
In the year 1672, _Cassini_ saw a Star in the _Neck_ of the Bull, which he thought was not visible in _Tycho_’s time; nor when _Bayer_ made his Figures.
[Sidenote: Cannot be Comets.]
404. Many Stars, besides those above-mentioned, have been observed to change their magnitudes: and as none of them could ever be perceived to have tails, ’tis plain they could not be Comets; especially as they had no parallax, even when largest and brightest. It would seem that the periodical Stars have vast clusters of dark spots, and very slow rotations on their Axis; by which means, they must disappear when the side covered with spots is turned towards us. And as for those which break out all of a sudden with such lustre, ’tis by no means improbable that they are Suns whose Fuel is almost spent, and again supplied by some of their Comets falling upon them, and occasioning an uncommon blaze and splendor for some time: which indeed appears to be the greatest use of the cometary part of any system[86].
[Sidenote: Some Stars change their Places.]
Some of the Stars, particularly _Arcturus_, have been observed to change their places above a minute of a degree with respect to others. But whether this be owing to any real motion in the Stars themselves, must require the observations of many ages to determine. If our solar System changeth its Place, with regard to absolute space, this must in process of time occasion an apparent change in the distances of the Stars from each other: and in such a case, the places of the nearest Stars to us being more affected than of those which are very remote, their relative positions must seem to alter, though the Stars themselves were really immoveable. On the other hand, if our own system be at rest, and any of the Stars in real motion, this must vary their positions; and the more so, the nearer they are to us, or the swifter their motions are; or the more proper the direction of their motion is, for our perception.
[Sidenote: The Ecliptic less oblique now to the Equator than formerly.]
405. The obliquity of the Ecliptic to the Equinoctial is found at present to be above a third part of a degree less than _Ptolemy_ found it. And most of the observers after him found it to decrease gradually down to _Tycho_’s time. If it be objected, that we cannot depend on the observations of the antients, because of the incorrectness of their Instruments; we have to answer, that both _Tycho_ and _Flamsteed_ are allowed to have been very good observers: and yet we find that _Flamsteed_ makes this obliquely 2-1/2 minutes of a degree less than _Tycho_ did, about 100 years before him: and as _Ptolemy_ was 1324 years before _Tycho_, so the gradual decrease answers nearly to the difference of time between these three Astronomers. If we consider, that the Earth is not a perfect sphere, but an oblate spheroid, having its Axis shorter than its Equatoreal diameter; and that the Sun and Moon are constantly acting obliquely upon the greater quantity of matter about the Equator, pulling it, as it were, towards a nearer and nearer co-incidence with the Ecliptic; it will not appear improbable that these actions should gradually diminish the Angle between those Planes. Nor is it less probable that the mutual attractions of all the Planets should have a tendency to bring the planes of all their Orbits to a co-incidence: but this change is too small to become sensible in many ages.
CHAP. XXI.
_Of the Division of Time. A perpetual Table of New Moons._ _The Times of the Birth and Death of_ CHRIST. _A Table of remarkable Æras or Events._
406. The parts of time are _Seconds_, _Minutes_, _Hours_, _Days_, _Years_, _Cycles_, _Ages_, and _Periods_.
[Sidenote: A Year.]
407. The original standard, or integral measure of Time, is a year; which is determined by the Revolution of some Celestial Body in its Orbit, _viz._ the _Sun_ or _Moon_.
[Sidenote: Tropical Year.]
408. The time measured by the Sun’s Revolution in the Ecliptic, from any Equinox or Solstice to the same again, is called the _Solar_ or _Tropical Year_, which contains 365 days 5 hours 48 minutes 57 seconds; and is the only proper or natural year, because it always keeps the same seasons to the same months.
[Sidenote: Sidereal year.]
409. The quantity of time, measured by the Sun’s Revolution, as from any fixed Star to the same Star again, is called the _Sidereal Year_; which contains 365 days 6 hours 9 minutes 14-1/2 seconds; and is 20 minutes 17-1/2 seconds longer than the true Solar Year.
[Sidenote: Lunar Year.]