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- Herausgeber: Science & Technology Publishing
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
Astronomy Explained Upon Sir Isaac Newton's Principles and Made Easy to Those Who Have Not Studied Mathematics is a comprehensive and accessible guide to the wonders of the universe, written by James Ferguson and first published in the 18th century. This classic work aims to demystify the complex principles of astronomy by presenting them in a clear and straightforward manner, specifically designed for readers without a background in mathematics. Drawing upon the revolutionary theories of Sir Isaac Newton, Ferguson explains the fundamental laws governing celestial bodies, including gravity, planetary motion, eclipses, tides, and the phases of the moon. The book is structured as a series of engaging dialogues and practical examples, making intricate scientific concepts approachable for the general public. Ferguson employs simple language, detailed illustrations, and everyday analogies to illuminate topics such as the structure of the solar system, the nature of comets, the measurement of time, and the use of astronomical instruments. He also addresses common misconceptions and provides historical context, tracing the development of astronomical thought from ancient times to the era of Newton. Throughout the text, Ferguson emphasizes observation and experimentation, encouraging readers to explore the night sky for themselves. The book includes instructions for constructing basic astronomical devices and offers guidance on how to make accurate observations without advanced equipment. By blending scientific rigor with clarity and enthusiasm, Astronomy Explained Upon Sir Isaac Newton's Principles remains a valuable resource for anyone interested in understanding the universe, regardless of their mathematical background. This enduring work not only celebrates the achievements of Newton but also inspires curiosity and wonder about the cosmos in readers of all ages.
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This book uses a number of astronomical symbols, including signs of the Zodiac (♈, ♉, ♊, ♋, ♌, ♍, ♎, ♏, ♐, ♑, ♒, ♓), and symbols for planets (☿, ♀, ⊕, ♂, ♃, ♄). If these characters do not display correctly, you may have to use an alternative font, such as Arial Unicode MS or DejaVu.
The ORRERY, made by JAMES FERGUSON.
N. 1. The Sun, 2. Mercury, 3. Venus, 4. The Earth, 5. The Moon, 6. The Sydereal Dial plate, 7. The Hour Circle, 8. ye Circle for ye. Moon’s Age, 9. The Moon’s Orbit, 10. ye Pointer, Shewing the Sun’s Place & Day of the Month, 11. The Ecliptic, 12. The Handle for turning ye whole machine
J. Ferguson inv. et delin.
G. Child. Sculp.
ASTRONOMYEXPLAINED UPONSir ISAAC NEWTON’sPRINCIPLES,AND MADE EASYTO THOSE WHO HAVE NOT STUDIEDMATHEMATICS.
THE CONTENTS.
CHAP. I.
Of Astronomy in general
Page 1
CHAP. II.
A brief Description of the
Solar System
5
CHAP. III.
The
Copernican
or
Solar System
demonstrated to be true
31
CHAP. IV.
The Phenomena of the Heavens as seen from different parts of the Earth
39
CHAP. V.
The Phenomena of the Heavens as seen from different parts of the Solar System
45
CHAP. VI.
The
Ptolemean
System refuted. The Motions and Phases of Mercury and Venus explained
50
CHAP. VII.
The physical Causes of the Motions of the Planets. The Excentricities of their Orbits. The times in which the Action of Gravity would bring them to the Sun.
Archimedes’s
ideal Problem for moving the Earth. The world not eternal
54
CHAP. VIII.
Of Light. It’s proportional quantities on the different Planets. It’s Refractions in Water and Air. The Atmosphere, it’s Weight and Properties. The Horizontal Moon
62
CHAP. IX.
The Method of finding the Distances of the Sun, Moon and Planets
73
CHAP. X.
The Circles of the Globe described. The different lengths of days and nights, and the vicissitude of Seasons, explained. The explanation of the Phenomena of Saturn’s Ring concluded
78
CHAP. XI.
The Method of finding the Longitude by the Eclipses of Jupiter’s Satellites: The amazing velocity of Light demonstrated by these Eclipses
87
CHAP. XII.
Of Solar and Sidereal Time
93
CHAP. XIII.
Of the Equation of Time
97
CHAP. XIV.
Of the Precession of the Equinoxes
108
CHAP. XV.
The Moon’s Surface mountainous: Her Phases described: Her Path, and the Paths of Jupiter’s Moons delineated: The proportions of the Diameters of their Orbits, and those of Saturn’s Moons to each other; and to the Diameter of the Sun
124
CHAP. XVI.
The Phenomena of the Harvest-Moon explained by a common Globe: The Years in which the Harvest-Moons are least and most beneficial, from 1751 to 1861. The long duration of Moon-light at the Poles in Winter Page
136
CHAP. XVII.
Of the ebbing and flowing of the Sea
147
CHAP. XVIII.
Of Eclipses: Their Number and Period. A large Catalogue of Ancient and Modern Eclipses
156
CHAP. XIX.
The Calculation of New and Full Moons and Eclipses. The geometrical Construction of Solar and Lunar Eclipses. The examination of ancient Eclipses
189
CHAP. XX.
Of the fixed Stars
230
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
248
CHAP. XXII.
A Description of the Astronomical Machinery serving to explain and illustrate the foregoing part of this Treatise
260
ERRATA.
In the Table facing Page 31, the Sun’s quantity of matter should be 227500. Page 40, l. last, for infinite read indefinite. Page 97, l. 20, for this read the next. Page 164, l. 2 from the bottom, for without any acceleration read as above, without any acceleration. Page 199, l. 16 for XIV read XV. Page 238, l. 16, for 40 read 406. Page 240, l. 15 from the bottom, for Tifri read Tisri, Page 249 l. 13; from the bottom for XVII read V.
CHAP. I. Of Astronomy in general.
1. Of all the sciences cultivated by mankind, Astronomy is acknowledged to be, and undoubtedly is, the most sublime, the most interesting, and the most useful. For, by knowledge derived from this science, not only the bulk of the Earth is discovered, the situation and extent of the countries and kingdoms upon it ascertained, trade and commerce carried on to the remotest parts of the world, and the various products of several countries distributed for the health, comfort, and conveniency of its inhabitants; but our very faculties are enlarged with the grandeur of the ideas it conveys, our minds exalted above the low contracted prejudices of the vulgar, and our understandings clearly convinced, and affected with the conviction, of the existence, wisdom, power, goodness, and superintendency of the SUPREME BEING! So that without an hyperbole,
2. From this branch of knowledge we also learn by what means or laws the Almighty carries on, and continues the admirable harmony, order, and connexion observable throughout the planetary system; and are led by very powerful arguments to form the pleasing deduction, that minds capable of such deep researches not only derive their origin from that adorable Being, but are also incited to aspire after a more perfect knowledge of his nature, and a stricter conformity to his will.
3. By Astronomy we discover that the Earth is at so great a distance from the Sun, that if seen from thence it would appear no bigger than a point; although it’s circumference is known to be 25,020 miles. Yet that distance is so small, compared with the distance of the Fixed Stars, that if the Orbit in which the Earth moves round the Sun were solid, and seen from the nearest Star, it would likewise appear no bigger than a point, although it is at least 162 millions of miles in diameter. For the Earth in going round the Sun is 162 millions of miles nearer to some of the Stars at one time of the year than at another; and yet their apparent magnitudes, situations, and distances from one another still remain the same; and a telescope which magnifies above 200 times does not sensibly magnify them: which proves them to be at least 400 thousand times farther from us than we are from the Sun.
4. It is not to be imagined that all the Stars are placed in one concave surface, so as to be equally distant from us; but that they are scattered at immense distances from one another through unlimited space. So that there may be as great a distance between any two neighbouring Stars, as between our Sun and those which are nearest to him. Therefore an Observer, who is nearest any fixed Star, will look upon it alone as a real Sun; and consider the rest as so many shining points, placed at equal distances from him in the Firmament.
5. By the help of telescopes we discover thousands of Stars which are invisible to the naked eye; and the better our glasses are, still the more become visible: so that we can set no limits either to their number or their distances. The celebrated Huygens carries his thoughts so far, as to believe it not impossible that there may be Stars at such inconceivable distances, that their light has not yet reached the Earth since it’s creation; although the velocity of light be a million of times greater than the velocity of a cannon bullet, as shall be demonstrated afterwards § 197, 216: and, as Mr. Addison very justly observes, this thought is far from being extravagant, when we consider that the Universe is the work of infinite power, prompted by infinite goodness; having an infinite space to exert itself in; so that our imaginations can set no bounds to it.
6. The Sun appears very bright and large in comparison of the Fixed Stars, because we keep constantly near the Sun, in comparison of our immense distance from the Stars. For, a spectator, placed as near to any Star as we are to the Sun, would see that Star a body as large and bright as the Sun appears to us: and a spectator, as far distant from the Sun as we are from the Stars, would see the Sun as small as we see a Star, divested of all its circumvolving Planets; and would reckon it one of the Stars in numbering them.
7. The Stars, being at such immense distances from the Sun, cannot possibly receive from him so strong a light as they seem to have; nor any brightness sufficient to make them visible to us. For the Sun’s rays must be so scattered and dissipated before they reach such remote objects, that they can never be transmitted back to our eyes, so as to render these objects visible by reflection. The Stars therefore shine with their own native and unborrowed lustre, as the Sun does; and since each particular Star, as well as the Sun, is confined to a particular portion of space, ’tis plain that the Stars are of the same nature with the Sun.
8. It is no ways probable that the Almighty, who always acts with infinite wisdom and does nothing in vain, should create so many glorious Suns, fit for so many important purposes, and place them at such distances from one another, without proper objects near enough to be benefited by their influences. Whoever imagines they were created only to give a faint glimmering light to the inhabitants of this Globe, must have a very superficial knowledge of Astronomy, and a mean opinion of the Divine Wisdom: since, by an infinitely less exertion of creating power, the Deity could have given our Earth much more light by one single additional Moon.
9. Instead then of one Sun and one World only in the Universe, as the unskilful in Astronomy imagine, that Science discovers to us such an inconceivable number of Suns, Systems, and Worlds, dispersed through boundless Space, that if our Sun, with all the Planets, Moons, and Comets belonging to it were annihilated, they would be no more missed out of the Creation than a grain of sand from the sea-shore. The space they possess being comparatively so small, that it would scarce be a sensible blank in the Universe; although Saturn, the outermost of our planets, revolves about the Sun in an Orbit of 4884 millions of miles in circumference, and some of our Comets make excursions upwards of ten thousand millions of miles beyond Saturn’s Orbit; and yet, at that amazing distance, they are incomparably nearer to the Sun than to any of the Stars; as is evident from their keeping clear of the attractive Power of all the Stars, and returning periodically by virtue of the Sun’s attraction.
10. From what we know of our own System it may be reasonably concluded that all the rest are with equal wisdom contrived, situated, and provided with accommodations for rational inhabitants. Let us therefore take a survey of the System to which we belong; the only one accessible to us; and from thence we shall be the better enabled to judge of the nature and end of the other Systems of the Universe. For although there is almost an infinite variety in all the parts of the Creation which we have opportunities of examining; yet there is a general analogy running through and connecting all the parts into one scheme, one design, one whole!
11. And then, to an attentive considerer, it will appear highly probable, that the Planets of our System, together with their attendants called Satellites or Moons, are much of the same nature with our Earth, and destined for the like purposes. For, they are solid opaque Globes, capable of supporting animals and vegetables. Some of them are bigger, some less, and some much about the size of our Earth. They all circulate round the Sun, as the Earth does, in a shorter or longer time according to their respective distances from him: and have, where it would not be inconvenient, regular returns of summer and winter, spring and autumn. They have warmer and colder climates, as the various productions of our Earth require: and, in such as afford a possibility of discovering it, we observe a regular motion round their Axes like that of our Earth, causing an alternate return of day and night; which is necessary for labour, rest, and vegetation, and that all parts of their surfaces may be exposed to the rays of the Sun.
12. Such of the Planets as are farthest from the Sun, and therefore enjoy least of his light, have that deficiency made up by several Moons, which constantly accompany, and revolve about them, as our Moon revolves about the Earth. The remotest Planet has, over and above, a broad Ring encompassing it; which like a lucid Zone in the Heavens reflects the Sun’s light very copiously on that Planet: so that if the remoter Planets have the Sun’s light fainter by day than we, they have an addition made to it morning and evening by one or more of their Moons, and a greater quantity of light in the night-time.
13. On the surface of the Moon, because it is nearer us than any other of the celestial Bodies are, we discover a nearer resemblance of our Earth. For, by the assistance of telescopes we observe the Moon to be full of high mountains, large valleys, and deep cavities. These similarities leave us no room to doubt but that all the Planets and Moons in the System are designed as commodious habitations for creatures endowed with capacities of knowing and adoring their beneficent Creator.
The Solar System
J. Ferguson delin.
J. Mynde Sculp.
14. Since the Fixed Stars are prodigious spheres of fire, like our Sun, and at inconceivable distances from one another, as well as from us, it is reasonable to conclude they are made for the same purposes that the Sun is; each to bestow light, heat, and vegetation on a certain number of inhabited Planets, kept by gravitation within the sphere of it’s activity.
15. What an august! what an amazing conception, if human imagination can conceive it, does this give of the works of the Creator! Thousands of thousands of Suns, multiplied without end, and ranged all around us, at immense distances from each other, attended by ten thousand times ten thousand Worlds, all in rapid motion, yet calm, regular, and harmonious, invariably keeping the paths prescribed them; and these Worlds peopled with myriads of intelligent beings, formed for endless progression in perfection and felicity.
16. If so much power, wisdom, goodness, and magnificence is displayed in the material Creation, which is the least considerable part of the Universe, how great, how wise, how good must HE be, who made and governs the Whole!
CHAP. II. A brief Description of theSolar System.
17. The Planets and Comets which move round the Sun as their center, constitute the Solar System. Those Planets which are nearer the Sun not only finish their circuits sooner, but likewise move faster in their respective Orbits than those which are more remote from him. Their motions are all performed from west to east, in Orbits nearly circular. Their names, distances, bulks, and periodical revolutions, are as follows.
18. The Sun , an immense globe of fire, is placed near the common center, or rather in the lower[2] focus, of the Orbits of all the Planets and Comets[3]; and turns round his axis in 25 days 6 hours, as is evident by the motion of spots seen on his surface. His diameter is computed to be 763,000 miles; and, by the various attractions of the circumvolving Planets, he is agitated by a small motion round the center of gravity of the System. All the Planets, as seen from him, move the same way, and according to the order of Signs in the graduated Circle ♈ ♉ ♎ ♋ &c. which represents the great Ecliptic in the Heavens: but, as seen from any one Planet, the rest appear sometimes to go backward, sometimes forward, and sometimes to stand still; not in circles nor ellipses, but in[4] looped curves which never return into themselves. The Comets come from all parts of the Heavens, and move in all sorts of directions.
19. Having mentioned the Sun’s turning round his axis, and as there will be frequent occasion to speak of the like motion of the Earth and other Planets, it is proper here to inform the young Tyro in Astronomy, that neither the Sun nor Planets have material axes to turn upon, and support them, as in the little imperfect Machines contrived to represent them. For the axis of a Planet is a line conceived to be drawn through it’s center, about which it revolves as on a real axis. The extremities of this line, terminating in opposite points of the Planet’s surface, are called its Poles. That which points towards the northern part of the Heavens is called the North Pole; and the other, pointing towards the southern part, is called the South Pole. A bowl whirled from one’s hand into the open air turns round such a line within itself, whilst it moves forward; and such are the lines we mean, when we speak of the Axes of the Heavenly bodies.
20. Let us suppose the Earth’s Orbit to be a thin, even, solid plane; cutting the Sun through the center, and extended out as far as the Starry Heavens, where it will mark the great Circle called the Ecliptic. This Circle we suppose to be divided into 12 equal parts, called Signs; each Sign into 30 equal parts, called Degrees; each Degree into 60 equal parts, called Minutes; and every Minute into 60 equal parts, called Seconds: so that a Second is the 60th part of a Minute; a Minute the 60th part of a Degree; and a Degree the 360th part of a Circle, or 30th part of a Sign. The Planes of the Orbits of all the other Planets likewise cut the Sun in halves; but extended to the Heavens, form Circles different from one another, and from the Ecliptic; one half of each being on the north side, and the other on the south side of it. Consequently the Orbit of each Planet crosses the Ecliptic in two opposite points, which are called the Planet’s Nodes. These Nodes are all in different parts of the Ecliptic; and therefore, if the planetary Tracks remained visible in the Heavens, they would in some measure resemble the different rutts of waggon-wheels crossing one another in different parts, but never going far asunder. That Node, or Intersection of the Orbit of any Planet with the Earth’s Orbit, from which the Planet ascends northward above the Ecliptic, is called the Ascending Node of the Planet; and the other, which is directly opposite thereto, is called it’s Descending Node. Saturn’s Ascending Node is in 21 deg. 13 min. of Cancer ♋, Jupiter’s in 7 deg. 29 min. of the same Sign, Mars’s in 17 deg. 17 min. of Taurus ♉, Venus’s in 13 deg. 59 min. of Gemini ♊, and Mercury’s in 14 deg. 43 min. of Taurus. Here we consider the Earth’s Orbit as the standard, and the Orbits of all the other Planets as oblique to it.
21. When we speak of the Planets Orbits, all that is meant is their Paths through the open and unresisting Space in which they move; and are kept in, by the attractive power of the Sun, and the projectile force impressed upon them at first: between which power and force there is so exact an adjustment, that without any solid Orbits to confine the Planets, they keep their courses, and at the end of every revolution find the points from whence they first set out, much more truly than can be imitated in the best machines made by human art.
22. Mercury, the nearest Planet to the Sun, goes round him (as in the circle marked ☿) in 87 days 23 hours of our time nearly; which is the length of his year. But, being seldom seen, and no spots appearing on his surface or disc, the time of his rotation on his axis, or the length of his days and nights, is as yet unknown. His distance from the Sun is computed to be 32 millions of miles, and his diameter 2600. In his course, round the Sun, he moves at the rate of 95 thousand miles every hour. His light and heat from the Sun are almost seven times as great as ours; and the Sun appears to him almost seven times as large as to us. The great heat on this Planet is no argument against it’s being inhabited; since the Almighty could as easily suit the bodies and constitutions of it’s inhabitants to the heat of their dwelling, as he has done ours to the temperature of our Earth. And it is very probable that the people there have such an opinion of us, as we have of the inhabitants of Jupiter and Saturn; namely, that we must be intolerably cold, and have very little light at so great a distance from the Sun.
23. This Planet appears to us with all the various phases of the Moon, when viewed at different times by a good telescope; save only that he never appears quite Full, because his enlightened side is never turned directly towards us but when he is so near the Sun as to be lost to our sight in it’s beams. And, as his enlightened side is always toward the Sun, it is plain that he shines not by any light of his own; for if he did, he would constantly appear round. That he moves about the Sun in an Orbit within the Earth’s Orbit is also plain (as will be more largely shewn by and by, § 141, & seq.) because he is never seen opposite to the Sun, nor above 56 times the Sun’s breadth from his center.
24. His Orbit is inclined seven degrees to the Ecliptic; and that Node § 20, from which he ascends northward above the Ecliptic is in the 14th degree of Taurus; the opposite, in the 14th degree of Scorpio. The Earth is in these points on the 5th of November and 4th of May, new style; and when Mercury comes to either of his Nodes at his[5] inferior Conjunction about these times, he will appear to pass over the disc or face of the Sun, like a dark round spot. But in all other parts of his Orbit his Conjunctions are invisible, because he either goes above or below the Sun.
25. Mr. Whiston has given us an account of several periods at which Mercury may be seen on the Sun’s disc, viz. In the year 1782, Nov. 12th, at 3 h. 44 m. in the afternoon: 1786, May 4th, at 6 h. 57 m. in the forenoon: 1789, Dec. 6th, at 3 h. 55 m. in the afternoon; and 1799, May 7th, at 2 h. 34 m. in the afternoon. There will be several intermediate Transits, but none of them visible at London.
26. Venus, the next Planet in order, is computed to be 59 millions of miles from the Sun; and by moving at the rate of 69 thousand miles every hour in her Orbit (as in the circle marked ♀), she goes round the Sun in 224 days 17 hours of our time nearly; in which, though it be the full length of her year, she has only 91⁄4 days, according to Bianchini’s observations; so that in her, every day and night together is as long as 241⁄3 days and nights with us. This odd quarter of a day in every year makes every fourth year a leap-year to Venus; as the like does to our Earth. Her diameter is 7906 miles; and by her diurnal motion the inhabitants about her Equator are carried 43 miles every hour: besides the 69,000 above-mentioned.
27. Her Orbit includes that of Mercury within it; for at her greatest Elongation, or apparent distance from the Sun, she is 96 times his breadth from his centre; which is almost double of Mercury’s. Her Orbit is included by the Earth’s; for if it were not, she might be seen as often in Opposition to the Sun as in Conjunction with him; but she was never seen 90 degrees, or a fourth part of a Circle, from the Sun.
28. When Venus appears west of the Sun she rises before him in the morning, and is called the Morning Star: when she appears east of the Sun she shines in the evening after he sets, and is then called the Evening Star: being each in it’s turn for 290 days. It may perhaps be surprising at first, that Venus should keep longer on the east or west of the Sun than the whole time of her Period round him. But the difficulty vanishes when we consider that the Earth is all the while going round the Sun the same way, though not so quick as Venus: and therefore her relative motion to the Earth must in every Period be as much slower than her absolute motion in her Orbit, as the Earth during that time advances forward in the Ecliptic; which is 220 degrees. To us she appears through a telescope in all the various shapes of the Moon.
29. The Axis of Venus is inclined 75 degrees to the Axis of her Orbit; which is 511⁄2 degrees more than our Earth’s Axis is inclined to the Axis of the Ecliptic: and therefore the variation of her seasons is much greater than of ours. The North Pole of her Axis inclines toward the 20th degree of Aquarius, our Earth’s to the beginning of Cancer; and therefore the northern parts of Venus have summer in the Signs where those of our Earth have winter, and vice versâ.
30. The [6]artificial day at each Pole of Venus is as long as 1121⁄2[7]natural days on our Earth.
31. The Sun’s greatest Declination on each side of her Equator amounts to 75 degrees; therefore her[8] Tropics are only 15 degrees from her Poles; and her [9]Polar Circles as far from her Equator. Consequently, the Tropics of Venus are between her Polar Circles and her Poles; contrary to what those of our Earth are.
32. As her annual Revolution contains only 91⁄4 of her days, the Sun will always appear to go through a Sign, or twelfth Part of her Orbit, in little more that three quarters of her natural day, or nearly in 183⁄4 of our days and nights.
33. Because her day is so great a part of her year, the Sun changes his Declination in one day so much, that if he passes vertically, or directly over head of any given place on the Tropic, the next day he will be 26 degrees from it: and whatever place he passes vertically over when in the Equator, one day’s revolution will remove him 361⁄4 degrees from it. So that the Sun changes his Declination every day in Venus about 14 degrees more at a mean rate, than he does in a quarter of a year on our Earth. This appears to be providentially ordered, for preventing the too great effects of the Sun’s heat (which is twice as great on Venus as on the Earth) so that he cannot shine perpendicularly on the same places for two days together; and by that means, the heated places have time to cool.
34. If the inhabitants about the North Pole of Venus fix their South, or Meridian Line, through that part of the Heavens where the Sun comes to his greatest Height, or North Declination, and call those the East and West points of their Horizon, which are 90 degrees on each side from that point where the Horizon is cut by the Meridian Line, these inhabitants will have the following remarkables.
The Sun will rise 221⁄2 degrees[10] north of the East, and going on 1121⁄2 degrees, as measured on the plane of the [11]Horizon, he will cross the Meridian at an altitude of 121⁄2 degrees; then making an entire revolution without setting, he will cross it again at an altitude of 481⁄2 degrees; at the next revolution he will cross the Meridian as he comes to his greatest height and declination, at the altitude of 75 degrees; being then only 15 degrees from the Zenith, or that point of the Heavens which is directly over head: and thence he will descend in the like spiral manner; crossing the Meridian first at the altitude of 481⁄2 degrees; next at the altitude of 121⁄2 degrees; and going on thence 1121⁄2 degrees, he will set 221⁄2 degrees north of the West; so that, after having been 45⁄8 revolutions above the Horizon, he descends below it to exhibit the like appearances at the South Pole.
35. At each Pole, the Sun continues half a year without setting in summer, and as long without rising in winter; consequently the polar inhabitants of Venus have only one day and one night in the year; as it is at the Poles of our Earth. But the difference between the heat of summer and cold of winter, or of mid-day and mid-night, on Venus, is much greater than on the Earth: because in Venus, as the Sun is for half a year together above the Horizon of each Pole in it’s turn, so he is for a considerable part of that time near the Zenith; and during the other half of the year, always below the Horizon, and for a great part of that time at least 70 degrees from it. Whereas, at the Poles of our Earth, although the Sun is for half a year together above the Horizon, yet he never ascends above, nor descends below it, more than 231⁄2 degrees. When the Sun is in the Equinoctial, or in that Circle which divides the northern half of the Heavens from the southern, he is seen with one half of his Disc above the Horizon of the North Pole, and the other half above the Horizon of the South Pole; so that his center is in the Horizon of both Poles: and then descending below the Horizon of one, he ascends gradually above that of the other. Hence, in a year, each Pole has one spring, one harvest, a summer as long as them both, and a winter equal in length to the other three seasons.
36. At the Polar Circles of Venus, the seasons are much the same as at the Equator, because there are only 15 degrees betwixt them, § 31; only the winters are not quite so long, nor the summers so short: but the four seasons come twice round every year.
37. At Venus’s Tropics, the Sun continues for about fifteen of our weeks together without setting in summer; and as long without rising in winter. Whilst he is more than 15 degrees from the Equator, he neither rises to the inhabitants of the one Tropic, nor sets to those of the other: whereas, at our terrestrial Tropics he rises and sets every day of the year.
38. At Venus’s Tropics, the Seasons are much the same as at her Poles; only the summers are a little longer, and the winters a little shorter.
39. At her Equator, the days and nights are always of the same length; and yet the diurnal and nocturnal Arches are very different, especially when the Sun’s declination is about the greatest: for then, his meridian altitude may sometimes be twice as great as his midnight depression, and at other times the reverse. When the Sun is at his greatest Declination, either North or South, his rays are as oblique at Venus’s Equator, as they are at London on the shortest day of winter. Therefore, at her Equator there are two winters, two summers, two springs, and two autumns every year. But because the Sun stays for some time near the Tropics, and passes so quickly over the Equator, every winter there will be almost twice as long as summer: the four seasons returning twice in that time, which consists only of 91⁄4 days.
40. Those parts of Venus which lie between the Poles and Tropics, and between the Tropics and Polar Circles, and also between the Polar Circles and Equator, partake more or less of the Phenomena of these Circles, as they are more or less distant from them.
41. From the quick change of the Sun’s declination it happens, that when he rises due east on any day, he will not set due west on that day, as with us; for if the place where he rises due east be on the Equator, he will set on that day almost west-north-west; or about 181⁄2 degrees north of the west. But if the place be in 45 degrees north latitude, then on the day that the Sun rises due east he will set north-west by west, or 33 degrees north of the west. And in 62 degrees north latitude when he rises in the east, he sets not in that revolution, but just touches the Horizon 10 degrees to the west of the north point; and ascends again, continuing for 31⁄4 revolutions above the Horizon without setting. Therefore, no place has the forenoon and afternoon of the same day equally long, unless it be on the Equator or at the Poles.
42. The Sun’s altitude at noon, or any other time of the day, and his amplitude at rising and setting, being so different at places on the same parallels of latitude, according to the different longitudes of those places, the longitude will be almost as easily found on Venus as the latitude is found on the Earth: which is an advantage we can never enjoy, because the daily change of the Sun’s declination is by much too small for that purpose.
43. On this Planet, wherever the Sun crosses the Equator in any year, he will have 9 degrees of declination from that place on the same day and hour next year; and will cross the Equator 90 degrees farther to the west; which makes the time of the Equinox a quarter of a day (almost equal to six of our days) later every year. Hence, although the spiral in which the Sun’s motion is performed, be of the same sort every year, yet it will not be the very same, because the Sun will not pass vertically over the same places till four annual revolutions are finished.
44. We may suppose that the inhabitants of Venus will be careful to add a day to some particular part of every fourth year; which will keep the same seasons to the same days. For, as the great annual change of the Equinoxes and Solstices shifts the seasons a quarter of a day every year, they would be shifted through all the days of the year in 36 years. But by means of this intercalary day, every fourth year will be a leap-year; which will bring her time to an even reckoning, and keep her Calendar always right.
45. Venus’s Orbit is inclined 31⁄2 degrees to the Earth’s; and crosses it in the 14th degree of Gemini and of Sagittarius; and therefore, when the Earth is about these points of the Ecliptic at the time that Venus is in her inferiour conjunction, she will appear like a spot on the Sun, and afford a more certain method of finding the distances of all the Planets from the Sun than any other yet known. But these appearances happen very seldom; and will only be thrice visible at London for three hundred years to come. The first time will be in the year 1761, June the 6th, at 5 hours 55 minutes in the morning. The second 1996, June the 9th, at 2 hours 13 minutes in the afternoon. And the third in the year 2004, June the 6th, at 7 hours 18 minutes in the forenoon. Excepting such Transits as these, she shews the same appearances to us regularly every eight years; her Conjunctions, Elongations, and Times of rising and setting being very nearly the same, on the same days, as before.
46. Venus may have a Satellite or Moon, although it be undiscovered by us: which will not appear very surprising, if we consider how inconveniently we are placed for seeing it. For it’s enlightened side can never be fully turned towards us but when Venus is beyond the Sun; and then, as Venus appears little bigger than an ordinary Star, her Moon may be too small to be perceptible at such a distance. When she is between us and the Sun, her full Moon has it’s dark side towards us; and then, we cannot see it any more than we can our own Moon at the time of Change. When Venus is at her greatest Elongation, we have but one half of the enlightened side of her Full Moon towards us; and even then it may be too far distant to be seen by us. But if she has a Moon, it may certainly be seen with her upon the Sun, in the year 1761, unless it’s Orbit be considerably inclined to the Ecliptic: for if it should be in conjunction or opposition at that time, we can hardly imagine that it moves so slow as to be hid by Venus all the six hours that she will appear on the Sun’s Disc.
47. The Earth is the next Planet above Venus in the System. It is 81 millions of miles from the Sun, and goes round him (as in the circle ⊕) in 365 days 5 hours 49 minutes, from any Equinox or Solstice to the same again: but from any fixed Star to the same again, as seen from the Sun, in 365 days 6 hours and 9 minutes; the former being the length of the Tropical year, and the latter the length of the Sidereal. It travels at the rate of 58 thousand miles every hour, which motion, though 120 times swifter than that of a cannon ball, is little more than half as swift as Mercury’s motion in his Orbit. The Earth’s diameter is 7970 miles; and by turning round it’s Axis every 24 hours from West to East, it causes an apparent diurnal motion of all the heavenly Bodies from East to West. By this rapid motion of the Earth on it’s Axis, the inhabitants about the Equator are carried 1042 miles every hour, whilst those on the parallel of London are carried only about 580, besides the 58 thousand miles by the annual motion above-mentioned, which is common to all places whatever.
48. The Earth’s Axis makes an angle of 231⁄2 degrees with the Axis of it’s Orbit; and keeps always the same oblique direction; inclining towards the same fixed Stars[12] throughout it’s annual course; which causes the returns of spring, summer, autumn, and winter; as will be explained at large in the tenth Chapter.
49. The Earth is round like a globe; as appears, 1. from it’s shadow in Eclipses of the Moon; which shadow is always bounded by a circular line § 314. 2. From our seeing the masts of a ship whilst the hull is hid by the convexity of the water. 3. From it’s having been sailed round by many navigators. The hills take off no more from the roundness of the Earth in comparison, than grains of dust do from the roundness of a common Globe.
50. The seas and unknown parts of the Earth (by a measurement of the best Maps) contain 160 million 522 thousand and 26 square miles; the inhabited parts 38 million 990 thousand 569: Europe 4 million 456 thousand and 65; Asia 10 million 768 thousand 823; Africa 9 million 654 thousand 807; America 14 million 110 thousand 874. In all, 199 million 512 thousand 595; which is the number of square miles on the whole surface of our Globe.
51. Dr. Long, in the first volume of his Astronomy, pag. 168, mentions an ingenious and easy method of finding nearly what proportion the land bears to the sea; which is, to take the papers of a large terrestrial globe, and after separating the land from the sea with a pair of scissars, to weigh them carefully in scales. This supposes the globe to be exactly delineated, and the papers all of equal thickness. The Doctor made the experiment on the papers of Mr. Senex’s seventeen inch globe; and found that the sea papers weighed 349 grains, and the land only 124: by which it appears that almost three fourth parts of the surface of our Earth between the Polar Circles are covered with water, and that little more than one fourth is dry land. The Doctor omitted weighing all within the Polar Circles; because there is no certain measurement of the land there, so as to know what proportion it bears to the sea.
52. The Moon is not a Planet, but only a Satellite or Attendant of the Earth, moving round the Earth from Change to Change in 29 days 12 hours and 44 minutes; and going round the Sun with it every year. The Moon’s diameter is 2180 miles; and her distance from the Earth 240 thousand. She goes round her Orbit in 27 days 7 hours 43 minutes, moving about 2290 miles every hour; and turns round her Axis exactly in the time that she goes round the Earth, which is the reason of her keeping always the same side towards us, and that her day and night taken together is as long as our lunar month.
53. The Moon is an opaque Globe like the Earth, and shines only by reflecting the light of the Sun: therefore whilst that half of her which is toward the Sun is enlightened, the other half must be dark and invisible. Hence, she disappears when she comes between us and the Sun; because her dark side is then toward us. When she is gone a little way forward, we see a little of her enlightened side; which still increases to our view, as she advances forward, until she comes to be opposite to the Sun; and then her whole enlightened side is towards the Earth, and she appears with a round, illumined Orb; which we call the Full Moon: her dark side being then turned away from the Earth. From the Full she seems to decrease gradually as she goes through the other half of her course; shewing us less and less of her enlightened side every day, till her next change or conjunction with the Sun, and then she disappears as before.
54. The continual changing of the Moon’s phases or shapes demonstrates that she shines not by any light of her own: for if she did, being globular, we should always see her with a round full Orb like the Sun. Her Orbit is represented in the Scheme by the little circle m, upon the Earth’s Orbit ⊕: but it is drawn fifty times too large in proportion to the Earth’s; and yet is almost too small to be seen in the Diagram.
55. The Moon has scarce any difference of seasons; her Axis being almost perpendicular to the Ecliptic. What is very singular, one half of her has no darkness at all; the Earth constantly affording it a strong light in the Sun’s absence; while the other half has a fortnight’s darkness and a fortnight’s light by turns.
56. Our Earth is a Moon to the Moon, waxing and waneing regularly, but appearing thirteen times as big, and affording her thirteen times as much light, as she does to us. When she changes to us, the Earth appears full to her; and when she is in her first quarter to us, the Earth is in it’s third quarter to her; and vice versâ.
57. But from one half of the Moon, the Earth is never seen at all: from the middle of the other half, it is always seen over head; turning round almost thirty times as quick as the Moon does. From the line which limits our view of the Moon, or all round what we call her edges, only one half of the Earth’s side next her is seen; the other half being hid below the Horizon. To her, the Earth seems to be the biggest Body in the Universe; for it appears thirteen times as big as she does to us.
58. The Moon has no such Atmosphere, or body of air surrounding her as we have: for if she had, we could never see her edge so well defined as it appears; but there would be a sort of a mist or haziness round her, which would make the Stars look fainter, when they were seen through it. But observation proves, that the Stars which disappear behind the Moon retain their full lustre until they seem to touch her very edge, and then vanish in a moment. This has been often observed by Astronomers, but particularly by Cassini[13] of the Star γ in the breast of Virgo, which appears single and round to the bare eye; but through a refracting Telescope of 16 feet appears to be two Stars so near together, that the distance between them seems to be but equal to one of their apparent diameters. The Moon was observed to pass over them on the 21st of April 1720, N. S. and as her dark edge drew near to them, it caused no change in their colour or Situation. At 25 min. 14 sec. past 12 at night, the most westerly of these Stars was hid by the dark edge of the Moon; and in 30 seconds afterward, the most easterly Star was hid: each of them disappearing behind the Moon in an instant, without any preceding diminution of magnitude or brightness; which by no means could have been the case if there were an Atmosphere round the Moon; for then, one of the Stars falling obliquely into it before the other, ought by refraction to have suffered some change in its colour, or in it’s distance from the other Star which was not yet entered into the Atmosphere. But no such alteration could be perceived though the observation was performed with the utmost attention to that particular; and was very proper to have made such a discovery. The faint light, which has been seen all around the Moon, in total Eclipses of the Sun, has been observed, during the time of darkness, to have it’s center coincident with the center of the Sun; and is therefore much more likely to arise from the Atmosphere of the Sun than from that of the Moon; for if it were the latter, it’s center would have gone along with the Moon’s.
59. If there were seas in the Moon, she could have no clouds, rains, nor storms as we have; because she has no such Atmosphere to support the vapours which occasion them. And every one knows, that when the Moon is above our Horizon in the night time, she is visible, unless the clouds of our Atmosphere hide her from our view; and all parts of her appear constantly with the same clear, serene, and calm aspect. But those dark parts of the Moon, which were formerly thought to be seas, are now found to be only vast deep cavities, and places which reflect not the Sun’s light so strongly as others, having many caverns and pits whose shadows fall within them, and are always dark on the sides next the Sun; which demonstrates their being hollow: and most of these pits have little knobs like hillocks standing within them, and casting shadows also; which cause these places to appear darker than others which have fewer, or less remarkable caverns. All these appearances shew that there are no seas in the Moon; for if there were any, their surfaces would appear smooth and even, like those on the Earth.
60. There being no Atmosphere about the Moon, the Heavens in the day time have the appearance of night to a Lunarian who turns his back toward the Sun; and when he does, the Stars appear as bright to him as they do in the night to us. For, it is entirely owing to our Atmosphere that the Heavens are bright about us in the day.
61. As the Earth turns round it’s Axis, the several continents, seas, and islands appear to the Moon’s inhabitants like so many spots of different forms and brightness, moving over it’s surface; but much fainter at some times than others, as our clouds cover them or leave them. By these spots the Lunarians can determine the time of the Earth’s diurnal motion, just as we do the motion of the Sun: and perhaps they measure their time by the motion of the Earth’s spots; for they cannot have a truer dial.
62. The Moon’s Axis is so nearly perpendicular to the Ecliptic, that the Sun never removes sensibly from her Equator: and the[14] obliquity of her Orbit, which is next to nothing as seen from the Sun, cannot cause any sensible declination of the Sun from her Equator. Yet her inhabitants are not destitute of means for determining the length of their year, though their method and ours must differ. For we can know the length of our year by the return of our Equinoxes; but the Lunarians, having always equal day and night, must have recourse to another method; and we may suppose, they measure their year by observing the Poles of our Earth; as one always begins to be enlightened, and the other disappears, at our Equinoxes; they being conveniently situated for observing great tracks of land about our Earth’s Poles, which are entirely unknown to us. Hence we may conclude, that the year is of the same absolute length both to the Earth and Moon, though very different as to the number of days: we having 3651⁄4 natural days, and the Lunarians only 127⁄19; every day and night in the Moon being as long as 291⁄2 on the Earth.
63. The Moon’s inhabitants on the side next the Earth may as easily find the longitude of their places as we can find the latitude of ours. For the Earth keeping constantly, or very nearly so, over one Meridian of the Moon, the east or west distances of places from that Meridian are as easily found, as we can find our distance from the Equator by the Altitude of our celestial Poles.
64. The Planet Mars is next in order, being the first above the Earth’s Orbit. His distance from the Sun is computed to be 123 millions of miles; and by travelling at the rate of 47 thousand miles every hour, as in the circle ♂, he goes round the Sun in 687 of our days and 17 hours; which is the length of his year, and contains 6671⁄4 of his days; every day and night together being 40 minutes longer than with us. His diameter is 4444 miles, and by his diurnal rotation the inhabitants about his Equator are carried 556 miles every hour. His quantity of light and heat is equal but to one half of ours; and the Sun appears but half as big to him as to us.
65. This Planet being but a fifth part so big as the Earth, if any Moon attends him, she must be very small, and has not yet been discovered by our best telescopes. He is of a fiery red colour, and by his Appulses to some of the fixed Stars, seems to be surrounded by a very gross Atmosphere. He appears sometimes gibbous, but never horned; which both shews that his Orbit includes the Earth’s within it, and that he shines not by his own light.
66. To Mars, our Earth and Moon appear like two Moons, a bigger and a less; changing places with one another, and appearing sometimes horned, sometimes half or three quarters illuminated, but never full; nor at most above a quarter of a degree from each other, although they are 240 thousand miles asunder.
67. Our Earth appears almost as big to Mars as Venus does to us, and at Mars it is never seen above 48 degrees from the Sun; sometimes it appears to pass over the Disc of the Sun, and so do Mercury and Venus: but Mercury can never be seen from Mars by such eyes as ours, unassisted by proper instruments; and Venus will be as seldom seen as we see Mercury. Jupiter and Saturn are as visible to Mars as to us. His Axis is perpendicular to the Ecliptic, and his Orbit is 2 degrees inclined to it.
68. Jupiter, the biggest of all the Planets, is still higher in the System, being about 424 millions of miles from the Sun: and going at the rate of 25 thousand miles every hour in his Orbit, as in the circle ♃ finishes his annual period in eleven of our years 314 days and 18 hours. He is above 1000 times as big as the Earth, for his diameter is 81,000 miles; which is more than ten times the diameter of the Earth.
69. Jupiter turns round his Axis in 9 hours 56 minutes; so that his year contains 10 thousand 464 days; and the diurnal velocity of his equatoreal parts is greater than the swiftness with which he moves in his annual Orbit; a singular circumstance, as far as we know. By this prodigious quick Rotation, his equatoreal inhabitants are carried 25 thousand 920 miles every hour (which is 920 miles an hour more than an inhabitant of our Earth moves in twenty-four hours) besides the 25 thousand above-mentioned, which is common to all parts of his surface, by his annual motion.
70. Jupiter is surrounded by faint substances, called Belts, in which so many changes appear, that they are generally thought to be clouds: for some of them have been first interrupted and broken, and then have vanished entirely. They have sometimes been observed of different breadths, and afterwards have all become nearly of the same breadth. Large spots have been seen in these Belts; and when a Belt vanishes, the contiguous spots disappear with it. The broken ends of some Belts have been generally observed to revolve in the same time with the spots; only those nearer the Equator in somewhat less time than those near the Poles; perhaps on account of the Sun’s greater heat near the Equator, which is parallel to the Belts and course of the spots. Several large spots, which appear round at one time, grow oblong by degrees, and then divide into two or three round spots. The periodical time of the spots near the Equator is 9 hours 50 minutes, but of those near the Poles 9 hours 56 minutes. See Dr.Smith’s Optics, § 1004 & seq.
