Index.
The numeral Figures refer to the Articles, and the small n to the Notes on the Articles.
A.
Acceleration of the Stars, 221.
Angle, what, 185.
Annual Parallax of the Stars, 196.
Anomaly, what, 239.
Antients, their superstitious notions of Eclipses, 329. Their method of dividing the Zodiac, 398.
Antipodes, what, 122.
Apsides, line of, 238.
Archimedes, his ideal Problem for moving the Earth, 159.
Areas described by the Planets, equal in times, 153.
Astronomy, the great advantages arising from it both in our religious and civil concerns, 1 Discovers the laws by which the Planets move, and are retained in their Orbits, 2
Atmosphere, the higher the thinner, 174. It’s prodigious expansion, ib. It’s whole weight on the Earth, 175. Generally thought to be heaviest when it is lightest, 176. Without it the Heavens would appear dark in the day-time, 177. Is the cause of twilight, ib. It’s height, ib. Refracts the Sun’s rays, 178. Causeth the Sun and Moon to appear above the Horizon when they are really below it, ib. Foggy, deceives us in the bulk and distance of objects, 185.
Attraction, 101-105. Decreases as the square of the distance increases, 106. Greater in the larger than in the smaller Planets, 158. Greater in the Sun than in all the Planets if put together, ib.
Axes of the Planets, what, 19. Their different positions with respect to one another, 120.
Axis of the Earth, it’s parallelism, 302. It’s position variable as seen from the Sun or Moon, 338. the Phenomena thence arising, 340.
B.
Bodies, on the Earth, lose of their weight the nearer they are to the Equator, 117. How they might lose all their weight, 118, How they become visible, 167.
C.
Calculator, (an Instrument) described, 436.
Calendar, how to inscribe the Golden Numbers rightly in it for shewing the days of New Moons, 423.
Cannon-Ball, it’s swiftness, 89. In what times it would fly from the Sun to the different Planets and fixed Stars, ib.
Cassini, his account of a double Star eclipsed by the Moon, 58. His Diagrams of the Paths of the Planets, 138.
Catalogue of the Eclipses, 327. Of the Constellations and Stars, 367. Of remarkable Æras and events, 433.
Celestial Globe improved, 438.
Centripetal and centrifugal forces, how they alternately overcome each other in the motions of the Planets, 152-154.
Changes in the Heavens, 403.
Chords, line of, how to make, 369.
Circles, of perpetual Apparition and Occultation, 128. Of the Sphere, 198. Contain 360 Degrees whether they be great or small, 207.
Civil Year, what, 411.
Columbus (Christopher) his story concerning an Eclipse, 330.
Clocks and Watches, an easy method of knowing whether they go true or false, 223. Why they seldom agree with the Sun if they go true, 228-245. How to regulate them by Equation Tables and a Meridian line, 225, 226.
Cloudy Stars, 402.
Cometarium (an Instrument) described, 437.
Constellations, antient, their number, 396. The number of Stars in each, according to different Astronomers, 399.
Cycle, Solar, Lunar, and Romish, 420. Of Easter, 425.
D.
Darkness at our Saviour’s crucifixion supernatural, 352, 432.
Day, natural and artificial, what, 417. And Night, always equally long at the Equator, 126. Natural, not compleated in an absolute turn of the Earth on it’s Axis, 222.
Degree, what, 207.
Digit, what, 321, n.
Direction, (Number of) 426.
Distances of the Planets from the Sun, an idea thereof, 89. A Table thereof, 98. How found, 190.
Diurnal and annual Motions of the Earth illustrated, 200, 202.
Dominical Letter, 427.
Double projectile force, a balance to a Quadruple Power of Gravity, 153. Star covered by the Moon, 58.
E.
Earth, it’s bulk but a point as seen from the Sun, 3 It’s Diameter, annual Period, and Distance from the Sun, 47. Turns round it’s Axis, ib. Velocity of it’s equatoreal Parts, ib. Velocity in it’s annual Orbit, ib. Inclination of it’s Axis, 48. Proof of it’s being globular, or nearly so, 49, 314. Measurement of it’s surface, 50, 51. Difference between it’s Equatoreal and Polar Diameters, 76. It’s motion round the Sun demonstrated by gravity, 108, 111. by Dr. Bradley’s observations, 113. by the Eclipses of Jupiter’s Satellites, 219. It’s diurnal motion highly probable from the absurdity that must follow upon supposing it not to move, 111. 120. and demonstrable from it’s figure, 116. this motion cannot be felt, 119. Objections against it’s motion answered, 112, 121. It has no such thing as an upper or under side, 122. in what case it might, 123. The swiftness of it’s motion in it’s Orbit compared with the velocity of light, 197. It’s diurnal and annual motions illustrated by an easy experiment, 200. Proved to be less than the Sun and bigger than the Moon, 315.
Easter Cycle, 425.
Eclipsareon (an Instrument) described, 442.
Eclipses, of Jupiter’s Satellites, how the Longitude is found by them, 207-218. they demonstrate the velocity of light, 216. Of the Sun and Moon, 312-327. Why they happen not in every month, 316. When they must be, 317. Their limits, ib. Their Period, 320, 326. A dissertation on their progress, 321-324. A large catalogue of them, 327. Historical ones, 328. More of the Sun than of the Moon, and why, 331. The proper Elements for their calculation and projection, 353-390.
Ecliptic, it’s Signs, their names and characters, 91. Makes different Angles with the Horizon every hour and minute, 275. how these Angles may be estimated by the position of the Moon’s horns, 260. It’s obliquity to the Equator less now than it was formerly, 405. How it’s Signs are numbered, 354.
Elongations, of the Planets, as seen by an observer at rest on the outside of all their Orbits, 133. Of Mercury and Venus as seen from the Earth, illustrated, 142. it’s quantity, 143. Of Mercury, Venus, the Earth, Mars, and Jupiter; it’s quantity as seen from Saturn, 147.
Epochas or Æras, 433.
Equation of time, 224-245. Of the Moon’s Place, 355. Of the Sun’s Place, ib. Of the Nodes, 363.
Equator, day and night always equal there, 126. Makes always the same Angle with the Horizon of the same place; the Ecliptic not, 274, 275.
Equinoctial Points in the Heavens, their precession, 246, a very different thing from the recession or anticipation of the Equinoxes on Earth, the one no ways occasioned by the other, 249.
Excentricities of the Planets Orbits, 155.
F.
Fallacies in judging of the bulk of objects by their apparent distance, 185; applied to the solution of the horizontal Moon, 187.
First Meridian, what, 207.
Fixed Stars, why they appear of less magnitude when viewed through a telescope than by the bare eye, 391. Their number, 392. Their division into different Classes and Constellations, 395-399.
G.
General Phenomena of a superior Planet as seen from an inferior, 149.
Gravity, demonstrable, 101-104. Keeps all bodies on the Earth to it’s surface, or brings them back when thrown upward; and constitutes their weight, 101, 122. Retains all the Planets in their Orbits, 103. Decreases as the square of the distance increases, 106. Proves the Earth’s annual motion, 108. Demonstrated to be greater in the larger Planets than in the smaller; and stronger in the Sun than in all the Planets together, 158. Hard to understand what it is, 160. Acts every moment, 162.
Globe, improved celestial, 438.
Great Year, 251.
H.
Harmony of the celestial motions, 111.
Harvest-Moon, 273-293. None at the Equator, 273. Remarkable at the Polar Circles, 285. In what years most and least advantageous, 292.
Heat, decreases as the square of the distance from the Sun increases, 169. Why not greatest when the Earth is nearest the Sun, 205. Why greater about three o’Clock in the afternoon than when the Sun is on the Meridian, 300.
Heavens, seem to turn round with different velocities as seen from the different Planets; and on different Axes as seen from most of them, 120. Only one Hemisphere of them seen at once from any one Planet’s surface, 125. The Sun’s Center the only point from which their true Motions could be seen, 135. Changes in them, 403.
Horizon, what, 125, n.
Horizontal-Moon explained, 187.
Horizontal Parallax, of the Moon, 190; of the Sun, 191; best observed at the Equator, 193.
Hour-Circles, what, 208.
Hour of time equal to 15 degrees of motion, ib. How divided by the Jews, Chaldeans, and Arabians, 419.
Huygenius, his thoughts concerning the distance of some Stars, 5
I. J.
Inclination of Venus’s Axis, 29. Of the Earth’s, 48. Of the Axis or Orbit of a Planet only relative, 201.
Inhabitants of the Earth (or any other Planet) stand on opposite sides with their feet toward one another, yet each thinks himself on the upper side, 122.
Julian Period, 430.
Jupiter, it’s distance, diameter, diurnal and annual revolutions, 67-69. The Phenomena of it’s Belts, 70. Has no difference of seasons, 71. Has four Moons, 72, their grand Period, 73, the Angles which their Orbits subtend as seen from the Earth, 74, most of them are eclipsed in every revolution, 75. The great difference between it’s equatoreal and polar Diameters, 76. The inclination of it’s Orbit, and place of it’s Ascending Node, 77. The Sun’s light 3000 times as strong on it as Full Moon-light is on the Earth, 85. Is probably inhabited, 86. The amazing strength required to put it in motion, 158. The figures of the Paths described by it’s Satellites, 269.
L.
Light, the inconceivable smallness of it’s particles, 165, and the dreadful mischief they would do if they were larger, 166. It’s surprising velocity, 166, compared with the swiftness of the Earth’s annual motion, 197. Decreases as the square of the distance from the luminous body increases, 169. Is refracted in passing through different Mediums, 171-173. Affords a proof of the Earth’s annual motion, 197, 219. In what time it comes from the Sun to the Earth, 216, this explained by a figure, 217.
Limits of Eclipses, 317.
Line, of the Nodes, what, 317; has a retrograde motion, 319. Of Sines and Chords, how to make, 369.
Long (Rev. Dr.) his method of comparing the quantity of the surface of dry land with that of the Sea, 51. His glass sphere, 126.
Longitude, how found, 207-213.
Lucid Spots in the Heavens, 401.
Lunar Cycle deficient, 422.
M.
Magellanic Clouds, 402.
Man, of a middle size, how much pressed by the weight of the Atmosphere, 175; why this pressure is not felt, ib.
Mars, it’s Diameter, Period, Distance, and other Phenomena, 64-67.
Matter, it’s properties, 99.
Mean Anomaly, what, 239.
Mercury, it’s Diameter, Period, Distance, &c. 22. Appears in all the shapes of the Moon, 23. When it will be seen on the Sun, 24. The inclination of it’s Orbit and Place of it’s Ascending Node, ib. It’s Path delineated, 138. Experiment to shew it’s Phases and apparent Motion, 142.
Mercury (Quicksilver) in the Barometer, why not affected by the Moon’s raising Tides in the Air, 311.
Meridian, first, 207. Line, how to draw one, 226.
Milky Way, what, 400.
Months, Jewish, Arabian, Egyptian, and Grecian, 415.
Moon, her Diameter and Period, 52. Her phases, 53, 255. Shines not by her own light, 54. Has no difference of seasons, 55. The Earth is a Moon to her, 56. Has no Atmosphere of any visible Density, 58; nor Seas, 59. How her inhabitants may be supposed to measure their year, 62. Her light compared with day-light, 85. The excentricity of her Orbit, 98. Is nearer the Earth now than she was formerly, 163. Appears bigger in the Horizon than at any considerable height above it, and why, 187; yet is seen much under the same Angle in both cases, 188. Her surface mountainous, 252: if smooth she could give us no light, ib. Why no hills appear round her edge, 253. Has no Twilight, 254. Appears not always quite round when full, 256. Her phases agreeably represented by a globular Stone viewed in Sun-shine when she is above the Horizon, and the observer placed as if he saw her on the top of the Stone, 258. Turns round her Axis, 262. The length of her Solar and Sidereal Day, ib. Her periodical and synodical revolution represented by the motions of the hour and minute hands of a Watch, 264. Her Path delineated, and shewn to be always concave to the Sun, 265-268. Her motion alternately retarded and accelerated, 267. Her gravity toward the Sun greater than toward the Earth at her Conjunction, and why she does not then abandon the Earth on that account, 268. Rises nearer the time of Sun-set when about the full in harvest for a whole week than when she is about the full at any other time of the year, and why, 273-284: this rising goes through a course of increasing and decreasing benefit to the farmers every 19 years, 292. Continues above the Horizon of the Poles for fourteen of our natural Days together, 293. Proved to be globular, 314. and to be less than the Earth, 315. Her Nodes, 317. ascending and descending, 318. their retrograde motion, 319. Her acceleration proved from antient Eclipses, 322, n. Her Apogee and Perigee, 336. Not invisible when she is totally eclipsed, and why, 346. How to calculate her Conjunctions, Oppositions, and Eclipses, 355-390. How to find her age in any Lunation by the Golden Number, 423.
Morning and Evening Star, what, 145.
Motion, naturally rectilineal, 100. Apparent, of the Planets as seen by a spectator at rest on the outside of all their Orbits, 133; and of the Heavens as seen from any Planet, 154.
N.
Natural Day, not compleated in the time that the Earth turns round it’s Axis, 222.
New and Full Moon, to calculate the times of 355.
New Stars, 403, cannot be Comets, 404.
New Style, it’s original, 414.
Nicias’s Eclipse, 328.
Nodes, of the Planet’s Orbits, their places in the Ecliptic, 20. Of the Moon’s Orbit, 317. their retrograde motion, 319.
Nonagesimal Degree, what, 259.
Number of Direction, 426.
O.
Objects, we often mistake their bulk by mistaking their distance, 185. Appear bigger when seen through a fog than through clear Air, and why, ib. this applied to the solution of the Horizontal Moon, 187.
Oblique Sphere, what, 131.
Olympiads, what, 323. n.
Orbits of the Planets not solid, 21.
Orrery described, 434, 435, 436.
P.
Parallax, Horizontal, what, 190.
Parallel Sphere, what, 131.
Path of the Moon, 265, 266, 267. Of Jupiter’s Moons, 269.
Pendulums, their vibrating slower at the Equator than near the Poles proves that the Earth turns on it’s Axis, 117.
Penumbra, what, 336. It’s velocity on the Earth in Solar Eclipses, 337.
Period of Eclipses, 320, 326.
Phases of the Moon, 252-268.
Planets, much of the same nature with the Earth, 11. Some have Moons belonging to them, 12. Move all the same way as seen from the Sun, but not as seen from one another, 18. Their Moons denote them to be inhabited, 86. The proportional breadth of the Sun’s Disc as seen from each of them, 87. Their proportional bulks as seen from the Sun, 88. An idea of their distances from the Sun, 89. Appear bigger and less by turns, and why, 90. Are kept in their Orbits by the power of gravity, 101, 150-158. Their motions very irregular as seen from the Earth, 137. The apparent motions of Mercury and Venus delineated by Pencils in an Orrery, 138. Elongations of all the rest as seen from Saturn, 147. Describe equal areas in equal times, 153. The excentricities of their Orbits, 155. In what times they would fall to the Sun by the power of gravity, 157. Disturb one another’s motions, the consequence thereof, 163. Appear dimmer when seen through telescopes than by the bare eye, the reason of this, 170.
Planetary Globe described, 439.
Polar Circles, 198.
Poles, of the Planets, what, 19. Of the world, what, 122. Celestial, seem to keep on the same points of the Heavens all the year, and why, 196.
Projectile Force, 150; if doubled would require a quadruple power of gravity to retain the Planets in their Orbits, 153. Is evidently an impulse from the hand of the Almighty, 161.
Precession of the Equinoxes, 246-251.
Ptolemean System absurd, 96, 140.
R.
Rays of Light, if not disturbed, move in straight lines, and hinder not one another’s motions, 168. Are refracted in passing through different mediums, 171.
Reflection of the Atmosphere causes the Twilight, 177.
Refraction of the Atmosphere bends the rays of light from straight lines, and keeps the Sun and Moon longer in sight than they would otherwise be, 178. A surprising instance of this, 183. Must be allowed for in taking the Altitudes of the celestial bodies, ib.
Right Sphere, 131.
S.
Satellites; the times of their revolutions round their primary Planets, 52, 73, 80. Their Orbits compared with each other, with the Orbits of the primary Planets, and with the Sun’s circumference, 271. What sort of Curves they describe, 272.
Saturn, with his Ring and Moon’s, their Phenomena, 78, 79, 82. The Sun’s light 1000 times as strong to him as the light of the Full Moon is to us, 85. The Phenomena of his Ring farther explained, 204.
Our blessed Saviour, the darkness at his crucifixion supernatural, 352. The prophetic year of his crucifixion found to agree with an astronomical calculation, 432.
Seasons, different, illustrated by an easy experiment, 200; by a figure, 202.
Shadow, what, 312.
Sidereal Time, what, 221; the number of Sidereal Days in a year exceeds the number of Solar Days by one, and why, 222. An easy method for regulating Clocks and Watches by it, 223.
Signs of the Zodiac, their names and characters, 91, 365. How they are numbered by Astronomers, 354.
Sines, line of, how to make, 369.
Smith, (Rev. Dr.) his companion between Moon-light and Day-light, 85. His demonstration that light decreases as the square of the distance from the luminous body increases, 169. (Mr. George) his Dissertation on the Progress of a Solar Eclipse, 321-324.
Solar Astronomer, the judgment he might be supposed to make concerning the Planets and Stars, 135, 136.
Sphere, parallel, oblique, and right, 131. It’s Circles, 198.
Spring and Neap Tides, 302.
Stars, their vast distance from the Earth, 3, 196. Probably not all at the same distance, 4 Shine by their own light, and are therefore Suns 7, probably to other worlds, 8 A demonstration that they do not move round the Earth, 111. Have an apparent slow motion round the Poles of the Ecliptic, and why, 251. A catalogue of them, 399. Cloudy, 402. New, 403. Some of them change their places, 404.
Starry Heavens have the same appearance from any part of the Solar System, 132.
Sun appears bigger than the Stars, and why, 4 Turns round his Axis, 18. His proportional breadth as seen from the different Planets, 87. Describes unequal arcs above and below the Horizon at different times, and why, 130. His Center the only place from which the true motions of the Planets could be seen, 135. Is for half a year together visible at each Pole in it’s turn, and as long in visible, 200, 294. Is nearer the Earth in Winter than in Summer, 205. Why his motion agrees so seldom with the motion of a well regulated Clock, 224-245. Would more than fill the Moon’s Orbit, 271. Proved to be much bigger than the Earth, and the Earth to be bigger than the Moon, 315. To calculate his true place, 360.
Systems, the Solar, 17-95; the Ptolemean, 96; the Tychonic, 97.
T.
Table, of the Periods, Revolutions, Magnitudes, Distances, &c. of the Planets, facing § 99. Of the Air’s rarity, compression, and expansion at different heights, 174. Of refractions, 182. For converting time into motion, and the reverse, 220. For shewing how much of the celestial Equator passes over the Meridian in any part of a mean Solar Day; and how much the Stars accelerate upon the mean Solar time for a month, 221. Of the first part of the Equation of time, 229; of the second part, 241. Of the precession of the Equinox, 247. Of the length of Sidereal, Julian, and Tropical Years, 251. Of the Sun’s place and Anomaly, following 251. Of the Equation of natural Days, following 251 Of the Conjunctions of the hour and minute hands of a Watch, 264. Of the Curves described by the Satellites, 272. Of the difference of time in the Moon’s rising and setting on the parallel of London every day during her course round the Ecliptic, 277. Of Eclipses, 327. For calculating New and Full Moons and Eclipses, following 390. Of the Constellations and number of the Stars, 399. Of the Jewish, Egyptian, Arabic, and Grecian months, 415. For inserting the Golden Numbers right in the Calendar, 423. Of the times of all the New Moons for 76 years, 424. Of remarkable Æras or Events, 433. Of the Golden Number, Number of Direction, Dominical Letter and Days of the Months, following 433.
Thales’s Eclipse, 323.
Thucydides’s Eclipse 324.
Tides, their Cause and Phenomena, 295-311.
Tide-Dial described, 441.
Trajectorium Lunare described, 440.
Tropics, 198.
Twilight, none in the Moon, 254.
Tychonic System absurd, 97.
U.
Universe, the Work of Almighty Power, 5, 161.
Up and down, only relative terms, 122.
Upper or under side of the Earth no such thing, 123.
V.
Velocity of Light compared with the velocity of the Earth in it’s annual Orbit, 197.
Venus, her bulk, distance, period, length of days and nights, 26. Shines not by her own light, ib. Is our morning and evening Star, 28. Her Axis, how situated, 29. Her surprising Phenomena, 29-43. The inclination of her Orbit, 45. When she will be seen on the Sun, ib. How it may probably be soon known if she has a Satellite, 46. Appears in all the Shapes of the Moon, 23, 141. An experiment to shew her phases and apparent motion, 141.
Vision, how caused, 167.
W.
Weather, not hottest when the Sun is nearest to us, and why, 205.
Weight, the cause of it, 122.
World not eternal, 164.
Y.
Year, 407, Great, 251, Tropical, 408, Sidereal, 400, Lunar, 410, Civil, 411, Bissextile, ib. Roman, 413, Jewish, Egyptian, Arabic, and Grecian, 415, how long it would be if the Sun moved round the Earth, 111.
Z.
Zodiac, what, 397. How divided by the antients, 398.
Zones, what, 199.
Footnotes
1. Dr. Young’s Night Thoughts.
2. If a thread be tied loosely round two pins stuck in a table, and moderately stretched by the point of a black lead pencil carried round by an even motion and light pressure of the hand, an oval or ellipsis will be described; the two points where the pins are fixed being called the foci or focuses thereof. The Orbits of all the Planets are elliptical, and the Sun is placed in or near to one of the foci of each of them: and that in which he is placed, is called the lower focus.
3. Astronomers are not far from the truth, when they reckon the Sun’s center the lower focus of all the Planetary Orbits. Though strictly speaking, if we consider the focus of Mercury’s Orbit to be in the Sun’s center, the focus of Venus’s Orbit will be in the common center of gravity of the Sun and Mercury; the focus of the Earth’s Orbit in the common center of gravity of the Sun, Mercury, and Venus; the focus of the Orbit of Mars in the common center of gravity of the Sun, Mercury, Venus, and the Earth; and so of the rest. Yet, the focuses of the Orbits of all the Planets, except Saturn, will not be sensibly removed from the center of the Sun; nor will the focus of Saturn’s Orbit recede sensibly from the common center of gravity of the Sun and Jupiter.
4. As represented in Plate III. Fig. I. and described in § 138.
5. When he is between the Earth and the Sun in the nearer part of his Orbit.
6. The time between the Sun’s rising and setting.
7. One entire revolution, or 24 hours.
8. These are lesser circles parallel to the Equator, and as many degrees from it, towards the Poles, as the Axis of the Planet is inclined to the Axis of it’s Orbit. When the Sun is advanced so far north or south of the Equator as to be directly over either Tropic, he goes no farther; but returns towards the other.
9. These are lesser circles round the Poles, and as far from them as the Tropics are from the Equator. The Poles are the very north and south points of the Planet.
10. A Degree is a 360th part of any Circle. See § 21.
11. The Limit of any inhabitant’s view, where the Sky seems to touch the Planet all round him.
12. This is not strictly true, as will appear when we come to treat of the Recession of the Equinoctial Points in the Heavens § 246; which recession is equal to the deviation of the Earth’s Axis from it’s parallelism: but this is rather too small to be sensible in an age, except to those who make very nice observations.
13. Memoirs d’Acad. ann. 1720.
14. The Moon’s Orbit crosses the Ecliptic in two opposite points called the Moon’s Nodes; so that one half of her Orbit is above the Ecliptic, and the other half below it. The Angle of it’s Obliquity is 51⁄3 degrees.
15. Cassini Elements d’Astronomie, Liv. ix. Chap. 3.
16. Optics, Art. 95.
17. Mr. Whiston, in his Astronomical Principles of Religion.
18. As will be demonstrated in the ninth Chapter.
19. Optics, B. I. § 1178.
20. Astronomy, B. II. §. 838.
21. Philosophy, Vol. I. p. 401.
22. Account of Sir Isaac Newton’s Philosophical Discoveries, B. III. c. 2. § 3.
23. Elements d’Astronomie, § 381.
24. The face of the Sun, Moon, or any Planet, as it appears to the eye, is called it’s Disc.
25. The utmost limit of a person’s view, where the Sky seems to touch the Earth all around, is called his Horizon; which shifts as the person changes his place.
26. The Plane of a Circle, or a thin circular Plate, being turned edgewise to the eye appears to be a straight line.
27. A Degree is the 360th part of a Circle.
28. Here we do not mean such a conjunction, as that the nearer Planet should hide all the rest from the observer’s sight; (for that would be impossible unless the intersections of all their Orbits were coincident, which they are not, See § 21.) but when they were all in a line crossing the standard Orbit at right Angles.
29. The Orrery fronting the Title-page.
30. To make the projectile force balance the gravitating power so exactly as that the body may move in a Circle, the projectile velocity of the body must be such as it would have acquired by gravity alone in falling through half the radius.
31. Astronomical Principles of Religion, p. 66.
32. Δὸς ποῦ στῶ, καὶ τὸν κόσμον κινήσω, i. e. Give me a place to stand on, and I shall move the Earth.
33. If the Sun was not agitated about the common center of gravity of the whole System, and the Planets did not act mutually upon one another, their Orbits would be elliptical, and the areas described by them would be exactly proportionate to the times of description § 153. But observations prove that these areas are not in such exact proportion, and are most varied when the greatest number of Planets are in any particular quarter of the Heavens. When any two Planets are in conjunction, their mutual attractions, which tend to bring them nearer to one another, draws the inferior one a little farther from the Sun, and the superior one a little nearer to him; by which means, the figure of their Orbits is somewhat altered; but this alteration is too small to be discovered in several ages.
34. Religious Philosopher, Vol. III. page 65.
35. This will be demonstrated in the eleventh Chapter.
36. A fine net-work membrane in the bottom of the eye.
37. Book I. Art. 57.
38. A medium, in this sense, is any transparent body, or that through which the rays of light can pass; as water, glass, diamond, air; and even a vacuum is sometimes called a Medium.
39. Newton’s System of the World, p. 120.
40. This is evident from pumps, since none can draw water higher than 33 foot.
41. Namely 10000 times the distance of Saturn from the Sun; p. 94.
42. See his Astronomy, p. 232.
43. As far as one can see round him on the Earth.
44.
Fig. V.
An Angle is the inclination of two right lines, as IH and KH, meeting in a point at H; and in describing an Angle by three letters, the middle letter always denotes the angular point: thus, the above lines IH and KH meeting each other at H, make the Angle IHK. And the point H is supposed to be the center of a Circle, the circumference of which contains 360 equal parts called degrees. A fourth part of a Circle, called a Quadrant, as GE, contains 90 degrees; and every Angle is measured by the number of degrees in the arc it cuts off; as the angle EHP is 45 degrees, the Angle EHF 33, &c: and so the Angle EHF is the same with the angle CHN, and also with the Angle AHM, because they all cut off the same arc or portion of the Quadrant EG; and so likewise the Angle EHF is greater than the Angle CHD or AHL, because it cuts off a greater arc.
The nearer an object is to the eye the bigger it appears, and under the greater Angle is it seen. To illustrate this a little, suppose an Arrow in the position IK, perpendicular to the right line HA drawn from the eye at H through the middle of the Arrow at O. It is plain that the Arrow is seen under the Angle IHK, and that HO, which is it’s distance from the eye, divides into halves both the Arrow and the Angle under which it is seen: viz. the Arrow into IO, OK, and the Angle into IHO and KHO: and this will be the case whatever distance the Arrow is placed at. Let now three Arrows, all of the same length with IK, be placed at the distances HA, HC, HE, still perpendicular to, and bisected by the right line HA; then will AB, CD, EF, be each equal to, and represent IO; and AB (the same as IO) will be seen from H under the Angle AHB; but CD (the same as IO) will be seen under the Angle CHD or AHL; and EF (the same as IO) will be seen under the Angle EHF, or CHN, or AHM. Also, EF or IO at the distance HE will appear as long as CN would at the distance HC, or as AM would at the distance HA: and CD or IO at the distance HC will appear as long as AL would at the distance HA. So that as an object approaches the eye, both it’s magnitude and the Angle under which it is seen increase; and as the object recedes, the contrary.
45. The fields which are beyond the gate rise gradually till they are just seen over it; and the arms, being red, are often mistaken for a house at a considerable distance in those fields.
I once met with a curious deception in a gentleman’s garden at Hackney, occasioned by a large pane of glass in the garden-wall at some distance from his house. The glass (through which the fields and sky were distinctly seen) reflected a very faint image of the house; but the image seemed to be in the Clouds near the Horizon, and at that distance looked as if it were a huge castle in the Air. Yet, the Angle under which the image appeared, was equal to that under which the house was seen: but the image being mentally referred a much greater distance than the house, appeared much bigger to the imagination.
46. The Sun and Moon subtend a greater Angle on the Meridian than in the Horizon, being nearer the Earth in the former case than the latter.
47. The Altitude of any celestial Phenomenon is an arc of the Sky intercepted between the Horizon and the Phenomenon. In Fig. VI. of Plate II. let HOX be a horizontal line, supposed to be extended from the eye at A to X, where the Sky and Earth seem to meet at the end of a long and level plain; and let S be the Sun. The arc XY will be the Sun’s height above the Horizon at X, and is found by the instrument EDC, which is a quadrantal board, or plate of metal, divided into 90 equal parts or degrees on its limb DPC; and has a couple of little brass plates, as a and b, with a small hole in each of them, called Sight-Holes, for looking through, parallel to the edge of the Quadrant whereon they stand. To the center E is fixed one end of a thread F, called the Plumb-Line, which has a small weight or plummet P fixed to it’s other end. Now, if an observer holds the Quadrant upright, without inclining it to either side, and so that the Horizon at X is seen through the sight-holes a and b, the plumb-line will cut or hang over the beginning of the degrees at o, in the edge EC; but if he elevates the Quadrant so as to look through the sight-holes at any part of the Heavens, suppose to the Sun at S; just so many degrees as he elevates the sight-hole b above the horizontal line HOX, so many degrees will the plumb-line cut in the limb CP of the Quadrant. For, let the observer’s eye at A be in the center of the celestial arc XYV (and he may be said to be in the center of the Sun’s apparent diurnal Orbit, let him be on what part of the Earth he will) in which arc the Sun is at that time, suppose 25 degrees high, and let the observer hold the Quadrant so that he may see the Sun through the sight-holes; the plumb-line freely playing on the quadrant will cut the 25th degree in the limb CP equal to the number of degrees of the Sun’s Altitude at the time of observation. N. B. Whoever looks at the Sun, must have a smoaked glass before his eyes to save them from hurt. The better way is not to look at the Sun through the sight-holes, but to hold the Quadrant facing the eye, at a little distance, and so that the Sun shining through one hole, the ray may be seen to fall on the other.
48. See the Note on § 185.
49. Here proper allowance must be made for the Refraction, which being about 34 minutes of a degree in the Horizon, will cause the Moon’s center to appear 34 minutes above the Horizon when her center is really in it.
50. By this is meant, that if a line be supposed to be drawn parallel to the Earth’s Axis in any part of it’s Orbit, the Axis keeps parallel to that line in every other part of it’s Orbit: as in Fig. I. of Plate V; where abcdefgh represents the Earth’s Orbit in an oblique view, and Ns the Earth’s Axis keeping always parallel to the line MN.
51. Smith’s Optics, § 1197.
52. All Circles appear ellipses in an oblique view, as is evident by looking obliquely at the rim of a bason. For the true figure of a Circle can only be seen when the eye is directly over it’s center. The more obliquely it is viewed, the more elliptical it appears, until the eye be in the same plane with it, and then it appears like a straight line.
53. Here we must suppose the Sun to be no bigger than an ordinary point (as ·) because he only covers a Circle half a degree in diameter in the Heavens; whereas in the figure he hides a whole sign at once from the Earth.
54. Here we must suppose the Earth to be a much smaller point than that in the preceding note marked for the Sun.
55. If the Earth were cut along the Equator, quite through the center, the flat surface of this section would be the plane of the Equator; as the paper contained within any Circle may be justly termed the plane of that Circle.
56. The two opposite points in which the Ecliptic crosses the Equinoctial, are called the Equinoctial Points: and the two points where the Ecliptic touches the Tropics (which are likewise opposite, and 90 degrees from the former) are called the Solstitial Points.
57. The Equinoctial Circle intersects the Ecliptic in two opposite points, called Aries and Libra, from the Signs which always keep in these points: They are called the Equinoctial Points, because when the Sun is in either of them, he is directly over the terrestrial Equator; and then the days and nights are equal.
58. In this discourse, we may consider the Orbits of all the Satellites as circular, with respect to their primary Planets; because the excentricities of their Orbits are too small to affect the Phenomena here described.
59. If a Globe be cut quite through upon any Circle, the flat surface where it is so divided, is the plane of that circle.
60. The Figure shews the Globe as if only elevated about 40 degrees, which was occasioned by an oversight in the drawing: but it is still sufficient to explain the Phenomena.
61. The Ecliptic, together with the fixed Stars, make 3661⁄4 apparent diurnal revolutions about the Earth in a year; the Sun only 3651⁄4. Therefore the Stars gain 3 minutes 56 seconds upon the Sun every day: so that a Sidereal day contains only 23 hours 56 minutes of mean Solar time; and a natural or Solar day 24 hours. Hence 12 Sidereal hours are 1 minute 58 seconds shorter than 12 Solar.
62. The Sun advances almost a degree in the Ecliptic in 24 hours, the same way that the Moon moves: and therefore, the Moon by advancing 131⁄6 degrees in that time goes little more than 12 degrees farther from the Sun than she was on the day before.
63. This center is as much nearer the Earth’s center than the Moon’s as the Earth is heavier, or contains a greater quantity of matter than the Moon, namely about 40 times. If both bodies were suspended on it they would hang in æquilibria. So that dividing 240,000 miles, the Moon’s distance from the Earth’s center, by 40 the excess of the Earth’s weight above the Moon’s, the quotient will be 6000 miles, which is the distance of the common center of gravity of the Earth and Moon from the Earth’s center.
64. The Penumbra is a faint kind of shadow all around the perfect shadow of the Planet or Satellite; and will be more fully explained by and by.
65. Which is the time that the Eclipse would be at the greatest obscuration, if the motions of the Sun and Moon were equable, or the same in all parts of their Orbits.
66. The above period of 18 years 11 days 7 hours 43 minutes, which was found out by the Chaldeans, and by them called Saros.
67. A Digit is a twelfth part of the diameter of the Sun or Moon.
68. There are two antient Eclipses of the Moon, recorded by Ptolemy from Hipparchus, which afford an undeniable proof of the Moon’s acceleration. The first of these was observed at Babylon, December the 22d, in the year before Christ 383: when the Moon began to be eclipsed about half an hour before the Sun rose, and the Eclipse was not over before the Moon set: but by our best Astronomical Tables, the Moon was set at Babylon half an hour before the Eclipse began; in which case, there could have been no possibility of observing it. The second Eclipse was observed at Alexandria, September the 22d, the year before Christ 201; where the Moon rose so much eclipsed, that the Eclipse must have begun about half an hour before she rose: whereas by our Tables the beginning of this Eclipse was not till about 10 minutes after the Moon rose at Alexandria. Had these Eclipses begun and ended while the Sun was below the Horizon, we might have imagined, that as the antients had no certain way of measuring time, they might have been so far mistaken in the hours, that we could not have laid any stress on the accounts given by them. But, as in the first Eclipse the Moon was set, and consequently the Sun risen, before it was over; and in the second Eclipse the Sun was set, and the Moon not risen, till some time after it began; these are such circumstances as the observers could not possibly be mistaken in. Mr. Struyk in the following Catalogue, notwithstanding the express words of Ptolemy, puts down these two Eclipses as observed at Athens; where they might have been seen as above, without any acceleration of the Moon’s motion: Athens being 20 degrees West of Babylon, and 7 degrees West of Alexandria.
69. Each Olympiad began at the time of Full Moon next after the Summer Solstice, and lasted four years, which were of unequal lengths because the time of Full Moon differs 11 days every year: so that they might sometimes begin on the next day after the Solstice, and at other times not till four weeks after it. The first Olympiad began in the year of the Julian Period 3938, which was 776 years before the first year of Christ, or 775 before the year of his birth; and the last Olympiad, which was the 293d, began A. D. 393. At the expiration of each Olympiad, the Olympic Games were celebrated in the Elean fields, near the river Alpheus in the Peloponnesus (now Morea) in honour of Jupiter Olympus. See Strauchius’s Breviarium Chronologium, p. 247-251.
70. The reader may probably find it difficult to understand why Mr. Smith should reckon this Eclipse to have been in the 4th year of the 48th Olympiad; as it was only in the end of the third year: and also why the 28th of May, in the 585th year before Christ should answer to the present 10th of that month. But we hope the following explanation will remove these difficulties.
The month of May (when the Sun was eclipsed) in the 585th year before the first year of Christ, which was a leap-year, fell in the latter end of the third year of the 48th Olympiad; and the fourth year of that Olympiad began at the Summer Solstice following: but perhaps Mr. Smith begins the years of the Olympiad from January, in order to make them correspond more readily with Julian years; and so reckons the month of May, when the Eclipse happened, to be in the fourth year of that Olympiad.
The Place or Longitude of the Sun at that time was ♉ 29° 43ʹ 17ʺ, to which same place the Sun returned (after 2300 years, viz.) A. D. 1716, on May, 9d. 5h. 6m. after noon: so that, with respect to the Sun’s place, the 9th of May, 1716 answers to the 28th of May in the 585th year before the first year of Christ; that is, the Sun had the same Longitude on both those days.
71. Before Christ 413, August 27.
72. Before Christ 168, June 20.
73. Struyk’s Eclipses are to the Old Style, all the rest to the New.
74. This Eclipse happened in the first year of the Peloponnesian war.
75. Although the Sun and Moon are spherical bodies, as seen from the Earth they appear to be circular planes, and so would the Earth if it were seen from the Moon. The apparently flat surfaces of the Sun and Moon are called their Disks by Astronomers.
76. A Digit is a twelfth part of the diameter of the Sun and Moon.
77. This is the same with the annual Argument of the Moon.
78. When the Romans divided the Empire, which was about 38 years before Christ, Spain fell to Augustus’s share: in memory of which, the Spaniards dated all their memorable events ab exordio Regni Augusti; as Christians do from the birth of our Saviour. But in process of time, only the initial letters AERA of these words were used instead of the words themselves. And thus, according to some, came the word ÆRA, which is made use of to signify a point of time from whence historians begin to reckon.
79. When the Sun’s Anomaly is 0 signs 0 degrees, or 6 signs 0 degrees, neither the Sun nor the Moon’s Anomaly have any Equation; which is the case in this Example.
80. See the Remark, p. 195.
81. Babylon is 42 deg. 46 min. east from the Meridian of London, which is equal to 2 hours 51 min. of time nearly. See § 220.
82. Our Saviour was born in a leap-year, and therefore every fourth year both before and after is a leap-year in the Old Stile: but the Tables begin with the year next after that of his birth.
83. When only one of the Nodes is mentioned, it is the Ascending Node that is meant, to which the Descending Node is exactly opposite.
84. When the Moon is North of the Ecliptic and going farther from it, her Latitude or Declination from the Ecliptic is called North Ascending: when she is North of the Ecliptic and going toward it, her Latitude is North Descending: when she is South of the Ecliptic and going farther from it, her Latitude is South Descending: and lastly, when she is South of the Ecliptic and going toward it, her Latitude is South Ascending.
85. See Page 193, Example II.
86. M. Maupertuis, in his dissertation on the figures of the Celestial Bodies (p. 61-63) is of opinion that some Stars, by their prodigious quick rotations on their Axes, may not only assume the figures of oblate spheroids, but that by the great centrifugal force, arising from such rotations, they may become of the figures of mill-stones; or be reduced to flat circular planes, so thin as to be quite invisible when their edges are turned towards us; as Saturn’s Ring is in such positions. But when very excentric Planets or Comets go round any flat Star, in Orbits much inclined to it’s Equator, the attraction of the Planets or Comets in their perihelions must alter the inclination of the Star; on which account it will appear more or less large and luminous as it’s broad side is more or less turned towards us. And thus he imagines we may account for the apparent changes of magnitude and lustre in those Stars, and likewise for their appearing and disappearing.
87. See this word explained in the note at the foot of page 194.
88. See the note on § 323.
89. Matt. xxvii. 45. Mark xv. 43. Luke xxiii. 44.
Directions to the Bookbinder.
The Orrery Plate is to front the Title Page.
| Plate I | fronting Page 5 |
| II | 39 |
| III | 49 |
| IV | 73 |
| V | 81 |
| VI | 97 |
| VII | 125 |
| VIII | 147 |
| IX | 147 |
| X | 157 |
| XI | 179 |
| XII | 203 |
| XIII | 279 |
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