The Marquis of Setstar's Observatory

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LECTURE II.

Of the Universe, and particularly of the Solar System.

THE last lecture was read at the Marquiss of Setstar's, who was so well pleased at these young gentlemen's meeting thus toimprove themselves, that he ordered them to be elegantly treated with tarts, sweatmeats, syllabubs, and such other dainties as his Lordships thought were most proper for youth: and I must observe, that the Marchioness did them the honour of her company, and was particularly pleased with the conversation of Master Telescope. As it was a moonlight night, her Ladyship, after supper, led them to the top of the tower, where his Lordship has an observatory furnished with all the instruments necessary for astronomical and philosophical observations; and the place itself is the best that can be conceived for enquiries of this kind, and for meditation. To see an extensive horizon thus shaded by the brow of night, and at intervals brightened up by the borrowed <18> light of the moon dancing among the clouds, was to me inexpressibly pleasing. Nothing was heard but a gentle breeze whispering the top of he battlements; the dying murmurs of a distant cascade; the melancholy hootings of the bird of night, who kept watch in an ivy tower near us; the mansion clock, that recorded the time; and old Echo, who repeated the hours from the side of a rock, where she has secluded herself ever since the deluge. Night, the nurse of Nature, had hushed all things else to silence. — But silence was soon broke by our Philosopher, who thus began his Lecture.

Look round, my dear friends, says he, you see the earth seems to be bounded at an equal distance from us every way, where it appears to meet the sky, that forms this beautiful arch or concave over our heads. Now that distance round, where we lose sight of the earth, is called the horizon; and when the sun, moon, and stars emerge from beneath and come into our sight, we say they are risen, or got above the horizon. For all this glorious canopy bespangled with lights, that bedeck the sky and illuminate the earth, as the Sun, the Fixed Stars, the Comets <19> and Planets, (to which last our Earth and the Moon belong) have all apparent motions, as may be perceived by the naked eye; tho' in fact none move but the planets and comets, as will be proved hereafter.

But besides the stars which we see, there are others not discernible by the naked eye, some of which are fixed stars, and some are bodies moving about the most distant planets, which were invisible and unknown to us before the discovery of prospective glasses. — Pray hand me the reflecting telescope — Here it is.

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And the refracting telescope, if you please. — Oh, here it is also.

If you use the reflecting telescope, you must turn this screw on the side of it till you can see the object you want to examine in the most perfect manner: and if you use the refracting telescope, you must move backwards or forwards this small part, till you have adjusted it to your sight. Then look at that part of the heavens where I have pointed them, orindeed any other part, and you will perceive more stars than you saw before with your eye alone. These are fixed stars, and are called fixed because they always keep the same distance one from another, and the same distance from the sun, which is supposed to be also fixed; and was he placed at the immense distance they are at, would probably appear no bigger than one of them. Hence some Philosophers have concluded, and I think not without reason, that every fixed <21> star is a sun, that has a system of planets revolving round it, like our solar system. And if so, how immensely great, how wonderfully glorious, is the structure of this universe, which contains many thousand worlds, large as ours, suspended in aether, rolling like the earth round their several suns, and filled with animals, plants, and minerals, all perhaps different from ours, but all intended to magnify the Almighty Architect; who weighed the mountains in his golden scales, who measured the ocean in the hollow of his hand, who drew out the heavens as a curtain, who maketh the clouds his chariot, and walketh upon the wings of the wind.

The fervour and air of piety with which he delivered this, silenced all his companions, and gave infinite satisfaction to the Marchioness. Master Wilson, who had before been very impertinent, began now to consider himself a fool in comparison of our Philosopher; and as Master Telescope had mentioned the solar system, he begged that he would explain it to him.

That I will with pleasure, replied the Philosopher; but first let me observe to you, that of these heavenly bodies some <22> are luminous, and lend us their own light, as doth the Sun and Fixed Stars; while others are opaque, and have no light of their own to give us; but reflect to us a part of the light they receive from the sun. This is particularly the case with respect to the planets and comets of our solar system, which all give us a portion of the light they have received, and we in return reflect to them a portion of ours: for I make no doubt, but those who inhabit the moon have as much of the sun's light reflected to them from the earth, as we have reflected to us from the moon.

The inhabitants of the moon! says Master Lovelace, with some emotion, Whither will you lead me? What, are the stories that have been told of the Man in the Moon then true?

I don't know what stories you have heard, replied the Philosopher; but it is no extravagant conjecture to suppose that the moon is inhabited, as well as the earth; though what sort of inhabitants they are, we on earth are unable to discover. As to my part, I am lost in this boundless abyss. It appears to me that the sun, which gives life to the world, is only a beam of the glory of God; and the <23> air, which supports that life, is, as it were, the breath of his nostrils.

Do thou, O God, support me while I gaze with astonishment at thy wonderful productions; since it is not idle impertinent curiosity that leads me to this enquiry, but a fervent desire to see only the skirts of thy glory, that I may magnify thy power and thy mercy to mankind.

Of the Solar System.

Our Solar System contains the sun in the centre, and the planets and commets moving about it. — Pray look at the figure on the other side, where I have drawn the sun, and the planets in their several orbits or circles, with their respective distances from the sun, and from each other; together with the orbit of a comet.

The planets, as I have already observed, are bodies that appear like stars, but are not luminous; that is, they have no light in themselves, tho' they give us light; for they shine by reflecting the light of the sun. Of these there are two kinds, the one called primary, and the other secondary planets.

There are six primary planets, and these are Mercury, Venus, the Earth, Mars, Ju <24> piter, and Saturn; all which move round the Sun, as you may see in the figure before you; whereas the secondary planets move round other planets. The Moon, you know, (which is one of the secondary planets) moves round the Earth; four moons, or Satellites, as they are frequently called, move round Jupiter; and five round Saturn. And thus has the Almighty provided light for those regions that lie at such an immense distance from the Sun.

The amazing distance which each planet is at from the Sun, may be seen by the time which it takes in its periodical revolution.

These move round the Sun from west to east in the time above mentioned. They are always to be found among the stars of those constellations that compose the twelve signs of the Zodiac; and in their progress do not describe a perfect circle, but an orbit a little inclining to an <25> oval: the reason whereof I shall give you in a future Lecture.

The comets move about the sun in a long slender oval of an amazing extent; one of the focus's being near the centre of the sun, and the shortest of the other far beyond the sphere of Saturn; so that the periodical revolutions of any are not performed in less than 70 or 80 years. See the Plate.

But let us quit these bodies, of which we know so little, and speak of our old companion the moon, with whom we ought to be better acquainted; since she not only lights us home in the night, but lends her aid to get our ships out of the docks, and to bring in and carry out our merchandize; for without the assistance of Lady Luna you would have no tides: but more of this hereafter. — A little more now, if you please, says Tom Wilson. What then, does the moon pour down water to occasion the tide? I am at a loss to understand you. No, replied our Philosopher, the moon does not pour down water to occasion the tides; that were impossible: but she by attracting the waters of the sea raises them higher, and that is the reason why the tides are always governed by the moon.

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The moon moves round the earth in the same manner as the earth does round the sun, and performs her synodical motion, as it is called, in 29 days, 12 hours, and 44 minutes, though the periodical is 27d. 7h. 43m. By this motion of the moon are occasioned the eclipses of the sun and moon, and the different appearances, aspects, or phases she at different times puts on: for when the earth is so situated between the sun and the moon, that we see all her enlightened parts it is full Moon; when the moon is so situated between the sun and the earth, that her enlightened parts are hid or turned from us, it is new Moon; and when her situation is such that only a portion of her enlightened part is hid from us, we see a horned Moon, a half Moon, or a gibbous Moon, according to the quantity of the enlightened part we can perceive.

The total or longest Eclipse of the Moon happens when the earth is directly between the sun and the moon, and prevents the light of the sun from falling upon and being reflected by the moon; as I'll shew you. We will suppose this orange to be the sun, this cricket-ball the earth, and this top the moon; now if you place them <27> in a strait line, with the ball in the middle, and then put your eye to the top, you'll find that the ball will entirely hide the orange from your view, and would prevent the rays of light (which always proceed in right lines) from falling upon it, whence it would ensue a total eclipse. But move the top, which represents the moon, a little on either side, and with your eye placed as before you will perceive a part of the orange, which will be now in such a position, that a strait line may be drawn from a part of the orange or sun, to a part of the top or moon, without touching the ball that represents the earth; and in this position the moon would be partly illuminated, and the eclipse be only partial.

An Eclipse of the Sun is occasioned by the moon's being betwixt the sun and the earth, and preventing the light of the sun from coming to that part of the earth which we inhabit. — If the moon hides from us the whole body of the sun, it is a total eclipse; but if the whole be not hid, it is a partial one. An Eclipse of the Sun never happens but at a new moon; nor one of the Moon but when she is at full.

The moon, as to matter and form, appears not much unlike our earth, as you <28> may perceive by this Map. The bright parts are supposed to be the high illumined tracts of land, as mountains, islands, &c. and the dark parts it is imagined are the seas, lakes, and vales, which reflect but little light. But of this there is no certainty.

The Earth, by its revolution about the sun in 365 days, 5 hours, and 49 minutes, measures out that space of time which we call a Year; and the line described by the earth in this annual revolution about the sun is called the Ecliptic. To give you a perfect idea of this and other circles necessary to be known, I have on the opposite side presented you with the figure of a Sphere.

The annual motion of the earth round the sun is from west to east, or so to speak more philosophically, it is according to the order of the signs of the Zodiac; which we shall hereafter explain.

But besides this annual motion or revolution about the sun in the line of the Ecliptic, the earth turns round upon its own axis in about 24 hours; so that it hath two motions at one and the same time.

The Marchioness, whose curiosity had kept her during the Lecture, desired to have this explained. That shall be done, <29> Madam, in a minute, says the little Philosopher, and I can never have a better opportunity; for I see the Duke of Galaxy is coming to make your ladyship a visit. His coach is just entering the iron gates, and will presently wheel round the circle, or rather oval, before the portico. Pray, Madam, fix your eye on one of the wheels, which you may do as it is moon-light,and you will perceive it turn round upon its own axis, at the same time that it runs rounds the circle before the house. This double motion of the wheel very fitly represents the two motions of the earth, which have heretofore been explained by the motion of a bowl on a bowling green; but I believe your Ladyship will think the instance or example before us is better. It is hard to reason from similitudes, because they generally fail in some part: all the members of a simile seldom correspond with the subject it is intended to illustrate; and, if I mistake not, that is the case with the bowl upon the green; which, though it aptly represents the earth's motion on its own axis, is far from representing its revolution about the sun, because the bias in the bowl will never induce it to form either a circle or an <30> oval; for the figure it describes is rather a parabola, or that sort of figure which a long fishing-rod forms when it is bent by drawing a fish out of the water.

Your Ladyship knows perfectly that the earth turning on its own axis makes the difference of day and night: you will therefore give me leave, Madam, to address my discourse to these young gentlemen and ladies, who may be ignorant of this branch of Philosophy.

That the turning of the earth on its own axis makes the difference of day and night is most certain; for in those parts of the earth which are turned towards the sun it will be day, and of consequence it must be night in those which are turned from it.

But the length of days and nights, and the variation of the seasons, are occasioned by the annual revolution of the earth about the sun in the Ecliptic; for as the earth in this course keeps its axis equally inclined every where to the plane of the Ecliptic and parallel to itself, and as the plane of the Ecliptic inclines 23 degrees and a half towards the Equator, the earth in this direction has sometimes one of its poles nearest the sun, and sometimes the other. Hence heat and cold, summer <31> and winter, and length of days and nights. Yet notwithstanding these effects of the sun, which gives us light and heat, his distance from us is so great, that a cannonball would be 25 years coming thence to the earth, even if it flew with the same velocity as it does when it is first discharged from the mouth of a cannon.

Here they were all amazed, and Lady Caroline said this doctrine could not be true; for if the sun was at that immense distance, his light could not reach us every morning, in the manner it does. — I beg your pardon, Madam, replied the Philosopher; your Ladyship's mistake arises from your not knowing, or at least not considering, the amazing velocity of light; which flies after the rate of 200,000 miles in a second of time; so that notwithstaning a cannon-ball would be 25 years in coming form the sun, the light finds its way to us in about eight minutes. But if you are so surprized at the sun's distance, Madam, what think you of that of the fixed stars, which are so far remote from us, that a cannon-ball, flying with the same velocity as when first discharged, would be 700,000 years in coming to the <32> earth? Yet many of these stars are seen, and even without the use of telescopes.

There are other things observable in our Solar System, which if attended to will excite our admiration: such are the dark spots which are seen on the Sun's surface, and which often change their place, number, and magnitude. Such also is the amazing Ring which encompasses the body of the planet Saturn at the distance of 21,000 miles; and such are the Belts that gird the body of Jupiter: concerning all which there are various conjectures, but conjectures in Philosophy are rarely to be admitted.

© 2017 The Newton Project

Professor Rob Iliffe
Director, AHRC Newton Papers Project

Scott Mandelbrote,
Fellow & Perne librarian, Peterhouse, Cambridge

Faculty of History, George Street, Oxford, OX1 2RL - newtonproject@history.ox.ac.uk

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