FAMILIAR SUBJECTS

ON

ASTRONOMY.

ASTRONOMY is that branch of natural philosophy which treats of the heavenly bodies. The determination of their magnitudes, distances, and the orbits which they describe, is called plane astronomy; and the investigation of the causes of their motions is called physical astronomy. The former discoveries are made from observations on their apparent magnitude and motions and the latter from analogy.

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, and goodness of the Supreme Being.

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 its circumference is known to be 25,020 miles; yet that distance is so small compared with the earth's distance from 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.

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 placed at inmense 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.

By the help of telescopes we discover thousands of stars which are invisible to the bare 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 carried 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 its creation; although the velocity of light be a million of times greater than the velocity of a cannon bullet; 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.

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.

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, it is plain that the stars are of the same nature with the sun.

Instead then of one sun and one world only in the universe, 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, by an eye that could take in the whole 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 4,884 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.

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 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!

And then, to an attentive observer, 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.

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.

On the surface of the moon, because it is nearer to 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.

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 its activity.-FERGUSON.

Of the Figure of the Earth, and its Magnitude.

The figure of the earth, as composed of land and water, is nearly spherical; the proof of this assertion will be the principal object of this chapter. The ancients held various opinions respecting the figure of the earth; some imagined it to be cylindrical, or in the form of a drum; but the general opinion was, that it was a vast extended plane, and that the horizon was the utmost limits of the earth, and the ocean the bounds of the horizon. These opinions were held in the infancy of astronomy; and, in the early ages of Christianity, some of the fathers went so far as to pronounce it heretical for any person to declare that there was such a thing as the antipodes. But, by the industry of succeeding ages, when astronomy and navigation were brought to a tolerable degree of perfection, and when it was observed that the moon was frequently eclipsed by the shadow of the earth, and that such shadow always appeared circular on the disc or face of the moon, in whatever position the shadow was projected, it necessarily followed that the earth, which cast the shadow, must be spherical; since nothing but a sphere, when turned in every position with respect to a luminous body, can cast a circular shadow; likewise all calculations of eclipses, and of the places of the planets, are made upon supposition that the earth is a sphere, and they all answer to the true times when accurately calculated. When an eclipse of the moon happens,

it is observed sooner by those who live eastward than by those who live westward; and, by frequent experience, astronomers have determined that, for every fifteen degrees difference of longitude, an eclipse begins so many hours sooner in the easternmost place, or later in the westernmost. If the earth were

a plane, eclipses would happen at the same time in all places, nor could one part of the world be deprived of the light of the sun while another part enjoyed the benefit of it. The voyages of the circumnavigators sufficiently prove that the earth is round from west to east. The first who attempted to circumnavigate the globe, was Magellan, a Portuguese, who sailed from Seville, in Spain, on the 10th of August, 1519; he did not live to return, but his ship arrived at St. Lucar, near Seville, on the 7th of September, 1522, without altering its direction, except to the north or south, as compelled by the winds, or intervening land. Since this period the circumnavigation of the globe has been performed at different times by Sir Francis Drake, Lord Anson, captain Cook, &c. The voyages of the circumnavigators have been frequently adduced by writers on geography and the globes to prove that the earth is a sphere: but when we reflect that all the circumnavigators sailed westward round the globe, (and not northward and southward round it) they might have performed the same voyages had the earth been in the form of a drum or cylinder: but the earth cannot be in the form of a cylinder, for if it were, then the difference of longitude between any two places would be equal to the meridional distance between the same places, as on a Mercator's chart, which is contrary to observation.-Again, if a ship sail in any part of the world and upon any course whatever, on her departure from the coast, all high towers or mountains gradually disappear, and persons on shore may see the masts of the ship after the hull is hid by the convexity of the water. (See Figure I. Plate XX.) If a vessel sail northward, in north latitude, the people on board may observe the polar star gradually to increase in altitude the farther they go: they may likewise observe new stars continually emerging above the horizon, which were before imperceptible; and at the same time, those stars which appear southward, will continue to diminish in altitude till they become invisible. The contrary phenomena will happen if the vessel sail southward: hence, the earth is spherical from north to south, and it has already been shown that it is spherical from east to west.

The arguments already adduced, clearly prove the rotundity of the earth, though common experience shows us that it is not strictly a geometrical sphere; for its surface is diversified with mountains and valleys: but these irregularities no more hinder the earth from being reckoned spherical, considering its mag