Page images
PDF
EPUB

days, and 18 hours. Its diameter is computed at 35,112 miles. When seen from the earth, its mean apparent diameter is three and seconds, and as seen from the Sun, 4 seconds of a degree. As the distance of this planet from the Sun is twice as great as that of Saturn, it can scarcely be distinguished without the aid of instruments. When the sky however is serene, it appears like a fixed star of the sixth magnitude, with a bluish white light, and a brilliancy between that of Venus and the Moon; but seen with a power of two or three hundred, its disk is visible and well defined.---The want of light arising from the distance of this planet from the Sun is supplied by six satellites, all of which were discovered by Dr. Herschel.

The first of those satellites is twenty-five and a half seconds from its primary, and revolves round it in 5 days, 21 hours, and 25 minutes; the second is nearly 34 seconds distant from the planet, and performs its revolution in 8 days, 17 hours, 1 minute and 19 seconds. The distance of the third satellite is 38,57 seconds, and the time of its periodical revolution is ten days, 23 hours, and four minutes. The distance of the fourth satellite is 44,22 seconds, and the time of its periodical revolution is 13 days, 11 hours, 5 minutes and 30 seconds. The distance of the fifth is one minute and 28 seconds, and its revolution is completed in 38 days, 1 hour, and 49 minutes. The sixth satellite, or the furthest from the centre of its primary, is at the distance of two minutes, and nearly 57 seconds, and therefore requires 107 days, 16 hours, and 40 minutes to complete one revolution. The second and fourth of these were discovered on the 11th of January, 1787, the other four were discovered in 1790, and 1794; but their distances, and times of periodical revolution, have not been so accurately ascertained as the other two. It is, however, a remarkable circumstance, that the whole of these satellites move in a retrograde direction, and in orbits lying in the same plane, and almost perpendicular to the ecliptic. When the Earth is in its perihelion, and Herschel in its aphelion, the latter becomes stationary, as seen from the Earth, when its elongation, or distance from the Sun is 8 signs, 17 degrees, and 37 minutes, his retrograda tions continue 151 days, and 12 hours. When the Earth is in its aphelion, and Herschel in its perihelion, it becomes stationary, at an elongation of 8 signs, 16 degrees, and 27 minutes, and its retrogradations continue 149 days and 18 hours.

ON COMETS.

Comets are a class of celestial bodies, which occasionally appear in the Heavens. They exhibit no visible or defined disk, but shine with a pale and cloudy light, accompanied with a tail or train, turned from the Sun. They traverse every part of the Heavens, and move in every possible direction,

When examined through a good telescope, a Comet resembles a mass of aqueous vapours, encircling an opaque nucleus, of different degrees of darkness in different Comets; though sometimes, as in the case of several discovered by Dr. Herschel, no nucleus can be

seen.

As the Comet advances towards the Sun, its faint and nebulous light becomes more brilliant, and its luminous train gradually increases in length.

When it reaches its perihelion, the intensity of its light, and the length of its tail reach their maximum, and then it sometimes shines with all the splendour of Venus. During its retreat from the perihelion, it is shorn of its splendour, and it gradually resumes its nebulous appearance; and its train decreases in magnitude, until it reaches such a distance from the Earth, that the attenuated light of the Sun which it reflects, ceases to make an impression on the organ of sight. Traversing unseen the remote portion of its orbit, the Comet wheels its etherial course far beyond the limits of the Solar System. What region it there visits, or upon what destination it is sent, the limited powers of man are unable to discover. After the lapse of years, we perceive

it again returning to our System, and tracing a portion of the same orbit round the Sun, which it had formerly

described.

Various opinions have been entertained by Astronomers respecting the tails of Comets. These tails or trains, sometimes occupy an immense space in the Hea

vens.

The Comet of 1681, stretched its tail across an arch of 104 degrees; and the tail of the Comet of 1769 subtended an angle of 60 degrees at Paris, 70 at Bologna, 97 at the Isle of Bourbon, and 90 degrees at Sea, between Teneriffe and Cadiz. These long trains of light are maintained by Newton, to be a thin vapour, raised by the heat of the Sun from the Comet.

If we knew their uses in our System, we could form more probable conjectures as to the chronology of their creation. They have been noticed from the earliest era of our Astronomical History, and if our modern Philosophers had not discovered, that some (at least,) leave us to return again into our System, and therefore describe a vast elliptical orbit round our Sun, we might have fancied that the periods of their first recorded appearances in our field of science, were the eras of their individual formation. But their recurring presence proves, that their first existence ascends into unexplored and unrecorded antiquity. Yet, from whence they came to us, we as little know as for what purpose. The Comet of 1682, re-appeared in 1759, in the interval describing an orbit in the form of an ellipsis, answering to a revolution of 27,937 days. It will therefore re-appear in November, 1835. In its greatest distance, it is supposed not to go above twice as far as Uranus. This is indeed a prodigious sweep of space, and it has been justly observed, that the vast distance to which some Comets roam, proves how very far the attraction of the Sun extends; for though they stretch themselves to such depths in the abyss of space, yet by virtue of the Solar power, they return into its effulgence. But it has been recently discovered, that three Comets (at least,) never leave the planetary system. One whose period is

three years and a quarter, is included within the orbit of Jupiter; another of six years and three quarters, extends not so far as Saturn; and a third of twenty years, is found not to pass beyond the circuit of Uranus.

The transient effect of a Comet passing near the Earth, could scarcely amount to any great convulsion, but if the Earth were actually to receive a shock by collision, from one of those bodies, the consequences would be awful. A new direction would be given to its rotatory motion, and the Globe would revolve around a new axis. The Seas, forsaking their ancient beds, would be hurled by their centrifugal force to the new equatorial regions: islands and continents, the abodes of men and animals, would be covered by the universal rush of the waters to the new Equator, and every vestige of human industry and genius at once destroyed.

Although the orbits of all the planets in the Solar System be crossed by five hundred different Comets, the chances against such an event however, are so very numerous, that there need be no dread of its occurrence; besides, that Almighty Arm which first created them, and described for them their various orbits-that Omnipotent Wisdom which directed the times of their periodical revolutions, still continues to guide and protect 'I the workmanship of his hands.

INTERROGATIONS FOR SECTION SECOND.

Of what does the SOLAR SYSTEM Consist?
What planets finish their circuits soonest?
Which moves with the greatest rapidity ?
In what direction do they move in their orbits ?

What is the form of the orbits described by the

planets?

Where is the Sun placed ?

In what time does he turn round on his own axis? How is that proved?

What is his diameter?

What is his mean apparent diameter as seen from the Earth?

How is his solidity calculated ?
What is the Ecliptic?

What is meant by the Nodes?
Which is the Ascending Node?
Which is the Descending Node ?

How many signs are they asunder?

What additional Astronomical discoveries were made

in the year 1805 ?

What planet is nearest the Sun ?

What reasons are given to suppose that this planet re

ceives its light from the Sun ?

What is the computed distance of Mercury from the Sun? What is its diameter?

In what time does it perform a revolution around the Sun?

What time on its own axis?

How many miles does it move in an hour in its motion round the Sun?

How many degrees is his orbit inclined towards the Ecliptic?

What planet is next to Mercury ?

What is the distance of this planet from the Sun ? How many miles in an hour, does Venus move in performing her revolution round the Sun ?

In what time does she perform her annual revolution? In what time does she performa revolution on her axis ? What is her diameter?

How is it known that the orbits of Mercury and Venus are included within that of the Earth?

How many degrees at most does Mercury depart from the Sun ?

How many Venus ?

What is meant by inferior conjunction ?

What is a transit?

What is the name of the third planet from the Sun ? What is its distance from the Sun?

What is its diameter?

In what time does it perform a revolution around the Sun?

« PreviousContinue »