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Saturn. ħ Till within 70 years Saturn was considered the most remote planet in our system. It shines with a pale, dead light. Its diameter is about 79,000 miles ; so that, in point of size, it is the second in the system. It exceeds the earth in bulk nearly 1,000 times. It turns on its axis in 10 hours 29 minutes. Its distance from the sun is 900 millions of miles; and it performs its journey round that luminary in a little less than 30 years, and consequently travels at the rate of 22,000 miles per hour.

Being between 9 and 10 times farther from the sun than the earth, it enjoys 90 times less light and heat; but the daylight there is not so small as we should suppose, for it has been calculated to be many hundred times greater than the light which we enjoy from our full moon.

The Great Creator of the universe seems to have indemnified the inhabitants of Saturn for their great distance from the sun, by giving them 7 moons, and also by surrounding the planet with two broad rings, which are probably of considerable importance in reflecting the light of the sun to the planet. These rings present a singular appearance when viewed through a telescope. The density of Saturn is about that of light wood. A cannon ball would take 85 years in passing from the sun to Saturn.

The breadth of the exterior ring of Saturn is 10,500 miles, and that of the interior 17,000. The interior ring is 19,000 m. distant from the body of the planet. The thickness of the rings is supposed to be about 100 miles. The rings rotate in a plane of their own in nearly the same time that the planet performs a revolution on its axis. The rings are clearly opaque bodies, for they throw a shadow on the body of the planet on the side nearest the sun, and receive the shadow of the planet on the opposite side.

Uranus. H This planet was discovered, March 13th, 1781, by Sir W. Herschel. It is the third of the planets in point of magnitude ; it has a diameter of 35,000 miles, and its volume is about 80 times that of the earth. Its distance from the sun is 1,800 millions of miles. It requires 84 years to perform its journey round that luminary, though it travels at the rate of nearly 16,000 miles per hour. The light and heat of the sun, at Uranus, is 368 times less than at the earth. Probably six satellites and certainly two attend this planet. A cannon ball would require 171 years in passing from it to the sun.

Before the discovery of this planet, astronomers conceived that a planet existed beyond the orbit of Saturn, for some inequalities in the motion of Jupiter and Saturn could not otherwise be accounted for. Herschel called this planet Georgium Sidus, in honour of George III., but foreign astronomers gave it the name of its great discoverer. Latterly the name Uranus -the most ancient of the heathen deities, and the father of Saturn-has been adopted, as being more in unison with the appellations of the other planets.

In two respects the satellites of Uranus offer remarkable peculiarities; they move in orbits nearly at right angles to the plane of the orbit of their primary, and in a direction from east to west.

The more easily to remember the relative distances of the planets, the following numbers, which are proportional to their mean distances from the sun, will be useful: Merc., Venus, Earth, Mars, Asteroids, Jupiter, Sat., Ura. 4 7 10 16 28 52 100 196

The mean distance of the earth being 95,000,000 miles, that of any other planet may be obtained by proportion.

The following illustration will convey to minds ur accustomed to contemplate millions of miles, a general impression of the relative magnitude and distance of the parts of our system.

Choose any well levelled field. On it place a globe, two feet in diameter; this will represent the Sun; Mercury will be represented by a grain of mustard seed on the circumference of a circle, 164 feet diameter for its orbit; Venus a pea, on a circle 310 feet in diameter ; the Earth also a pea, on a circle of 430 feet; Mars a rather large pin's head, on a circle of 654 feet; Juno, Ceres, Vesta, and Pallas, grains of sand, in orbits of from 1,000 to 1,200 feet; Jupiter a moderate sized

orange, in a circle nearly half a mile across ; Saturn a small orange, on a circle of four-fifths of a mile; and Herschel a full sized cherry, or small plum, upon the circumference of a circle more than a mile and a half in diameter.

These views ought to humble man. How insignificant is this earth, the theatre of so many passions, and so much contention! How much blood is sometimes shed for the possession of a mere point !

COMETS.

Comets appear in very various aspects. The head consists of a nebulous mass of light containing a bright spot in its centre, called the nucleus. The more diffuse light surrounding the nucleus is called the coma, or hair, from which the word comet is derived. The tail consists of a stream of light proceeding from the head, generally directed towards the side most remote from the sun. It is often slightly curved, bending towards the region which the comet has left, and is usually most fully developed just after the comet has passed the perihelion. A tail is by no means an invariable appendage of a comet.

Comets are not like planets confined to the zodiacal belt, they move in all parts of the heavens, and they proceed in all directions, some pursuing a retrograde and others a direct course.

Comets revolve in extremely eccentric orbits, so that at one time they are very near the sun and at another very remote from it.

Those comets which have elliptic orbits, make regular revolutions round the sun in fixed periods, but there are some which seem to move in a curve that does not return into itself. These comets having come within the reach of the sun's attraction, move round him, again launch forth into boundless space, again to perform a temporary revolution round the sun of some other system.

Comets appear to consist of matter entirely gaseous.

The proof of this is pretty decisive. They have been found to make no sensible derangement (by attraction) in the motions of Jupiter's satellites, near which they have passed, while they themselves have been considerably diverted from their course. Stars of the 16th magni

tude have been seen through the nucleus of some of them. Also they present no phases, which shows that light passes freely through them.

The most remarkable comets that have appeared in modern times are those of 1680 and 1811.

The comet of 1680 was seen by the illustrious Newton. He calculated that its tail was 123 millions of miles long, and that when nearest to the sun it was exposed to a heat 2,000 times greater than that of red hot iron. This comet is supposed to have been the same as that which appeared about the time of Cæsar's death (B.C. 44).

The comet of 1811 continued visible to the naked eye for more than three months. Its brilliant tail, at its greatest elongation, had an extent of 108 millions of miles by a breadth of 15 millions.

The precise nature of the orbits and the period of time occupied in traversing them, have been ascertained in the case of three comets, which have been named after the astronomers who investigated their courses and predicted their return-Halley's, Biela's, and Encke's.

Halley's comet appeared in the year 1682, and it has twice visited this part of the system since Halley's time, namely, in 1759 and 1835. It has a period of 75 or 76 years.

Biela's comet describes its orbit in 6 yrs. Encke's comet has a period of 3 yrs. Both Encke's and Biela's comets are destitute of tail and nucleus.

A very interesting fact has been noticed in the case of Encke's comet. The time in which it completes a revolution round the sun is undergoing a progressive diminution, owing to the diminution of the size of its orbit. It is hence inferred that the comet meets in its passage through the system with a resisting medium, and that it will eventually be precipitated upon the sun's surface.

PROBLEM XXI. To mark the Places of the Planets on the Globe, from

having their longitude and latitude. 1. Look on the right-hand page of White's Ephemeris for the day of the month.

2. Find out the column marked at the top with the character of the planet whose place you are seeking ; then, in that column, opposite to the day of the month, is the longitude of the planet for that day at noon.

Mars,

3. The latitude is given, at the top of the page, for 5 days in every month, and seldom exceeds 2 or 3 degrees.

4. Find the longitude and latitude upon the globe and put on a small patch with the character of the planet ;and thus may all the planets be marked upon the globe for any day of the year.

Page 32 of White's Ephemeris is appropriated to the planet Uranus : its variations in longitude and latitude are so small, that they are given for only the first day in each month.

EXAMPLES. 1. What is the situation of the inferior planets for May 13, 1828 ?

Mercury, 8 10° 54' 1° 17' S. Venus,

5 7 533 3 N. 2. What is the situation of the superior planets on the same day?

1 . 134 1 10 45'S. Jupiter,

m

7 491 92 N. Saturn,

06 16 21 0 2 S. Uranus,

2 18 0 34 S. 3. Required the situation of all the planets for the first day of every month, during the present year.

PROBLEM XXII. To find the Right Ascension and Declination of the Planets,

_their Rising, Culminating, Setting, Amplitude, Azimuth, Altitude, &c. for a given day and place.

The situation of the planets being marked upon the globe for the given day, their right ascension, declination, &c., may be found the same way as for the fixed stars.

Examples.-1. Required the right ascension and declination of all the planets on November 13th, 1828.

RIGHT ASCENSION. DECLINATION.
Mercury,
228° 0'

180s.
Venus,
188 0

1 39 s.

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