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MARS, VESTA, JUNO, AND PALLAS.

111

LESSON 50.

Mars, Vesta, Juno, Pallas, and Ceres.

Eccentric, deviating from the centre.

Eccentricity, the distance between the centre of an ellipse and the focus.

MARS, the first of the exterior planets, is distinguished from the rest by the redness of its colour, which has been attributed to the density of his atmosphere. He revolves round the sun in about two years, at the mean distance of one hundred and forty-four millions of miles, and turns on his axis in a little less than twenty-five hours. The time of his diurnal rotation was discovered by means of a large spot seen on his surface, when in that part of his orbit which is opposite to the sun and the earth. The telescopic appearance of Mars is exceedingly variable; but the predominant brightness of his polar regions, leads to the supposition, that, like those of the earth, they are covered with perpetual snow. The proportion of light and heat, received at Mars from the sun, is less than one half of that enjoyed by the earth.

The planet next to Mars in the solar system is Vesta. It shines with a pure and white light, and is visible in a clear evening without the aid of a telescope. It revolves round the sun, in about three years and eight months, at the mean distance of two hundred and twenty-three millions of miles. Vesta was first discovered by Dr. Olbers, of Bremen, in Lower Saxony, March 29, 1807.

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Juno was discovered by Mr. Harding, near Bremen, September 1, 1804. It completes its revolution in about four years and four months, at a mean distance from the sun of about two hundred and fifty-three millions of miles. It is distinguished from all the other planets by the great eccentricity of its orbit; and the effect of this is such that it passes over one half of its orbit in half the time that it employs in describing the other half. From the same cause its greatest distance from the sun is double the least distance, the difference between the two distances being about one hundred and twenty-seven millions of miles.

Pallas was discovered by Dr. Olbers, March 28, 1802. It completes its revolution in about four years and seven months, and its orbit is nearly as eccentric as that of Juno. Its mean

110

ECLIPSES OF THE SUN.

moon be in one of her nodes, she will, in most cases, cover the whole disk of the sun and produce a total eclipse; if she be any where within about sixteen degrees of a node, a partial eclipse will be produced. When a bright luminous ring appears round the dark body of the moon during an eclipse of the sun, it is called an annular eclipse. This kind of eclipse is occasioned by the moon being at her greatest distance from the earth at the time of an eclipse; in which situation, the vertex or point of the cone of the moon's shadow does not reach the surface of the earth. A total eclipse of the sun is a very curious and uncommon spectacle; and total darkness cannot last more than three or four minutes. Of one that was observed in Portugal more than one hundred and fifty years ago, it is said that the darkness was greater than that of night;—that some of the largest stars made their appearance;-and that birds were so terrified that they fell to the ground. A very remarkable total eclipse took place in New England June 16, 1806. The day was clear; several stars were visible; the birds were greatly agitated; and a gloom spread over the landscape. The first gleam of light, contrasted with the previous darkness, seemed like the usual meridian day.

QUESTIONS.-1. What is an eclipse of the moon? 2. Describe the earth's shadow. 3. When does an eclipse of the moon happen? 4. Why is she not eclipsed at every full moon? 5. How near a node

sun?

must she be in order to be eclipsed? 6. How long may an eclipse of the moon last? 7. From the knowledge of what circumstances are lunar eclipses calculated? 8. Over what part of the earth are they visible? 9. What is the cause of an eclipse of the sun? 10. When does an eclipse of the sun happen? 11. Why can it not happen at other times? 12. When will the moon produce a total eclipse of the 13. Partial? 14. When is an eclipse of the sun called annular?-why? 15. What occasions this kind of eclipses? 16. How tong may a total eclipse of the sun last? [NOTE. The diameters of the sun and moon are supposed to be divided into 12 equal parts, called digits. They are said to have as many digits eclipsed as 12th parts involved in darkness.] 17. Look at fig. 45. and illustrate an eclipse of the moon. 18. At fig. 44. and illustrate an eclipse of the sun.

MARS, VESTA, JUNO, AND PALLAS.

111

LESSON 50.

Mars, Vesta, Juno, Pallas, and Ceres.

Eccentric, deviating from the centre.

Eccentric'ity, the distance between the centre of an ellipse and

the focus.

MARS, the first of the exterior planets, is distinguished from the rest by the redness of its colour, which has been attributed to the density of his atmosphere. He revolves round the sun in about two years, at the mean distance of one hundred and forty-four millions of miles, and turns on his axis in a little less than twenty-five hours. The time of his diurnal rotation was discovered by means of a large spot seen on his surface, when in that part of his orbit which is opposite to the sun and the earth. The telescopic appearance of Mars is exceedingly variable; but the predominant brightness of his polar regions, leads to the supposition, that, like those of the earth, they are covered with perpetual snow. The proportion of light and heat, received at Mars from the sun, is less than one half of that enjoyed by the earth.

The planet next to Mars in the solar system is Vesta. It shines with a pure and white light, and is visible in a clear evening without the aid of a telescope. It revolves round

the sun, in about three years and eight months, at the mean distance of two hundred and twenty-three millions of miles. Vesta was first discovered by Dr. Olbers, of Bremen, in Lower Saxony, March 29, 1807.

Juno was discovered by Mr. Harding, near Bremen, September 1, 1804. It completes its revolution in about four years and four months, at a mean distance from the sun of about two hundred and fifty-three millions of miles. It is distinguished from all the other planets by the great eccentricity of its orbit; and the effect of this is such that it passes over one half of its orbit in half the time that it employs in describing the other half. From the same cause its greatest distance from the sun is double the least distance, the difference between the two distances being about one hundred and twenty-seven millions of miles.

Pallas was discovered by Dr. Olbers, March 28, 1802. It completes its revolution in about four years and seven months, and its orbit is nearly as eccentric as that of Juno. Its mean

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distance from the sun is two hundred and sixty-three millions of miles. Its atmosphere seems to be dense and cloudy.

The planet Ceres was discovered by Piazzi, at Palermo, in Sicily, January 1, 1801. It is apparently surrounded by a dense atmosphere, and is of a ruddy appearance. Its mean distance from the sun, and its revolution in its orbit is nearly the same as that of Pallas. These newly discovered planets exhibit various changes in appearance and size; so that their real magnitude has not been ascertained with certainty.

From some irregularities, observed in the motions of the old planets, some astronomers had been led to suppose, long before the discovery of the four new planets, that a planet existed between the orbits of Mars and Jupiter. Dr. Olbers, before he made his last discovery, conceived that these small celestial bodies were merely the fragments of a larger planet, which had been burst asunder by some internal convulsion, and that several more might be discovered. With the intention, therefore, of detecting other fragments of the supposed planet, he examined, thrice every year, the little stars in certain constellations, till his labours were crowned with success by the discovery of the new planet Vesta. The opinion, that these four small planets have been separated from one original planet, by some convulsion in nature, has been maintained by Dr. Brewster with much ingenuity and plausibility. He supposes, moreover, that the phenomena of the meteoric stones, which have fallen on the earth from the atmosphere, may have been occasioned by the bursting of this planet.

QUESTIONS. 1. By what is Mars distinguished from the rest of the planets? 2. In what time does Mars revolve round the sun? 3. At what mean distance? 4. What is the time of his diurnal rotation, and how was it discovered? 5. What is the telescopic appearance of Mars? 6. Proportion of light and heat? 7. What is the appearance of Vesta? 8. When, where, and by whom were each of the new planets discovered? 9. What is the distance of each from the sun? 10. By what is Juno distinguished from all the other planets? 11. What supposition did some astronomers make before the discovery of the new planets? 12. What was the conjecture of Dr. Olbers, and to what did it lead? 13. To what does Dr. Brewster think the phenomena of meteoria stones may be attributed?

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JUPITER is the largest of all the planets. His diameter is eighty-nine thousand miles. He revolves round the sun at the mean distance of four hundred and ninety millions of miles, completes a revolution in a little less than twelve years, and turns on his axis in the short interval of nine hours and fifty-six minutes. With the exception of Venus, Jupiter is the most brilliant of the planets, and, when viewed through a telescope, its surface is remarkable for being always covered with a number of belts or stripes of various shades. They are not regular or constant in their appearance, and their breadth is also variable, one belt growing narrower while another in its neighbourhood becomes broader, as if one had flowed into the other. Sometimes one or more spots are formed between the belts, which increase until the whole are united in one large dusky band. Bright spots also may be discovered on Jupiter's surface, which are more permanent than the belts, and re-appear after unequal intervals of time. For the cause of these appearances, we are referred by eminent philosophers, to his swift diurnal motion, to the changes in the density of his atmosphere, as occasioned by variations of temperature, and to other incidental agencies. The axis of Jupiter is perpendicular to the plane of his orbit; his inhabitants, therefore, will experience no change of seasons, nor difference in the length of their days and nights. At the equator there will be perpetual summer, and at the poles unceasing winter. The degree of light and heat is about twenty-five times less than at the earth.

The satellites of Jupiter are invisible to the naked eye, but through a telescope they make a beautiful appearance. As our moon turns round the earth, enlightening the nights by reflecting the rays of the sun, so these also enlighten the nights of Jupiter, and move round him in different periods of time, proportioned to their several distances. They often pass behind the body of the planet, and also into its shadow, and are eclipsed. These eclipses are of use for ascertaining the longitude of places. They have led to the discovery, that light is about eight minutes in coming from the sun to

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