LECTURES ON ASTRONOMY.
BY PROF. ORSON PRATT.
LECTURE NINTH.
MARS.
The next planet in the order of distance after Venus is the earth; but as this planet has already received a lengthy description, we will pass on to the next in order, namely, Mars. The planet Mars revolves in an orbit 145,000,000 of miles from the sun; consequently its orbit is 50,000,000 of miles exterior to the orbit of the earth. Mars is 4100 in diameter. Its apparent diameter varies according to the position which it occupies in its orbit. When in conjunction with the sun, or in a line drawn from the earth through the sun, and extended to the orbit of Mars, it is 190,000,000 of miles farther from the earth than when in opposition, or in that part of its orbit, situated in a line drawn from the sun through the earth. The surface of Mars in opposition appears about 25 times larger, than when in conjunction.
The orbit of Mars is 901,000,000 of miles in circumference; through this distance it moves in about 687 days. Its average rate of velocity is about 54,650 miles every hour; this is about 13,500 miles slower every hour than what the earth moves. – Mars rotates upon an axis from west to east in 24 hours and 37 minutes; its axis being inclined from the perpendicular to the plane of its orbit 30 deg., 18 min.; this is nearly 7 deg. greater inclination than the earth’s axis has; consequently its seasons will be somewhat more rigorous or intense than ours; each of the seasons will also be nearly double the length of ours. The inclination of the orbit of Mars to the ecliptic is 1 deg. 51 min. 6.2 sec.; hence it will never be seen to exceed four times the apparent diameter of the sun from the ecliptic.
The synodical period of Mars, or the time which it occupies in going from opposition round to the same point again, is about 2 years and 50 days. – About 36 days before Mars attains to its opposition, it will begin to retrograde and continue apparently to move contrary to the order of the signs for about 36 days before Mars attains to its opposition, it will begin to retrograde and continue apparently to move contrary to the order of the signs for about 36 days, after the opposition; the arc of retrogradation is equal to about 16 deg. 12 min. All the superior planets, or those bodies which are more distant from the sun than the earth, when at and near their oppositions, have apparently retrograde movements. The greater the distance of the body from the earth, the less will be the arc of retrogradation, and the longer the period of its apparent description, and the more frequently will such retrogradations happen.
The eccentricity of the orbit of Mars is 13,463,000 miles; consequently it is nearly 27,000,000 miles nearer the sun at its perihelion than at its aphelion. From the telescopic appearances of Mars, it is probable that its surface consists of land and water; it is also very evident that it is surrounded by a very dense and extensive atmosphere, in which numerous clouds float, as in the atmosphere of the earth. It is further evident that snows are congealed in the atmosphere of Mars and precipitated upon its surface in the polar regions, which is indicated by the brightness of those regions, after being exposed to their long winter of six months: the brightness of these sposts is gradually diminished by a long exposure to the summer rays of the sun. The quantity of light on that planet, received from the sun, is not quite one-half as much as we enjoy.
The mass or the quantity of matter, contained in Mars is 2,680,337 times less than the quantity contained in the sun. The density of Mars is about 19-20 of the density of the earth; that is, about 4 3-4 times as dense as water. It would take about 7 globes like Mars to weigh as much as the earth; 1 pound of matter on the earth’s surface will weigh about 1-2 pound on the surface of Mars.
The Asteroids.
The Asteroids are small planetary bodies revolving around the sun, between the orbits of Mars and Jupiter. None of these bodies were discovered until the present century. The great distance between the orbits of Mars and Jupiter, compared with the distances between the other planetary orbits, gave rise to the idea that there should be a planet situated in this great interval. Prof. Bode had detected a law relating to the distances of planetary orbits from each other. As you recede from the sun, each planetary orbit is found to be nearly one-half the distance from the orbit of Mercury which the next succeeding one has. Thus, the interval between the orbits of Venus and Mercury is one-half the interval between those of the Earth and Mercury; and the interval between the orbits of the Earth and Mercury is about one-half the interval between those of Mars and Mercury; the interval between the orbits of Jupiter and Mercury is about one-half the interval between those of Saturn and Mercury; and so on. But when this law was applied to the interval between Mars and Jupiter, it was found to fail. In order that the law might hold good, it was calculated that a planet ought to be situated from Mars about one-third the distance between that planet and Jupiter.
Astronomers were so thoroughly convicted of the existence of such a body, that they actually called a convention in the year 1800, and resolved to carefully search for the suspected new world. A zone of the heavens extending several degrees on each side of the ecliptic, was divided into 24 equal parts, and parceled out to as many observers, each of whom was required to thoroughly examine the portion assigned to him. This plan was successful. Piazzi, on the evening of the first day of the year 1801 discovered the planet Ceres. After ascertaining its distance from the sun, it was found to occupy the position between the orbits of Mars and Jupiter, require 1 by Bode’s law of planetary distances. This discovery was hailed with joy by the whole astronomical world; the great chain in the solar system was filled; the discrepancy in Bode’s law vanished, and the harmony in the whole planetary system seemed to be completed.
But scarcely had astronomers time to congratulate one another in relation to this great discovery, when they were startled at the announcement of Dr. Olbers, of Bremen, who had, on the evening of the 28th of March, 1802, discovered another planet having its mean distance and periodic time almost identical with that of Ceres. Here was an anomaly presented in the solar system—two planets having about the same distances and periods, and whose elliptic orbits actually intersected each other, that is, each planet in different points of its path was alternately nearer and then further from the sun than the other. This new planet was called Pallas.
Dr. Olbers conjectured that these two minute bodies might be fragments of some greater planet, which, by some unknown cause, had been burst asunder or broken in pieces. If such a catastrophe ever happened, it must have taken place at one of the points of the intersections of their orbits. — The force necessary to burst a planet, and project the fragments in different directions so as to pursue elliptic orbits of various degrees of eccentricity, can be calculated. The larger fragments of such a planet would deviate from the original path, less than the smaller ones; however great the inclinations of their orbits, or however eccentric the ellipses, yet they would all intersect each other at the point where the explosion happened.
Under the influence of this bold hypothesis, astronomers pointed their telescopes to the opposite constellations, Cetus and Virgo, where the nodes of the two orbits lie, as the most likely place to discover other fragments, and on the 2d of September, 1804, Mr. Harding, of Lilienthal, discovered the planet Juno near one of the points of the intersection of the other two orbits.
Dr. Olbers, finding his theory assuming the air of reality, continued his researches with still greater zeal; and on the 29th of March, 1807, he discovered the fourth of these supposed fragments in the constellation Virgo, not far from the point of the intersection of the other three orbits. This planet was named Vesta. The researches continued for nearly 40 years, and no other fragments were discovered; and it began to be supposed that all the small bodies revolving in this region were detected.
But on the 8th of December, 1845, Professor Hencke, of Dreisen, discovered another asteroid, which was called Astrea; and on July 1st, 1847, the same observer detected another, which was called Hebe. This great success in discovering new worlds in the vicinity of our own system, excited other astronomers to commence a diligent research for these supposed fragmentary bodies. Mr. Hind of London, on the 13th of August, 1847, discovered a seventh, which was called Iris; and on the 18th of October the same year, he discovered the eighth, which is called Flora. And on the 25th of April, 1848, Mr. Graham, of Ireland, discovered the ninth asteroid, which is called Metis. Thus, within the short period of less than 2 1-2 years, five new asteroids were detected and added to the group of worlds revolving between the orbits of Mars and Jupiter.
All of these bodies are extremely small. Vesta, which is probably one of the largest, is believed to be only 250 miles in diameter. Juno’s diameter is stated to be only about 79 miles, and Ceres, 163 miles. It is extremely difficult to measure these minute worlds at so great a distance; and consequently these approximations may be far from the truth.
Juno is supposed to have a rotation upon an axis in about 27 hours; but this is uncertain; the rotation of the other asteroids has not as yet been detected. The distances from the sun and the periodic times of these nine bodies, have already been calculated. The mean distances at which they revolve around the sun is nearly the same; and they perform their revolutions in nearly the same periods.
The orbits of the older planets are inclined at a very small angle to the ecliptic, but the orbits of several of the new planets are inclined at a considerable angle to the ecliptic; that of Pallas being the greatest, amounting to 34 deg. 37 min. 33.1 sec – The eccentricity of some of these orbits is much greater than that of the old planets. Juno, Pallas, Iris, Hebe, and Astrea, have the greatest eccentricities, amounting to nearly one quarter of their mean distance.
The hypothesis which considers these bodies as the fragments of a planet which has been bursted, is sustained by a considerable amount of evidence, arising from the anomalies and apparent irregularities, observed in this system of bodies. The inclination of the orbits—their eccentricities—the position of the nodes and aphelia—and many other peculiarities,–seem to indicate that these bodies have diverged from one common node, and therefore that they were originally one single planet. When, however, we shall have arrived to a knowledge of the great laws, that operated in the construction of the solar system, we shall then, perhaps, see that the apparent anomalies and irregularities of the asteroidal system are among the possible results of the workings of the grand mechanical laws of the universe, ordained by the great Architect of nature to display in endless variety his wisdom, power and goodness.
Jupiter.
The next planet beyond Pallas is Jupiter; this is the largest planet in the system. Its distance from the sun is 495,000,000 of miles, and the circumference of its orbit is 3,110,000,000 of miles. It completes one revolution in 4332 1-2 days; its average velocity is nearly 30,000 miles every hour. A faint idea of the great distance around the circumference of this planet’s orbit, may be acquired by supposing a rail car to travel without intermission at the rate of 500 miles per day; with such a velocity it would require over 16430 years to perform the grand journey. When Jupiter is nearest to the earth, at the time of its opposition to the sun, its distance is 400,000,000 of miles from us. A steam carriage, moving at the rate of 20 miles per hour, would require about 2300 years to pass over that distance. Even light itself, though it darts 192,000 miles every second, would require 34 minutes and 43 seconds to come from the nearest point of Jupiter’s orbit to us. When Jupiter is in conjunction with the sun, it is 590,000,000 of miles from us; light will pass over this distance in 51 minutes and 13 seconds. If the light of Jupiter were to be extinguished at the moment of its conjunction with the sun, we should not be aware of the fact until 51m. 13s. after the conjunction; whereas, if its light were to be extinguished at the moment of its opposition, we should be aware of it in 34m. 43s. after; or in other words, let two planets be situated in Jupiter’s orbit, one in conjunction and the other in opposition with the sun, and let the light of both planets be extinguished at the same instant, we should continue to see the planet in conjunction 16m. and 30s. after the one in opposition had disappeared. This can be demonstrated by calculating the exact moment of the eclipses of Jupiter’s moons, when in conjunction and opposition; and it will be found invariably that the eclipses will happen 16 1-2 minutes later when in conjunction than when in opposition. And as Jupiter is 190,000,000 of miles further off when in conjunction than in opposition, it follows that it must take light 16 1-2 minutes to traverse that distance. It was in this manner that the velocity of light was first discovered.
The diameter of Jupiter is 87,000 miles, and its circumference is over 273,000 miles. It has been found to revolve around its axis in the short space of 9 hours and 56 minutes. This is determined by telescopic observations of certain permanent spots upon its disc which are seen to be carried across the same from east to west, remaining visible 4 hours and 58 minutes, and then disappearing for the same length of time. The equatorial regions of Jupiter must move with a velocity of over 27,000 miles every hour; this is very nearly equal to its velocity in its orbit. As that hemisphere of Jupiter which is turned from the sun rotates in the same direction as the orbitual motion, the velocity from west to east, at the time of their midnight, will be greatly accelerated, amounting to 950 miles a minute. – And as that hemisphere which is turned towards the sun rotates in a direction from east to west contrary to the orbitual motion, the velocity from west to east, arising from the orbit motion, will, at the time of their noon, be greatly diminished, amounting to no more than 50 miles a minute. The inhabitants of Jupiter, therefore, will be carried from west to east, 900 miles a minute swifter at their midnight than at their noon. From noon till midnight, (which is a period of only about 5 hours) the velocity will increase at an average rate of about 3 miles a minute, or 16 rods per second. The decrease from midnight to noon will be in the same proportion.
One year on Jupiter is nearly 12 of our years; during this time Jupiter makes 10,470 revolutions upon its axis; consequently there are 10,470 days of about 10 hours long in one of Jupiter’s years. – The rapid rotation of this planet will have the effect to make all bodies for many degrees each side of its equator, lighter than what they would be if there were no rotation. Gravity at the surface of this planet is more than 3 times as great as at the surface of the earth; this is owing to the bulk and quantity of matter in Jupiter; its quantity of matter is 371 times as much as is contained in the earth; while its bulk is 1323 times greater. A body weighing one pound on the earth would, if transported to Jupiter, weigh 3 pounds and 1 ounce–. The centrifugal force diminishes the weight of bodies about one-thirteenth, that is, a body which would weigh 13 pounds, if the planet had no rotation, would weigh only 12 pounds with the rotation- While a clock pendulum would make 4 vibrations on the earth, it would in the same time make 7 vibrations on the surface of Jupiter. A body would fall through space of 49 feet 3 inches in one second of time on the surface of Jupiter, if it had no rotation; this fall will be diminished at its equator 3 feet 8 inches by the centrifugal force of rotation. The density of Jupiter is about 2.7 of that of the earth, being 1 4-10 times heavier than a globe of water of the same size. The density of Jupiter and the Sun is about alike. It would take nearly 1048 such worlds as Jupiter to weigh as much as the sun.
The inclination of the plane of Jupiter’s orbit to that of the equator is 3d. 5 l-2 m; this inclination is so small that it will not produce any sensible variety of seasons. The torrid zone of that planet will be only 6d.11m. broad. But as the length of a degree on that planet is 755 miles, the breadth of the torrid zone will be about 4600 miles, and the diameter of its polar circles will be about the same length. At the poles there will be nearly 6 years day and 6 years night, while the days and nights in the torrid and the most part of the temperate zones will not vary much from 5 hours each.
The inclination of the orbit to the ecliptic is 1d. 18m. 51.3s. The eccentricity of the orbit is 23,000810,000 of miles; consequently it is nearer the sun by almost 48,000,000 of miles when at its perihelion, than when at its aphelion. Its apparent diameter when in opposition is 47 1-2s. Its mean are of retragradation is 9d. 54m., and its mean duration about 121 days. The equatorial diameter of Jupiter is 6300 miles longer than the polar diameter; this is occasioned by its rapid rotation upon its axis which would have a tendency to draw away the matter from the polar regions and form a protuberance in the equatorial. Water, in running from its poles towards its equator, would ascend on an average, over one mile in perpendicular heights, for every eleven miles progression. Should the planet cease to rotate, its equatorial oceans would rush to the north and south, forming two great polar oceans several thousand miles in depth.
The apparent diameter of the sun as seen from Jupiter is only 6m. 9s., while at the earth his apparent diameter subtends an angle of 32m. 3s., which is over 5 times greater; the sun’s disc, therefore, will appear at Jupiter about 27 1-6 times less than what he appears to us; consequently the intensity of solar light and heat on the surface of Jupiter will be 27 1-6 times less than on the earth.
A large and powerful telescope will expand the disc of Jupiter to about the size of the full moon; and it is as clearly and distinctly seen as the full moon to the naked eye. His disc is distinctly marked with belts of light and darkness, extending from west to east around the whole circumference of the planet; the darker belts are portions of the surface of the planet; the brighter ones are believed to be clouds, floating in its atmosphere. The brighter belts are subject to many changes, revealing sometimes more and at other times less of the dark surface beneath. These belts, being parallel to his equator, are no doubt produced by the great atmospheric currents from east to west, occasioned by the rapid rotation of that planet from west to east, combined with the northerly and southerly currents to and from the poles. These currents near the surface of the tropical regions of Jupiter will be much more deflected to the east than the trade winds of our globe, because of the great velocity of the rotation; while, for the same cause, the upper currents towards the poles will be much more deflected to the west than the similar ones of our globe. Therefore, the clouds would have a tendency to arrange themselves in zones or belts parallel to its equator, as they are actually seen by the telescope. The narrowest of these belts, that can be distinctly seen, will be about one thousand miles in width. Some of the broadest occupy, at least, one-eigth part of the breadth of the disc, and consequently must be 11,000 miles broad.
Jupiter is attended by four satellites or moons. – The distance from the surface of the planet to the first is nearly 220,000 miles, and its magnitude is about one-sixth greater than our own; therefore its apparent magnitude will be greater than that of the full moon. The distance from the planet’s surface to the second satellite is 375,000 miles, and its real magnitude is nearly equal to our moon, therefore its apparent disc will be nearly 3 times less than that of the full moon. The distance of the third satellite is 624,000 miles; its real magnitude is somewhat over one-half greater than that of our moon; its apparent magnitude, therefore, will be about one third of that of the full moon. The distance of the fourth satellite from the surface of its primary, is about 1,131,000 miles; its real diameter is about one third greater than the moon; therefore the apparent magnitude of its disc will be about 13 times less than the full moon.
The nearest moon to Jupiter revolves around him in 1 day, 18 hours, 27 min. 33.506 s. The second satellite performs its revolution around the primary in 3d. 13h. 14m. 36.393s. The third, in 7d. 3h. 42m. 33.362s; and the fourth in 16d. 16h. 31m. 49.702s. Each moon, during its period, exhibits all the phases seen in our moon; so that the inhabitants of the primary see each moon, during its period, of the shape of a thin crescent, afterwards halved, then gibbous and full. The periods of these satellites are such that all of them can never beon the same side of Jupiter at the same time; one at least, must be on the opposite side from the other three; hence, there will always be one moon near its full when the other three happen to be near their change, or in conjunction. But sometimes there will be two, and sometimes three moons near their full. All of these appendages will serve to render the nocturnal scenery of the heavens as seen from Jupiter, grand and delightful.
All bodies on the surfaces of Jupiter’s satellites will weigh much less than what they would weigh at the surface of our earth; for instance, 1 pound of terrestrial matter, if transported to the surfaces of those satellites, would on the first, or on the one nearest to the primary, weigh but 1 oz.11.36 dr. On the second satellite it would weigh 2 oz. 0.27dr. – On the third it would weigh 2 oz. 14.10 dr. On the fourth, it would weigh 1 oz 14.34 dr.
If the density of the earth be equal to 1, the density of the first satellite will be equal to .20332 -The density of the second will be .48629. That of the third .42534; and that of the fourth 32713.
If the volume of the earth be taken as 1, the volume of the first satellite will be equal to .0316835. The volume of the second, .01776216. That of the third, .0773472; and that of the fourth, .0484780.
If the mass of the earth be taken as 1, the mass of the first satellite will be equal to .006441774.- The mass of the second .008637727. That of the third, .032899202; and that of the fourth, .015858697.
These satellites rotate upon their axes from west to east, precisely in the same time that they revolve around Jupiter; and consequently, like our moon, they always turn the same hemisphere towards their primary.
[page break]
Newspaper clip
[Transcribed by Nora Fowers, Dorrie Lee, and Mauri Pratt; Jan. 2014]