Saturn: The remotest of the planets known to the ancients. Mean distance, 886,000,000 miles, with a variation on either side, owing to the eccentricity of the orbit, of 50,000,000 miles. Period, 291⁄2 years; synodic period, 378 days; mean diameter, 73,000 miles; surface, 82 times, and volume, 760 times that of the Earth; mass only 95 times the Earth's; while its density is only one-eighth that of the Earth, or five-sevenths that of water, by far the least dense of all the planets. Axial rotation is about 10 h. 14 m. Surface markings, belts less distinct, and less variable than those of Jupiter.

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The wonderful ring-system, surrounded by three thin, flat concentric rings, generally referred to as A, B and C, — A being the exterior one. The appearance of Saturn, a great puzzle for nearly fifty years after the time of Galileo; in 1610, he announced that "the outermost planet is triple," but not long afterwards, when the rings were turned edgewise to the Earth and were invisible to him, he asked "whether Saturn has devoured his children, according to the legend." Huyghens, in 1655, was the first to explain what the the rings were; and Cassini, twenty years later, discovered the ring was double, two bright portions with a dark division between them. The crape" ring, C, discovered by Bond in 1850. Dimensions of rings Outer ring, A, exterior diameter, 168,000 miles, and about 10,000 miles wide. The division between A and B is 1,600 miles wide. Ring B is 16,500 miles in width, and it is much brighter than A, especially at the outer edge. "Gauze," or Crape," ring C, semi-transparent, about as wide as outer ring A, about 10,000 miles between planet's equator and inner edge of gauze ring. Thickness of ring not more than 100 miles, proved by disappearance of rings when turned edgewise to the Earth; next disappearance in summer of 1907; phases of the rings. Structure of the rings: Clerk Maxwell proved they cannot be continuous solid or liquid sheets, but must be an aggregation of small particles; in other words, a swarm of meteors. This demonstrated by the spectroscopic observations of Keeler in 1895, a wonderful triumph for spectroscopic methods.

Eight (or possibly nine) satellites.

Uranus: The first planet to be "discovered," found by Herschel in 1781.

Mean distance, 1,800 millions of miles; period, 84 years; synodic period, 369 days; diameter, 32,000 miles; four satellites, Ariel, Umbriel, Oberon, and Titania. Peculiarity of the satellites their orbits are inclined 82°.2 to the plane of the ecliptic, and they revolve in a direction opposite to that of all the other motions in the solar system.

Neptune: Its discovery the greatest triumph of mathematical astronomy; discovered in 1846 as the result of the calculations of Leverrier and Adams; the data derived from the perturbations of Uranus.

Distance, 2,800 millions of miles (in defiance of Bode's Law); period, 164 years; diameter, 35,000 miles; volume, 90 times the Earth's; mass, 18 times; one satellite.

REFERENCES:

See Lecture IV; also, Proctor, Saturn and its System.

QUESTIONS ON LECTURE V:

1. Give a short description of the planet Jupiter. What do you think of the inhabitability of Jupiter?

2. Write an account of Saturn and his ring system.

3. Describe the spectroscopic method whereby it has been proven that Saturn's rings consist of a multitude of small solid particles.

4. Give an account of the discovery of Neptune.

LECTURE VI

METEORS AND COMETS

General Characteristics: Occasionally bodies fall on the Earth from the sky, bodies which at times weigh as much as several tons. During its flight through the air, such a body is called a meteor, and the pieces which fall from it are called meteorites, aerolites or meteor stones. If the fall occurs at night, a fire ball is seen, which moves with a speed which depends on its real velocity and the position of the observer. The fire ball is followed usually by a luminous train, which sometimes lasts for a considerable time after the meteor has disappeared. The motion is more or less regular, seldom being straight, and at times the meteor seems to throw off fragments and change its

course more or less abruptly. If the fire ball is not too far distant, these explosions are accompanied by sharp detonations.

The aerolites which fall to the Earth are nearly all stones, though a few consist of nearly pure iron, crusted over with a hard metallic oxide, and usually as glossy as if varnished.

When a meteor is observed by a number of persons at different points data may be furnished which will give its path. The meteor usually appears at an altitude from eighty to one hundred miles, and disappears at an altitude of between five and ten miles. The length of the path may be anywhere from fifty to five hundred miles, depending on its inclination to the Earth's surface. The velocity ranges from ten to forty miles a second when it appears first, to one or two miles a second when it finally disappears. The average velocity is about twenty-six miles per second; which goes to prove that these bodies, whatever their origin may have been, are now moving in space like comets under the attraction of the Sun.

The heat and light of the meteor is caused by the destruction of the body's velocity; the energy of motion is changed into heat by the friction of the air, the temperature, as Lord Kelvin has shown, being independent of the density of the air.

Shooting Stars: The number of these bodies is very great. A single watcher sees on the average four to eight hourly, and if sufficient observers were present to cover the whole sky about sixty per hour would be detected. It has been estimated that the number which daily enter the Earth's atmosphere large enough to be visible amount to over twenty-five millions. For the most part, these bodies are much like the stars in brightness, a few are even as bright as Venus and Jupiter. These bodies disappear at an elevation of about fifty miles, not being able to penetrate as deeply into the denser atmosphere as the aerolites. They are all very small. There is no absolutely certain way of getting at their masses, but it seems highly probable that the average shooting star does not weigh more than a single grain, the largest of them weighing as much as quarter of an ounce! Since the Earth is continually receiving meteoric matter, it is consequently growing larger. The increased size of the Earth would manifest itself by lengthening the day, but the whole effect would not amount to one-thousandth of a second in a million years.

Meteoric Showers: At times the shooting stars, instead of appearing here and there in the sky at intervals of several minutes, and moving in all directions, appear in thousands for a few hours. At such times they appear to radiate from a small area in the sky, their paths produced backwards appear to come from a common point, called the "radiant." Thus we have showers called the "Leonids," whose radiant is in the constellation of Leo, the Andromedes (also called the Beilids), the Perseids, the Geminids, etc. The simple explanation of this radiating feature is that it is simply an effect of perspection; the real paths of the meteors are parallel straight lines, but being projected on the sky their "vanishing point" is the radiant.

The meteor swarm pursues a regular orbit about the Sun, and hence the Earth can only encounter it when her orbit cuts the path of the meteors. As a result, the shower must occur on the same day of the year, except as, in process of time, the meteor's orbits slowly shift their position on account of perturbations. The Leonids, therefore, appear about the 13th or 14th of November, the Andromedes about the 27th of the same month, and the Perseids early in August.

History of November meteors, why they did not appear in 1899. The connection of meteors with comets.

Comets: Their appearance formerly supposed to presage war, pestilence and death of princes. About seven hundred on the list, about four hundred being recorded previous to the invention of the telescope in 1609. While telescopic comets are very numerous, as many as five to eight have been discovered in a year, — brilliant ones are comparatively rare. Since 1880 there have been ten, the great comet of 1882 being the most brilliant. Comets are known (a) by the name of their discoverer, or computer of motion; (b) by the year and order of discovery, as, 1903, a; and by the order of perihelion passage, as, 1889 V.

The determination of a comet's orbit, the hyperbola, parabola and eclipse. The orbits of 350 have so far been computed, about twenty have been seen at more than one return.

While comets are evidently subjected to the attraction of gravity, as shown by their orbits about the Sun, they also exhibit evidence of being acted upon by powerful repulsive forces emanating from the Sun. While they shine. in nart. at

course more or less abruptly. If the fire ball is not too far distant, these explosions are accompanied by sharp detonations.

The aerolites which fall to the Earth are nearly all stones, though a few consist of nearly pure iron, crusted over with a hard metallic oxide, and usually as glossy as if varnished.

When a meteor is observed by a number of persons at different points data may be furnished which will give its path. The meteor usually appears at an altitude from eighty to one hundred miles, and disappears at an altitude of between five and ten miles. The length of the path may be anywhere from fifty to five hundred miles, depending on its inclination to the Earth's surface. The velocity ranges from ten to forty miles a second when it appears first, to one or two miles a second when it finally disappears. The average velocity is about twenty-six miles per second; which goes to prove that these bodies, whatever their origin may have been, are now moving in space like comets under the attraction of the Sun.

The heat and light of the meteor is caused by the destruction of the body's velocity; the energy of motion is changed into heat by the friction of the air, the temperature, as Lord Kelvin has shown, being independent of the density of the air.

Shooting Stars: The number of these bodies is very great. A single watcher sees on the average four to eight hourly, and if sufficient observers were present to cover the whole sky about sixty per hour would be detected. It has been estimated that the number which daily enter the Earth's atmosphere large enough to be visible amount to over twenty-five millions. For the most part, these bodies are much like the stars in brightness, a few are even as bright as Venus and Jupiter. These bodies disappear at an elevation of about fifty miles, not being able to penetrate as deeply into the denser atmosphere as the aerolites. They are all very small. There is no absolutely certain way of getting at their masses, but it seems highly probable that the average shooting star does not weigh more than a single grain, the largest of them weighing as much as quarter of an ounce! Since the Earth is continually receiving meteoric matter, it is consequently growing larger. The increased size of the Earth would manifest itself by lengthening the day, but the whole effect would not amount to one-thousandth of a second in a million years.