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THE SOLAR SYSTEM.
The solar or planetary system consists of the sun, with eight planets circulating around him at varying distances, in oval figures not greatly differing from the circular form. Six of these planets have other secondary and smaller bodies circulating around their primary, forming separate systems, and all receiving their light and heat from the sun.
The surface of the sun presents a mottled appearance, with here and there in definite positions certain black spots, the nature of which is not yet certainly known. They vary in numbers, reaching a maximum about every 11 years. They are found in the region about the sun's equator. As the sun rotates on its axis in 25 days 6 hours, the spots appear to pass across the disc in 13 days, moving from east to west, disappearing at the western edge, and, supposing the spot to last long enough, reappearing again at the eastern limb in 13 or 14 days. They are of irregular shape, vary greatly in size, and consist of two distinct parts, a nucleus quite black and a penumbra less dark, the nucleus being at a lower level than the penumbra, and both lower than the bright surface.
The distance of the sun from the earth is about 93,000,000 miles. In comparing distances expressed in such high numbers it is not very easy to get a clear idea of their relation to each other. It will answer the purpose better to take this distance as the unit and express all others in terms of it.
Mercury is the nearest planet to the sun, and is the smallest in the system. Its mean distance in terms of our unit being 0.387, the next will be Venus at mean distance 0.723, then comes the earth at the unit distance.
As seen from the earth, all the heavenly bodies appear to rise in the east and set in the west, but we know that this appearance is caused by the earth turning on its axis from west to east, which makes the sun, moon and stars apparently move in the opposite direction.
Mercury and Venus, moving in orbits lying between the sun and earth, will never be seen very far from the sun, being sometimes to the east of it as evening stars, and then west as morning stars. Their apparent excursions are technically known as Elongations, east or west. Mercury's period of revolution round the sun-in other words, his year-consists of 88 solar days, and he turns on his axis in the same time; he therefore always presents the same face to the sun, like the moon with respect to the earth, and also, like the moon, exhibits similar phases to the earth, like the moon in her first quarter if the elongation be in the east, and like her last quarter if in the west, passing from new to full while moving from inferior to superior conjunction, and from full to new again while passing from superior to inferior conjunction.
A planet is said to be in conjunction with another body when it has the same longitude and is seen in the same direction in the heavens.
The nearness of Mercury to the sun makes him a difficult object to pick up except under favorable circumstances, such as when he sets an hour and a half after the sun and rises an hour and a half before the sun. He may on these occasions be identified by his strongly sparkling light and his movement with regard to the stars.
Venus, nearly twice as far from the sun as Mercury, is almost exactly the same size as the earth, and presents phenomena similar to those of Mercury. Her year consists of 225 days. The disc of Venus is not telescopically interesting, as owing to her dense, cloud-laden atmosphere, but very few markings are visible, rendering the determination of her axial notation uncertain. By many astronomers her day is supposed to be very nearly equal to that of the earth. At the time of her greatest brilliancy Venus appears as a beautiful crescent, like the moon when about 3 days old.
If we could observe the planetary system from the sun the movements of the planets would appear extremely simple, but as we must observe them from the earth itself a planet and, therefore, a moving observatory-the motions appear most complex. sometimes direct and sometimes retrograde; though they are all moving in one direction round the sun. As far as is known at present neither Mercury nor Venus has any satellites.
Our own planet has one moon, the mean distance of which from the earth is about 239,000 miles-near enough for us to get a more complete knowledge of its surface conditions than of any other member of the solar system. When viewed through a telescope charts of very great accuracy can be made of her surface, having no appreciable atmosphere to obscure the more delicate markings. With a powerful telescope the moon may be brought apparently within about 500 miles, so that any changes which occur over large tracts of country could be easily distinguished; cities perhaps from cultivated fields. The moon's surface shows signs
16 Th. 17 Fr. 18 Sa.
4 48 12 11 277 35 11 Sa. 4 4912 11 367 34 12Sun 4 5012 11 447 34 13 Mo. 4 5012 11 527 34 14Tu. 4 5112 11 597 33 15W. 4 52 12 12 67 32 4 53 12 12 127 32 4 53 12 12 187 31 4 5412 12 237 30 19 Sun 4 5512 12 287 29 20 Mo. 4 56 12 12 327 28 21 Tu. 4 57 12 12 367 27 22 W. 4 58 12 12 397 27 23 Th. 4 59 12 12 417 27 4 5912 12 437 27 012 12 457 26 112 12 467 25 112 12 467 25 212 12 467 24 312 12 447 23 412 12 437 22 412 12 417 21
24 Fr. 25 Sa.
26 Sun 5 27 Mo. 5 28 Tu. 5 29 W. 5 30 Th. 5 31 Fr. 5
Highest Flood Tide July 4, 5, 6, 7, 8, A. M., 1.6 ft.
stat. Antares sets 12 30 Cujam (Herc.) S. 8 P.M. Vega S. 10 9 P.M. Kaus Media S. 9 46 P.M. 3 Kaus Australis S. 9 45 PM
Forked lightning at night,
Sunshine and shower, won't last half an hour;
Will show you what the day will do.
THE SOLAR SYSTEM-Continued.
of tremendous volcanic action; craters are found all over the surface, varying greatly in size and crowded closely together. The most conspicuous of these craters is Tycho, with a breadth of 50 miles and 20,000 feet deep, having a central cone about 5,000 feet high. It will be noticed that we only see one side of the moon, and from this fact we infer that she must turn on her axis in the same time that she turns round the earth. There being no sign of water or vapor on her disc, and consequently no atmosphere, the surface must be exposed to the most violent changes of temperature; heated in the lunar day of half a month to a very high temperature indeed, and in the other half subjected to cold equal to that of interstellar space. Her diameter is 2,160 miles, or a trifle more; and her rate of motion through the firmament 13° 10′ 35′′ per day and 32′ 56′′ per hour. Thus she travels over a space slightly exceeding her own diameter in one hour. The actual time she takes to move around the earth is 27d. 7h. 43m. 11.46s., but the time from one new moon to another is 29d. 12h, 44m. 3s. When seen through a telescope the surface of the moon presents many objects of great beauty and interest. Quite a small instrument will show the chief "seas." plains, mountain ranges, ring-plains and crater pits. We see, as it were, a cold, dead world apparently devoid of water or atmosphere. Deep down in the clefts and valleys it is possible that some small amount of moisture may exist, and with it a very low form of vegetation, but it is certain that the water has long since left the regions popularly known as seas. Owing to the absence of moisture all details on the lunar surface stand out with startling distinctness, and the shadows of the mountain peaks are sharp and clear.
When the moon passes between the earth and sun and the three bodies lie on a line joining their centres, there will be an eclipse of the sun, which may be either total or annular-total if the moon happens to be near perigee, and annular if near to to apogee; the moon in this latter case, being farther from the earth, looks smaller than the sun and leaves at the middle of the eclipse a ring of light from the sun round the dark circular disc of the moon. When the centres of the sun and moon are not coincident, but differing by something less than the moon's semi-diameter, the eclipse will be partial. Solar eclipses are visible only as total at certain places on the globe, and will be partial in localities off the line of totality, owing to the change in the observer's position placing the moon out of the direct line between him and the sun.
When the earth passes between the sun and moon, the earth's shadow sweeps across the moon, causing a lunar eclipse, which may be either total or partial as in the case of the solar eclipses, but with this difference-that the eclipse will be visible alike to all observers where the moon is above the horizon at the time. The obscuration of the moon's disc is barely ever complete, but owing to the sun's light being scattered by the clouds in the earth's atmosphere, the moon appears usually as a dark copper-colored disc. It will be seen that solar eclipses can take place at new moon, and lunar eclipses only at full moon.
When the moon is full in that part of the ecliptic called the vernal equinox, her motion towards the north is most rapid, and in north latitudes nearly parallel to the horizon; then her tendency to rise later each night is compensated by the northward part of her motion, and she will in that case rise about the same time, nearly full, for two or three successive nights at the time of sunset, and thus give light to get in the harvest. Hence the full moon which happens nearest to September 22 is called the harvest moon. In the United Kingdom, however, the harvest is usually gathered in some three or four weeks before this time.
Beyond the advantage we get from her light the moon is most important to us as a tide-producing body.
The lunar tides, according to the results of recent investigation, are by friction slowly interfering with the earth's rotation, and therefore lengthening our day. This must go on for countless centuries till the earth's time of rotation is equal to a lunation, when the lunar tides will no longer exist, and our seas be disturbed only by the weak action of the solar tides.
The earth in its orbital motion round the sun has the effect of making the latter body apparently describe a complete revolution among the stars in the course of a year, half of its path being above the earth's equator, as in summer, and half below, as in winter. The sun does not, of course, move, but the earth moves in a direction opposite to the apparent motion of the sun, in a plane called the ecliptic, passing through the sun's centre. The earth's equator is inclined 23 with respect to the ecliptic, and this tilt it is which causes our seasons. The earth's equator, therefore, in consequence of this tilt, cuts the ecliptic in two opposite points, the vernal and autumnal equinoxes; and when the sun is at either of these points the days and nights all over the globe are of equal duration. The earth travels over one-half of the ecliptic in summer, and the other half in
1 54 12 27
2 Sun 5 3 Mo. 4Tu. 5
h mh m shmh mh mh mh m
12 44 8 38
12 55 9 39
1 5610 38
5W. 5 912 12 207 15 6Th. 5 10 12 12 147 14
Rises 12 24
6 53 7
5 11 12 12
8 16 1 10
7 30 7 49
8 Sa. 5 12 12 12
8 36 1 53
8 5 8 35
Kaus Bo’lis (xSag.) S.9 22
9th Sunday after Trinity
S. 11 34 P.M.
10 16 P.M.
south. 11 58
y Sagittæ S. 10 22 P.M.
3 44 2 50
-3°41' Total ec.; par. at Balto. Deneb (Cygni) S. 940 P.M. Markab rises 7 11 P.M.
11 50 7 33 1 1712 51
6 546 40 12 53 9 28 3 22 3 16
Highest Flood Tide August 1, 2, 3, 4, 5, A. I., 1.5 ft.
12th Sunday after Trinity (Venus) sets 8 20 P.M.
Observe on what day in August the first heavy fog occurs, and you may expect a hard frost on the same day in October.
When it rains in August, it rains honey and wine.
A fog in August indicates a severe winter and plenty of snow.
Bearded frost is a forerunner of snow.
Heavy white frost indicates warmer weather.
If ice crack much, expect frost to continue.
THE SOLAR SYSTEM-Continued.
winter. It will be noticed in comparing the times of the beginning of the seasons that in the summer half, from spring to autumn, the earth is delayed in traversing this part some eight days longer than the winter half, from autumn to spring. This is caused by the eccentricity of the orbit and the law of areas-a law which requires that the imaginary line joining the earth and sun shall sweep over equal areas in equal times. The earth when farthest from the sun, as in the suminer solstice, must move more slowly than at the winter solstice, when the radius vector, as this imaginary line is called, is shorter. In relation to the sun the earth is a very small body, the diameter of the sun being 866.400 miles, that of the earth only 7,918 miles. The earth revolves around the sun, in the ecliptic, with an average velocity of 18 miles a second, and rotates on its axis once in 24 hours. Mercury and Venus, being nearer to the sun, have a velocity of 29 and 21 miles a second respectively.
The distance of Mars from the sun is 1.524, taking the earth's distance as unity, and his diameter, in terms of the earth's, 0.6-rather more than half that of the earth. His year consists of 687 days, and period of rotation, or day, equals 24h. 37m. Mars. like Mercury and Venus, exhibits phases, but in a much less degree than the inferior planets.
Unlike Mercury and Venus, Mars exhibits many well-defined markings, which enable his rotation period to be well ascertained, besides white spots at the poles of rotation. These are supposed to consist of snow-a reasonable supposition if. as is probably the case, Mars has a considerable atmosphere. When it is summer to the Martians in the northern hemisphere, the white spot about that pole dwindles considerably in extent, if it does not disappear entirely. The continents and seas, as the markings are called, have been charted and their positions accurately laid down. Many excellent photographs of Mars have also been obtained showing the white polar spots and other markings very plainly.
In 1877 Professor Asaph Hall discovered that Mars is attended by two very small satellites, both too faint to be seen except in the most powerful telescopes. Mars is an interesting object for telescopic study, as his actual surface may be seen. Dark and light patches, long dark streaks often called "canals"-and white polar caps are amongst the most prominent and easily distinguished features on his disc.
Between Mars and the next conspicuous planet, Jupiter, there are a number of small planets, or asteroids, amounting at the present time to some 700. Of this number Vesta, Juno, Ceres and Pallas are the largest the largest, Vesta, when in opposition, being just visible to the naked eye. Her diameter is 239 miles. Her light-reflecting capacity is very great, and she is the most brilliant of all the asteroids.
The planet Jupiter, distant from the sun 5.20 times that of the earth, is the largest planet in the system, having a diameter eleven times that of the earth. His year is equivalent to twelve of our years, and his day 9h. 56m. long. Though so much larger than the earth, his density is but 14th of the latter. His orbit is only inclined 1° 20' to the ecliptic. On viewing Jupiter through a telescope the first thing that strikes the observer is that his disc is not circular, but oval, the polar diameter being 84,570 miles and the equatorial 90,190 miles.
This form, of an oblate spheroid, is due to his rapid axial rotation, which causes a flattening of the poles and bulging outwards at the equator. The same effect is produced on all the planets in more or less degree. Jupiter has four principal satellites about the same size as our moon.
The distance of the planet Saturn from the sun is nine and one-half times that of the earth, having a diameter of 76,470 miles, or nearly nine times that of the earth; his day consists of 10 hours 38 minutes, and his year 29% of our years. His rings consist of three bright ones and one interior dusky ring, through which the body of the planet can be seen. Some recent observations seem to show that another dusky ring, outside the bright ones, also exists. The rings appear to consist of three flat annular discs of extreme thinness, inclined at 28 to the ecliptic, and extending to a distance a little greater than the radius of the planet. It has been shown that these rings cannot be a coherent mass, or in one piece, whether solid or liquid, as in that case it would be difficult to account for their stability. On the contrary, they probably consist of an immense number of minute bodies revolving like satellites round the body of the planet.
Saturn has ten satellites, situated outside the rings, some of which are visible in low-power telescopes.