our time (71), and his diurnal revolution 9 hours (128), it follows that he has 92,683 natural days in his year. 29 years 175 days= 10,760 days of our time; × 24=258,240 hours÷10 hours, the time of Saturn's revolution, = 24,594 3, the number of days in his year. So 84 years, 27 days, the periodic time of Uranus=36,687 days, or 880,488 hours; which÷94 hours, the time of the planet's diurnal revolution = 92,683, the number of natural days in his year. 155. As going from the earth's center is to ascend (page 27), and the equator of an oblate spheroid is further from the center than the poles, it follows, that the earth being an oblate spheroid, we must ascend somewhat in going from either pole to the equator. A river, therefore, running for a great distance toward the equator, would actually ascend; or, in other words, run up hill -the centrifugal force generated by the earth's motion driving the water on toward the equator. WATER RUNNING UP HILL The Mississippi is said to be higher at its mouth than it is some thousands of miles north of it. If its bed conforms at all to the general figure of the earth, this must cer tainly be the case, as may be demonstrated by the aid of the annexed diagram. Let A B represent the polar, and CD the equatorial diameters. The entire difference between them is 26 miles, or 13 miles on each side. The two circles represent this difference. Now as the earth's circumference is 25,000 miles, the distance from the poles to the equator (being onefourth of that distance) must be 6,250 miles; and in that 6,250 miles the ascent is 13 miles, or over two miles to every 1,000 toward the equator. The Mississippi runs from the 50th to the 30th degree of north latitude inclusive, or 21 degrees; which, at 69 miles to a degree, would amount to about 1,500 miles. If, then, it runs a distance equivalent to 1,500 miles directly south (in a winding course of about 8,000), theory requires that it should be about three miles higher at its mouth than it is 1.500 miles directly north There is some philosophy, therefore, in saying that if a river runs for a great distance from either pole toward the equator, it must run up hill. A B 156. Should the earth cease to rotate upon its axis, the waters about the equator would at once rush toward the poles, flooding them to the depth of 64 miles, and receding from the equator to the saine amount. So far as the solid portions of the earth would perinit, it would at once become a perfect sphere. (See page 17, and also Art. 153 and note.) 157. It has already been stated (77), that the orbits of all the planets were ellipses; but they are not all alike eccentric. The orbit of Mercury is quite elliptical, while 155. What curious fact follows from the earth's oblateness? (What in Lance given? Illustrate by diagram.) 156. What would be the effect should the earth cease to rotate ? that of Venus is nearly a circle. The student should observe that the eccentricity is not the deviation from a circle, but the distance from the center of an ellipse to either focus (see page 23 and cuts). The eccentricity of the orbits of the principal planets is as follows: 158. The equinoctial points have already been defined (125) as two points in the earth's orbit where the equinoctial or celestial equator (20) cuts the sun's center. They are in opposite sides of the ecliptic, or 180° apart (see 119 and cut). The vernal equinox is the point from which both celestial longitude and right ascension are reckoned (20 and 91); but not being marked by any fixed object in the heavens, it is reached just when the sun comes to be exactly over the earth's equator, or in the equinoctial 159. But it is found by long and careful observation that the earth reaches the equinoctial point about 22 minutes and 23 seconds earlier every year than on the year preceding. This is equal to an arc of 501" in the ecliptic. In this manner the equinoctial points are slowly receding westward, D m PRECESSION OF THE EQUINOXES, * B 157. What said of the orbits of Mercury and Venus? Of eccentricity? 158. Are the equinoctial points marked by any fixed object in the heavens How know when reached? 159. Are they stationary or not? Reached how much earlier annually or falling back upon the ecliptic, at the rate of 501" a year, or 1° every 713 years. This would amount to 30°, or one whole sign in 2,140 years, and to the entire circle of the ecliptic in 25,868 years. This very interesting phenomenon may be explained by the preceding diagram. Let the point A represent the vernal equinox, reached, for instance, at 12 o'clock on the 20th of March. The next year the sun will be in the equinoctial 22 minutes 23 seconds earlier, at which time the earth will be 501" on the ecliptic, back of the point where the sun was in the equinoctial the year before. The next year the same will occur again; and thus the equinoctial point will recede westward little by little, as shown by the small lines from A to B, and from C to D. It is in reference to the stars going forward, or seeming to precede the equinoxes, that the phenomenon was called the Precession of the Equinoxes. But in reference to the motion of the equinoxes themselves, it is rather & recession. 160. The cause of this wonderful motion was unknown, until Newton proved that it was a necessary consequence of the rotation of the earth, combined with its elliptical figure, and the unequal attraction of the sun and moon on its polar and equatorial regions. There being more matter about the earth's equator than at the poles, the former is more strongly attracted than the latter, which causes a slight gyratory or wabbling motion of the poles of the earth around those of the ecliptic, like the pin of a top about its center of motion, when it spins a little obliquely to the base. 161. One marked effect of this recession of the equi noxes is an increase of longitude in all the heavenly bodies. As the vernal equinox is the zero or starting point, if that recedes westward, it increases the distance between it and all bodies east of it; or, in other words, increases their longitude to the amount of its recession. Hence catalogues of stars, and maps, showing their longitude, need to be corrected at least every 50 years, otherwise their longitude, as laid down, will be too little to indicate their true position. Allowing the world to have stood at this date (1867) 5,871 years, the equinoxes have receded already through about 75° of longitude. At the same time the constellations have gone forward How much in angular measurement? Revolving which way? At what rate? How long for 10? For 300? For the whole circle of the eclipti! (Illustrate by diagram.) 160. Cause of recession? Who discovered? 161. Effect of recession upon longitude? Explain how effected. Sig and constellations? eastward, and left the signs which bear their names. Hence the sign Aries actually covers the constellation Pisces. 162. Another effect of the recession of the equinoxes is, that it gives to the pole of the earth a corresponding revolution around the pole of the ecliptic in 25,868 years. Let the line A A in the figure represent the plane of the ecliptic; DB, the poles of the ecliptic; CC, the poles of the earth; and DD, the equinoctial. EE is the obliquity of the ecliptic. The star C at the top represents the pole star, and the curve line passing to the right from it may represent the circular orbit of the north pole of the heavens around the north pole of the ecliptic. 163. This gyratory motion of the north pole of the heavens, while it keeps at the distance of 23° 28' from B E E the pole of the ecliptic, will cause it to change its place in the heavens to the amount of 46° 56′ in 12,934 years; thus alternately approaching toward and receding from the stars, at every revolution of the equinoxes around the ecliptic. Thus the place of the pole is in constant but very slow motion around the pole of the ecliptic. 164. The Nutation of the earth's axis is another small and slow gyratory motion, by which, if subsisting alone, the pole would describe among the stars, in the period of about 19 years, a minute ellipse, having its longer axis equal to 18", and its shorter about 14"; the longer axis pointing toward the pole of the ecliptic. It is on account of these varied motions shifting the point from which Longitude and right ascension are reckoned, and also the pole of the heavens, that it becomes necessary, in de 162. What other effect of recession? (Illustrate by diagram.) 163. What effect upon the apparent distance of the stars from the north pole of the heavens? 164. What is Natation? What meant by epoch, and why necessary to state? 1 TELESCOPIC VIEWS OF THE PLANETS- -MERCURY. 83 scribing the place of a star or planet, by any of these standards, to state the epoch or time, and also whether it be mean right ascension-i. e., right ascension after having been corrected for the recession of the equinox, the zero point. 165. The Colures are two great circles crossing at the poles of the ecliptic at right angles. One passes through the equinoxes, and is thence called the Equinoctial Colure; the other passes through the solstices, and is called the Solstitial Colure. They are to the heavens what four meridians, each 90° apart, would be to the earth. Petrograde Sead Now, balqualen Motion Eve begin wred Periw CHAPTER III. TELESCOPIC VIEWS OF THE PLANETS. 166. By the aid of telescopes, we discover myriads of objects in the heavens that are entirely invisible to the naked eye; while objects naturally visible are immensely magnified, and seem to be brought much nearer the ob server. This impression of nearness is an intellectual conclusion drawn from the fact of the increased distinctness of the object; as we judge of the distance of objects, in a great measure, by their dimness or distinctness. MERCURY. 167. Under favorable circumstances, Mercury is visible to the naked eye, but yet is seldom seen, owing to his nearness to the sun. During a few days in March and April, and August and September, he may be seen for several minutes in the morning or evening twilight, when 165. What are the colures? Describe. 166. Effect of the teloscope upon vision? Upon distant objects? (Why appear nearer ?) 167. Can Mercury be seen by the naked eye? Is he often seen? Why Dot! When may he be seen? How appear? |