A Text-Book of Astronomy

Renew：Part 25
Author：George C. Comstock
Update：08-15-2023
Tags：Other Books
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Chapter List

Chapter 1
Chapter V), these numbers all change slowly with the lapse of time, and on the average the right ascension of each star of the table must be increased by one twentieth of a minute for each year after 1900--i. e., in 1910 the right ascension of the first star of the table will be 0h. 38.6m. + (10/20)m. = 0h. 39.1m. The declinations also change slightly, but as they are only intended to help in finding the star on the star maps, their change may be ignored.
Chapter IV), has completely supplanted the older Ptolemaic doctrine, and an ill.u.s.tration of the simple manner in which it accounts for the apparently complicated motions of a planet among the stars is found in Figs. 14 and 15, the first of which represents the apparent motion of the planet Mars through the constellations Aries and Pisces during the latter part of the year 1894, while the second shows the true motions of Mars and the earth in their orbits about the sun during the same period. The straight line in Fig. 14, with cross ruling upon it, is a part of the ecliptic, and the numbers placed opposite it represent the distance, in degrees, from the vernal equinox. In Fig. 15 the straight line represents the direction from the sun toward the vernal equinox, and the angle which this line makes with the line joining earth and sun is called the earth"s longitude. The imaginary line joining the earth and sun is called the earth"s radius vector, and the pupil should note that the longitude and length of the radius vector taken together show the direction and distance of the earth from the sun--i. e., they fix the relative positions of the two bodies. The same is nearly true for Mars and would be wholly true if the orbit of Mars lay in the same plane with that of the earth. How does Fig. 14 show that the orbit of Mars does not lie exactly in the same plane with the orbit of the earth?
Chapter III for their periodic times.) Would it be possible for two planets at different distances from the sun to move around their orbits in the same time?
Chapter V) is a striking ill.u.s.tration of a perturbation of slightly different character from the above, and another is found in connection with the plane of the moon"s...o...b..t. It will be remembered that the moon in its motion among the stars never goes far from the ecliptic, but in a complete circuit of the heavens crosses it twice, once in going from south to north and once in the opposite direction.
Chapter II, how these angles may be measured, and it is apparent from the figure that the difference between any two of these angles--e. g., the angles at _1_ and _2_--is equal to the angle at the center, _O_, between the points _1_ and _2_. By measuring these angular distances of the star from the zenith, the astronomer finds the angles at the center of the earth between the stations _1_, _2_, _3_, etc., at which his observations are made. If the meridian were a perfect circle the change of zenith distance of the star, as one traveled along a meridian from the equator to the pole, would be perfectly uniform--the same number of degrees for each hundred miles traveled--and observations made in many parts of the earth show that this is very nearly true, but that, on the whole, as we approach the pole it is necessary to travel a little greater distance than is required for a given change in the angle at the equator. The earth is, in fact, flattened at the poles to the amount of about 27 miles in the length of its diameter, and by this amount, as well as by smaller variations due to mountains and valleys, the shape of the earth differs from a perfect sphere. These astronomical measurements of the curvature of the earth"s surface furnish by far the most satisfactory proof that it is very approximately a sphere, and furnish as its equatorial diameter 7,926 miles.
Chapter V, the year which measures the course of the seasons is shorter than this on account of the precession of the equinoxes. It is called a tropical year with reference to the circuit which the sun makes from one tropic to the other and back again.
Chapter III has learned for himself a fact of which careful account must now be taken. The apparent paths of the moon and sun among the stars are great circles which lie near each other, but are not exactly the same; and since these great circles are only the intersections of the sky with the planes of the earth"s...o...b..t and the moon"s...o...b..t, we see that these planes are slightly inclined to each other and must therefore intersect along some line pa.s.sing through the center of the earth. This line, _N" N""_, is shown in the figure, and if we suppose the surface of the paper to represent the plane of the earth"s...o...b..t, we shall have to suppose the moon"s...o...b..t to be tipped around this line, so that the left side of the orbit lies above and the right side below the surface of the paper. But since the earth"s shadow lies in the plane of its...o...b..t--i. e., in the surface of the paper--the full moon of March, _M"_, must have pa.s.sed below the shadow, and the new moon, _M_, must have cast its shadow above the earth, so that neither a lunar nor a solar eclipse could occur in that month. But toward the end of May the earth and moon have reached a position where the line _N" N""_ points almost directly toward the sun, in line with the shadow cones which hide it. Note that the line _N" N""_ remains very nearly parallel to its original position, while the earth is moving along its...o...b..t. The full moon will now be very near this line and therefore very close to the plane of the earth"s...o...b..t, if not actually in it, and must pa.s.s through the shadow of the earth and be eclipsed. So also the new moon will cast its shadow in the plane of the ecliptic, and this shadow, falling upon the earth, produced the total solar eclipse of May 28, 1900.
Chapter X.
Chapter XIV. A partial explanation of these discordances and other similar ones is that the eye is most strongly affected by greenish-yellow light, while the photographic plate responds most strongly to violet light; the photograph, therefore, represents things which the eye has little capacity for seeing, and _vice versa_.
Chapter III has in hand data from which much may be learned about the earth"s satellite.
Chapter 12
Chapter IV) as a fractional part of the sun"s ma.s.s, and we have only to invert the fraction in order to find that the sun"s ma.s.s is 329,000 times that of the earth and moon combined, or 333,000 times that of the earth alone.
Chapter 14
Chapter III, and Figs. 16 and 17 of that chapter may be completed so as to represent all of the planets by drawing in Fig. 16 two circles with radii of 7.9 and 12.4 centimeters respectively, to represent the orbits of the planets Ura.n.u.s and Neptune, which are more remote from the sun than Saturn, and by introducing a little inside the orbit of Jupiter about 500 ellipses of different sizes, shapes, and positions to represent a group of minor planets or asteroids as they are often called. It is convenient to regard these asteroids as composing by themselves a cla.s.s of very small planets, while the remaining 8 larger planets fall naturally into two other cla.s.ses, a group of medium-sized ones--Mercury, Venus, Earth, and Mars--called inner planets by reason of their nearness to the sun; and the outer planets--Jupiter, Saturn, Ura.n.u.s, Neptune--each of which is much larger and more ma.s.sive than any planet of the inner group. Compare in Figs. 84 and 85 their relative sizes. The earth, _E_, is introduced into Fig. 85 as a connecting link between the two figures.
Chapter X; but, as noted there, this method, like every other, has its weak side, and the result may be a good many thousands of miles in error.
Chapter 17
Chapter 18
Chapter 19
Chapter IV hold true, and we may expect to find each component of a double star moving in a conic section of some kind, having its focus at the common center of gravity of the two stars.
Chapter 21
Chapter 22
Chapter X, is the means by which the solar output of radiant energy is maintained from year to year. Its amount, only a few feet per annum, is far too small to be measured with any telescope; but it is c.u.mulative, working century after century in the same direction, and, given time enough, it will produce in the future, and must have produced in the past, enormous transformations in the sun"s bulk and equally significant changes in its physical condition.
Chapter XIII. The most systematic development of this idea is due to Lockyer, who looks upon all the celestial bodies--sun, moon and planets, stars, nebulae, and comets--as being only collections of meteoric matter in different stages of development, and who has sought by means of their spectra to cla.s.sify these bodies and to determine their stage of advancement. While the fundamental ideas involved in this "meteoritic hypothesis" are not seriously controverted, the detailed application of its principles is open to more question, and for the most part those astronomers who hold that in the present state of knowledge stellar spectra furnish a key to a star"s age or degree of advancement do not venture beyond broad general statements.
Chapter 25