fixed body, like a fixed star, for then the interval would be CHAP. II. 24 hours of sidereal time. But as the interval is always more than 24 hours, it shows that the general motion of the moon is eastward among the stars, with a daily motion varying from 10 to 16 degrees,* traveling, or appearing to travel, through the whole circle of the heavens (360°) in a little more than 27 days. nomy. Thus, these observations, however imperfectly and rudely Chief obtaken, at once disclose the important fact, that the sun and ject of astromoon are in constant change of position, in relation to the stars, and to each other; and, we may add, that the chief object and study of astronomy, is, to discover the reality, the causes, the nature, and extent of such motions. Other movable and wandering (27.) Besides the sun and moon, several other bodies were noticed as coming to the meridian at very unequal intervals of time-intervals not differing so much from 24 bodies. sidereal hours as the moon, but, unlike the sun and moon, the intervals were sometimes more, sometimes less, and sometimes equal to 24 sidereal hours. These facts show that these bodies have a real, or apparent motion, among the stars, which is sometimes westward, sometimes eastward, and sometimes stationary; but, on the whole, the eastward motion preponderates; and, like the sun and moon, they finally perform revolutions through the heavens from west to east. Only four such bodies (stars) were known to the ancients, Wandering namely, Venus, Mars, Jupiter, and Saturn. stars known to the an. discoveries These stars are a portion of the planets belonging to our cients. solar system, and, by subsequent research, it was found that Modern the Earth was also one of the number. As we come down to more modern times, several other planets have been discovered, namely, Mercury, Uranus, Vesta, Juno, Ceres, Pallas, and, very recently (1846), the planet Neptune.† * Four minutes above 24 hours corresponds to one degree of arc. We have not mentioned the names of these planets in the order in which they stand in the system, but rather in the order of their discovery. As yet, we have really no idea of a planet, or a planetary system. CHAP. II. Observa determine We shall again examine the meridian passages of the sun, moon, and planets, and deduce other important facts concerning them, besides that of their apparent, or real motions among the fixed stars. (28.) But let us return to the fixed stars. We have tions which several times mentioned the fact, that the same star returns the meridian to the same meridian again and again, after every interval of distances of 24 sidereal hours. So two different stars come to the meri the stars. dian at constant and invariable intervals of time from each other; and by such intervals we decide how far, or how many degrees, one star is east or west of another. For instance, if a certain fixed star was observed to pass the meridian when the sidereal clock marked 8 hours, and another star was observed to pass at 9, just one sidereal hour after, then we know that the latter star is on a celestial meridian, just 15 degrees eastward of the meridian of the first mentioned star. Correspon- As 24 hours corresponds to the whole circle, 360 degrees, be therefore one hour corresponds to 15 degrees; and 4 minutes, in time, to one degree of arc. Hence, whatever be the observed interval of time between the passing of two stars over the meridian, that interval will determine the actual difference of the meridians running through the stars; and when we know the position of any one, in relation to any celestial meridian, we know the positions of all whose meridian observations have been thus compared. dence and degrees. cension. Right as- The position of a star, in relation to a particular celestial meridian, is called Right Ascension, and may be expressed either in time or degrees.. Astronomers have chosen that It is true, we might mention every fact, and every particular respecting each planet; such as its period of revolution, size, distance from the sun, &c. ; but such facts, arbitrarily stated, would not convey the science of astronomy to the reader, for they can be told alike to the man and to the child to the intellectual and to the dull-to the learned and to the unlearned. To constitute true knowledge-to acquire true science- the pupil must not only know the fact, but how that fact was discovered, or deduced from other facts. Hence we shall mainly construct our theories from observations, as we pass along, and teach the pupil to decide the case from the facts, evidences, and circumstances presented. meridian, for the first meridian, which passes through the sun's center at the instant the sun crosses the celestial equator in the spring, on the 20th of March. Right ascension is measured from the first meridian, eastward, on the equator, all the way round the circle, from 0 to 360 degrees, or from 0 h. to 24 h. The reason why right ascension is not called longitude will be explained hereafter. CHAP. II. First meri dian. right ascen sions of the (29.) If we observe and note the difference of sidereal To find the time between the coming of a star to the meridian, and the coming of any other celestial body, as the sun, moon, planet, sun, or comet, such difference, applied to the right ascension of the and planets, star, will give the right ascension of the body. But every astronomer regulates, or aims to regulate, his sidereal clock, so that it shall show 0 h. 0 m. Os. when the equinox is on the meridian; and, if it does so, and runs regularly, then the time that any body passes the meridian by the clock, will give the right ascension of the body in time, without any correction or calculation; but, practically, this is never the case: a clock is never exact, nor can it ever run exactly to any given rate or graduation. We have thus shown how to determine the right ascensions of the heavenly bodies. We shall explain how to find their positions in declination, in the next chapter. moon, CHAPTER III. REFRACTION. POSITION OF THE EQUINOX, AND OBLIQUITY OF HOW FOUND BY OBSERVATION. (30.) To determine the angular distance of the stars from CHAP. I. the pole, the observer must first know the distance of his zenith from the same point. As any zenith is 90 degrees from the true horizon, if the observer can find the altitude of the pole above the horizon CHAP. III. (which is the latitude of the place of observation), he, of Prepara. tions for de course, knows the distance between the zenith and the pole. As the north pole is but an imaginary point, no star being termining there, we cannot directly observe its altitude. But there is a very bright star near the pole, called the Polar Star, which, by original as all other stars in the same region, apparently revolves the latitude observa tions. The mural circle round the pole, and comes to the meridian twice in 24 sidereal hours; once above the pole, and once below it; and it is evident that the altitude of the pole itself must be midway between the greatest and least altitudes of the same star, provided the apparent motion of the star round the pole is really in a circle; but before we examine this fact, we will show how altitudes can be taken by the mural circle. H Fig. 2. N (31.) The mural, or wall circle, is a large metallic circle, firmly fastened to a wall, so that its plane shall coincide with the plane of the meridian. A perpendicular line through the center, Z N, (Fig. 2), represents the zenith and nadir points; and at right angles to this, through the center, is the horizontal line, Hh. How to ob- A telescope, Tt, and an index bar, I i, at right angles to meri- the telescope, are firmly fixed together, and made to revolve serve dian tudes. alti on the center of the mural circle. The circle is graduated from the zenith and nadir points, each way, to the horizon, from 0 to 90 degrees. When the telescope is directed to the horizon, the index points, I and i, will be at Z and N, and, of course, show 0° of altitude. When the telescope is turned perpendicular to Z, the index bar will be horizontal, and indicate 90 degrees of altitude. When the telescope is pointed toward any star, as in the figure, the index points, I and i, will show the position of the CHAP. III. telescope, or its angle from the horizon, which is the altitude of the star. a transit in. As the telescope, and index of this instrument, can revolve Mural cir. freely round the whole circle, we can measure altitudes with cle also it equally well from the north or the south; but as it turns strument. only in the plane of the meridian, we can observe only meridian altitudes with it. This instrument has been called a transit circle, and, says Sir John Herschel, "The mural circle is, in fact, at the same time, a transit instrument; and, if furnished with a proper system of vertical wires in the focus of its telescope, may be used as such. As the axis, however, is only supported at one end, it has not the strength and permanence necessary for the more delicate purposes of a transit; nor can it be verified, as a transit may, by the reversal of the two ends of its axis, east for west. Nothing, however, prevents a divided. circle being permanently fastened on the axis of a transit instrument, near to one of its extremities, so as to revolve with it, the reading off being performed by a microscope fixed on one of its piers. Such an instrument is called a transit circle, or a meridian circle, and serves for the simultaneous determination of the right ascensions and polar distances of objects observed with it; the time of transit being noted by the clock, and the circle being read off by the lateral microscope." (32.) To measure altitudes in all directions, we must have another instrument, or a modification of this. Conceive this instrument to turn on a perpendicular axis, parallel to Z N, in place of being fixed against a wall; and conceive, also, that the perpendicular axis rests on the center of a horizontal circle, and on that circle carries a horizontal index, to measure azimuth angles. This instrument, so modified, is called an altitude and azimuth instrument, because it can measure altitudes and azimuths at the same time. (33.) After astronomy is a little advanced, and the angular distance of each particular star, sun, moon, and planet, Altitude and azimuth instrument. |