32 



THE POPULAR EDUCATOR. 



to indicate Oh. Om. Os. when the first point of Ariea is on the 

 meridian, and has of course passed round the twenty-four 

 hours when that point returns to the meridian. 



Now right ascension is also measured from the first point in 

 Aries, and hence we shall find that the clock will show us the right 

 ascension of any star. In one hour the Earth has revolved 

 through 15, or 5 l jth of 360, and any star on the meridian then 

 must have 15, or 1 hour of right ascension; similarly, every 

 4 minutes of sidereal time indicates 1. 



We see, then, that we can by a single 

 observation with this instrument fix 

 the place of any star. The graduated 

 circle will give us its polar distance, 

 and deducting this from 90 we have 

 its declination ; while by noting the 

 time by the clock we at once learn its 

 right ascension. Thus, if a star crossed 

 the meridian at 13h. 13m. 80s., and its 

 distance from the pole was 35, we 

 should at once know its declination 

 was 55 N., and its right ascension 

 13h. 13m. 30s., or 198 22^'. By 

 observations of this kind the places 

 of all the stars have been noted, and 

 catalogues compiled giving their posi- 

 tions. In the eye-piece of the telescope 

 there are usually five or seven equi- 

 distant and parallel wires, and by 

 noting the time of the star passing 

 over each, and taking the mean, we 

 obtain the true time more exactly than 

 by one observation taken singly. 



The only other kind of mounting for 

 a telescope we can explain now is that 

 known as the equatorial. With any ordi- 

 nary stand as, for instance, that for the altazimuth instru- 

 ment two motions are requisite to keep a star in the field of 

 view ; both the observer's hands are therefore constantly occu- 

 pied with the handles intended to impart a slow motion to 

 the telescope, since, owing to the rapid rotation of the Earth 

 on its axis, the star would in a very few minutes pass out 

 of the field of view if these adjustments were neglected. 



All the heavenly bodies, how- 

 over, appear to revolve round 

 the pole ; hence if we mount 

 our telescope so that its axis 

 shall point towards the pole, 

 one motion will be sufficient to 

 enable us to follow a star and 

 keep it in view. 



One of the modes in which a 

 telescope may be thus equa- 

 torially mounted is shown in 

 Fig. 41. This is known as the 

 English equatorial. The axis, 

 instead of being vertical, is in- 

 clined to the horizon at such an 

 angle that it points to the pole ; 

 the inclination, therefore, has to 

 vary with the latitude of the 

 place. An instrument of this 

 kind is, however, usually made 

 a fixture, and then the lower 

 end of the axis turns in a 

 bearing set in a block of 

 masonry, A, while a strong sup- 

 port, B, bent over at the upper part, carries the other end. 



The telescope is attached to a pivot which turns in the polar 

 axis, and is carefully adjusted so as to be at right angles to it. 

 The telescope thus moves in a plane parallel to the axis, and 

 sweeps along a meridian. A graduated circle, D, is attached to 

 it, and read off by means of a microscope or a vernier at E. 

 The edge of this circle is cut into teeth, which catch in an 

 endless screw on the rod G, and in this way a slow motion 

 may be imparted to the telescope. By pressing the handle a 

 downwards, the screw is removed from the teeth, and the 

 tolescope then moves freely to allow of a coarse adjustment 

 or rapid motion. 



A similarly divided circle, F, is affixed to the lower end of 

 the axis c C, and serves to give the right ascension of any 

 object, the graduations on it reading from I. to xxiv. hours. 

 A slow-motion handle is usually affixed to this, similar to that 

 shown at Q. 



When the telescope is directed to any celestial object, the 

 circle D may be clamped, as the star can be kept in the field 

 by merely turning the circle F. In the best instruments a 

 driving-clock, regulated to keep sidereal 

 time, is added, and this causes the circle 

 F to revolve in exactly 24 hours. All 

 difficulty in adjusting the telescope is 

 thus removed, as it constantly remains 

 directed to the object without any 

 alteration. It is by an arrangement of 

 this kind that photographs of tho 

 heavenly bodies are taken. 



The mode of using the equatorial 

 requires a little explanation. The circle 

 F is so adjusted as to read when 

 the telescope is directed to the meridian, 

 and a sidereal clock is required to note 

 the time of making the observation. 

 Now, suppose that we want to note tha 

 position of any star, we get it exactly in 

 the centre of the field, for which pur- 

 pose fine cross-wires are usually placed 

 in the eye-piece. We then note the 

 exact time, and read off the two circles. 

 Suppose, for example, that the time is 

 lOh. 45m., and tho reading of the 

 declination circle is 37 P 20', and that 

 of the other 3h. 40m. ; the star then 

 had passed the meridian 3 hours and 

 40 minutes at the moment of making 

 the observation ; it was on it, therefore, at 7h. 5m., and this 

 is its right ascension, while its declination is 37 20', and thus 

 we know exactly its position. 



One great advantage which the equatorial possesses over the 

 transit instrument arises from the fact that with it observa- 

 tions may be made in any part of the heavens, while with the 

 other a star can only be observed when on the meridian. 



A different mode of mounting 

 an equatorial is represented in 

 Fig. 42. This instrument, which 

 is called the " star-finder," is 

 mounted on the German, or 

 Fraunhofer's system, which is 

 that more usually adopted. The 

 advantages it possesses over the 

 English form arc that stars near 

 tho pole may bo observed with 

 it, which the axis in the other 

 prevents ; and further, that only 

 one support is required, and 

 thus there is far less difficulty 

 in fixing it accurately. Tho 

 instrument shown here is 

 portable, but in observatories 

 the larger instruments mounted 

 on the German plan are usually 

 fixed to stone pedestals. 



To nearly all telescopes of 

 high power, a small one is usually 

 affixed to serve as a finder, H 

 (Fig. 41). The power of this is 



but small, but it takes in a large field, the centre of which is 

 marked by means of cross-wires, and it is so adjusted that when 

 the star is thus in the centre of its field, it is also in the centre 

 of the field of the large instrument. In all powerful instru- 

 ments the extent of sky seen at one time is very small, and 

 therefore considerable difficulty would be experienced in finding 

 any required star were it not for tho finder. 



There are many other instruments used by the astrono- 

 mer, as the mural circle, the zenith- sector, etc., but we must 

 not stay to notice them here. They consist for the most 

 part of telescopes mounted in a peculiar way for some special 

 purpose. 



