402 



THE POPULAR EDUCATOR. 



the heavens is by giving its declination and right ascension, as 

 described in our last lesson, the distances being reckoned from 

 the equinoctial. Sometimes, however, these distances are mea- 

 sured from the ecliptic, and are then called the latitude and 

 longitude. Parallels of latitude are frequently drawn on the 

 celestial globe to enable the latitude to be found without diffi- 

 culty ; the pole of the ecliptic is, of course, the centre of these 

 circles. Longitude, like right ascension, is reckoned from the 

 point Aries, and, like it, is reckoned only in one direction, from 

 to .960. Terrestrial longitude, on the other hand, is reckoned 

 from to 180, east or west. 



In Fig. 13, if I represents the place of any heavenly body, 

 and o be the point T in the ecliptic, then the number of degrees 

 in the arc i R will give its latitude, and the number in o B will 

 give its longitude. 



Another way in which the position of a star may be described 

 is by giving its altitude and azimuth ; as, however, these vary 

 greatly from hour to hour, the exact time must be given like- 

 wise, and then they afford an ensy way of ascertaining its posi- 

 tion in the heavens, without reference to a globe or to any other 

 stars. It is also easier for the amateur to note these measure- 

 ments than to describe the position in either of the other ways 

 mentioned. 



To determine the altitude, a great circlo of the heavens is 

 supposed to pass through the star and also through the zenith, 

 or part of the sky vertically over head, and the nadir, which is 

 diametrically opposite to the zenith, and is the part of the 

 heavens directly under our feet. We then measure the arc of 

 this circle contained between the horizon and the star, and this 

 is its elevation above the horizon, or, as it is called, its altitude. 

 It is, in fact, the angle contained between lines drawn from the 

 observer to the star, and to the point of the horizon directly 

 under it. 



Having measured this, we must then ascertain the distance 

 of the point where this circle cuts the horizon from the north 

 or south points, and this distance is called its azimuth. As 

 will easily be understood, to an observer at the poles the 

 altitude of a star will exactly correspond with its declination, 

 and remain unaltered, while the azimuth will be continually 

 changing, as it is reckoned from a point of the earth, and not 

 from a fixed point in the sky. At the equator, the azimuth 

 remains the same, but at all other places both vary constantly. 



A telescope may easily be mounted so as to indicate at onca 

 the altitude and azimuth of any star, and an instrument of this 

 kind will be found very useful. In actual practice a great 

 many modifications and additions are made, but the form of 

 mounting sketched in Fig. 1-1 will servo well to explain the con- 

 struction, and guide the student should he resolve on making 

 such an instrument for himself. The base consists of a flat 

 circle of wood, and is fitted with levels and levelling screws, so 

 that it may be placed perfectly horizontal. This circle is accu- 

 rately divided into degrees, reckoning each way from the north 

 and south points, and in use must be adjusted so that these 

 points are due north and south. 



The telescope and stand is fixed to another circle, which 

 turns on the lower one, and has marked on it a line exactly 

 corresponding with the direction of the tube. A microscope, A, 

 placed at the extremity of this, serves to read off the azimuth. 



A similarly graduated circle is attached to the tube of the 

 telescope, so as to turn vertically with it. The divisions' on this 

 are so placed that, when the tube is perfectly horizontal, a 

 second microscope placed at B shall read 0, and thus when the 

 cube is pointed to any star, and clamped in that position, its 

 altitude and azimuth can at once be ascertained. A telescope 

 thus mounted is called an alt-azimuth instrument. 



As the sky appears to be in constant rotation, it will easily 

 be seen that these measures are continually altering; by 

 noticing, however, the exact time of observation, we shall be 

 able to assign the place of the star on the globe. 



To do this we must first bring the globe into such a position 

 as exactly to represent the appearance of the sky at that par- 

 ticular time. This is a problem of very easy solution, but 

 attention should be paid to it, as it will very frequently prove a 

 great help to the student to place the globe in this position 

 when he is endeavouring to become familiar with the constella- 

 tions. If the globe is correctly adjusted, and placed so that 

 its brass meridian points exactly north and south, any desired 

 star can easily be found. The only difficulty is that we look at 



the globe from the outside, while we gaze on the vault of 

 heaven from a point within it, and thus the relative positions 

 of the stars are -reversed. If, however, we take an ordinary 

 pencil with a flat end, and place it on the star's place on the 

 globe, so that its head may be directed backwards to the centre 

 of the globe, the pencil will point to the star, and thus it may, 

 after a little practice, be found without difficulty. 



Now we have already learnt that at the equator the pole-star 

 appears to be in the horizon, and as we recede from it this star 

 increases in altitude ; the first thing, then, which we have to 

 do is to elevate the pole above the horizon as many degrees as 

 are equal to the latitude of the place at which we are. We will 

 suppose that London is the place of observation, and, as its 

 latitude is 51^, we will_elevate the pole this amount above the 

 north point of the horizon. We next find the position of the 

 sun in the ecliptic on the day by referring to the wooden 

 horizon of the globe. Let us suppose the time of observation 

 is eleven o'clock in the evening of the 3rd of June. We first 

 find June 3rd on the horizon, and opposite to it we shall find 

 the 13th degree in Gemini. Now refer to the ecliptic, and, 

 having found this place on it, bring it to the brass meridian, 

 and adjust the brass circle round the pole, called the hour 

 circle, so tt at XII. on that shall be under the meridian. The 

 globe then represents the position of the heavens at noon on 

 the day, but it is constantly turning towards the west, and we 

 must, therefore, turn it westward till the hour circle shows that 

 eleven hours have been passed over, and the hour XI. comes 

 under the meridian. Fixing the globe in this position we now 

 place it in the open air, so that the north pole points to the 

 north or to the pole-star, and we have it then accurately 

 representing the visible sky. 



When the globe has been set thus, we fix the quadrant of 

 altitude to the brass meridian, exactly in the zenith. As one 

 side of the brass meridian is graduated towards the pole, and 

 the other from it, this is easily done by fixing it at the degree 

 which marks the latitude, that is, at 51 .|, and by bringing the 

 graduated edge of it to any star we can at once read off the 

 altitude ; the point, too, where it cuts the horizon will show its 

 azimuth. Thus, with the globe in this position, we will find the 

 altitude and azimuth of the bright star Regulus in the constel- 

 lation Leo. On bringing the quadrant of altitude against its 

 centre, we find its elevation above the horizon to be about 12 L. 

 This, therefore, is its altitude at the hour named, a trifling 

 allowance being made for refraction, the extent of which for 

 each degree of elevation is shown in refraction tables. On 

 referring now to the inner circle of the wooden horizon, we 

 shall find that the degree of it indicated by the quadrant is 84 

 west of the north point ; this, then, is its azimuth and its place 

 is fully known when we say that at eleven o'clock in the 

 evening of the 3rd of June its altitude is 12-J- , and its azimuth 



i wcrft of north. 



We will give one more problem of the same kind. On the 

 15th of November, at half -past seven in the evening, a bright 

 star is observed at London, whoso altitude is about 17, and 

 its azimuth is 85 east of north ; find the star. On referring, 

 as before, to the globe, we shall learn that the star is Aldebaran, 

 situated in the head of Taurus, or the Bull, and in close 

 proximity to the V-shaped cluster of the Hyades. This the 

 student should verify for himself by reference to the globe. 



Occasionally, the polar distance of a star is given, that is, 

 the arc contained between it and the pole. A moment's thought, 

 however, shows that this is merely the complement of the 

 declination, or the amount required to make it up to 90. In a 

 similar way, the amplitude of any object is the complement of 

 its azimuth, or its distance from the east and west points of the 

 horizon. It is only applied to the sun or a star when rising or 

 setting, and signifies the arc of the horizon included between 

 their position at that time and the east or west points. At the 

 period of the equinoxes the sun rises due east and sets due 

 west ; in the winter months, however, he rises some distance to 

 the south of the east point, and sets a similar distance to the 

 south of the west ; while in the summer, when he has north 

 declination, he rises and sets to the north of these points. The 

 ixact position of its rising and setting on any given day can 

 very easily be found. We have only to elevate the pole of the 

 globe to the latitude of the place, and then, having found the 

 sun's place in the ecliptic, turn the globe slowly round, and 

 note where this point cuts the eastern horizon in rising and the 



