sis 



TRANSIT. OR TRANSIT INSTRtJMKNT. 



TRANSIT, OR TRANSIT INSTRCMKXT. 



applying the transit lnfa-umnttoetul observation, the adjust- 

 t of the lord of the vertical circle muat be examined, which we will 

 suppose to read altitude* when the illuminated end U wert, and zenith 

 distance* when the axis U reversed. Having levelled the axii approxi- 

 mately, bring the bubble of the small level to the middle by the Krewi 

 U a which hold the tail-piece, fix these firmly, and then direct the 

 telescope on come point, which U to be placed exactly on the horizontal 

 wire if there U but one, or between the wire* if there are two. Read 

 off the vernier, which we will luppoee to give an altitude of 4*. 

 Reverie toe axU and repeat the former 'observation ; and now read the 

 vernier, which we will suppose to give a rcnith distance of 87*. The 

 urn of these reading* U 91*, while it ought to be 90*, showing an 

 exec** of 4 a degree in each reading. Now set the vernier at 86 30', 

 clamp the screw at &, and, by the tail-piece screws at a bring the 

 object to its proper place between the wires ; finally bring the bubble 

 of the small level to the middle by the antagonist capstan-headed 

 screws, which are seen towards the end of the level at o. The instru- 

 ment will now show true altitudes and zenith distances within 1 ' if the 

 operation has been nicely performed. When the vernier reads polar 

 distances, the vernier must be set to the polar distance of a known 

 star, the telescope brought ,to the star, and the bubble of the small 

 level afterwards Drought to the middle. The latitude of the place may 

 generally be supposed pretty well known : if not, direct the telescope 

 to the pole star, or t Ursjc Minoris, when near their upjwr or lower 

 culminations, or to the sun, or to any star which the observer can 

 identify when it is nearly south or north. The altitude or zenith 

 distance of a known object will give a latitude near enough for finding 

 a star from its catalogued place, or for instrumental corrections. 



We will now proceed to the azimuthol correction. If the time U 

 known from any other observations, the middle wire of the transit 

 may be nude to bisect a star at the time when by calculation it should 

 pass ; and if this star be Polaris or 8 Unto Minoris, the instrument 

 will be very nearly in the meridian ; but if the observer has nothing 

 but a transit instrument and a chronometer, he must place it a* near 

 as he can guess in the meridian,* and level the axis pretty carefully. 

 A moderate knowledge of the heavens will tell what known star is 

 likely to pass soon ; and the instrument having been set to the proper 

 altitude (or sweeping for it), the observer must wait patiently till it 

 enters the field. The observation is then made in the manner already 

 described. If the star is near the zenith, this alone will give an 

 approximate clock-error. Now compute the time a star near the pole 

 or horizon should pass, and bisect the star by the middle wire at the 

 computed tune. If the error in the position of the instrument La too 

 Urge to be corrected by the azimuth screw, the stand must be shifted 

 bodily a sufficient angle, and the instrument levelled afresh. In two 

 or three trials it U easy to get the error within command of the 

 azimuth-screw ; and the observer, if he dislike calculation, may con- 

 tinue getting his clock-error by a star near the zenith, and then 

 bisecting a star near the pole or horizon at the calculated time with 

 the azimuth-screw, until all stars, high and low, give the same clock- 

 error. The instrument is then in the meridian, and the clock-error is 

 the true one. This tentative process may however be considerably 

 abridged by a little easy calculation, which we will proceed to explain, 



The errors of collimation and inclination being supposed to be 

 annulled, either by adjustment or calculation, the line of sight describes 

 a great circle passing through the zenith and not far from the pole. 

 On drawing the figure, and supposing the deviation to be to the east of 

 the south and west of the north, it will be seen that the effect of the 

 error is to cause all stars between the south horizon and the zenith to 

 pan too early, as well as all stars tub polo ; while stars between the 

 zenith and pole pass too late. The effect upon the passage of a star is 



sin. zenith distance 

 proportional to ^ of 8tar ' If x "* """a"* 1 - **> the 



declination^ 



amount of the deviation to the east of the south, measured in units of 

 16*, and the latitude of the place and the declination of the star be 

 denoted respectively by 9 and J, the correction to be added to the 

 observed passage of each star in respect of the error of deviation is 

 sin (> - *) 



~j~f~ x W-7 Now suppose two stars, i and ', to be observed, 



/^ _ t\ 

 which differ a good deal in declination, and let the values of sin . 



for these stars be p and p' : then + ;: and '+;/* are the times at 

 which the stars would have been observed if the instrument had been 

 in the meridian. The interval therefore between their transits thus 

 corrected (and also corrected for the rate of the clock, if necessary) 

 will be equal to the difference of their right ascensions, which may be 

 taken from the ' Nautical Almanac/ if the stars are contained in its 

 lint, or must be computed from some good catalogue, if they are not 



If a Urn be drawn from Potato between the fifth tnd ilxth tr of the 

 Crest Ecu, and a point Uken In tbli line (boat Ij ' from folirii, the tr.niit 

 directed to this point will careelr bo 1 ' frum the meridian. With a litUc 

 pnctlee one may cone nearer than UU at one*. 



t Tbroachout e Khali sanm the latitude to be north : when the drrhnn- 

 Uon U tooth, the iMrn of t U changed, and ( ) become* (+). North of 

 Ik* senltb the numerator U negative, and the correction U to be subtracted 

 between senilh and pole. Below the pole the eoa ) become* negative (reckoned 

 thnmfb tbc pole), and the correction Imam addttlre, u at Ant. 



Let the right ascensions of the two stars be a and a', and we have the 

 following equation : 



ty + ^4- (t+jw) -/-. 



If the value of a revolution of live azimuth screw is known, it U 

 easy to correct for this error at once with considerable accuracy. If x 

 exceed I' or 2', it must be reduced to those limits. Should je be 

 negative, the deviation is to the west of the south. 



The accuracy of the determination of x depends, cattrit paribut, 

 ujion the stars which are used for the computation, that is, upon il;. 

 value of the denominator of the above {notion, which should be as 

 large as possible. The most favourable condition in, that both stars 

 should be near the pole, one above and the other below : t Ursw 

 Minoris and Cephei 61 Hevelii present this combination. It is always 

 desirable that one of the stars should be pretty near the pole Polaris, 

 if possible. If the instrument is nearly in the meridian, it is bettor 

 not to touch the azimuth screw, but to determine the value of x by 



grouping the stars together, in which *j"J ?_ ' is nearly of the same 



, . - 5 



magnitude with the same sign. The value of x may afterwards be 

 used to correct each star separately, and the transits thus corrected 

 will be very nearly such as tcoiiid have been made if the instrument 

 had been exactly placed in the meridian. 



In fixed observatories the meridian error is obtained when possible 

 from consecutive transits of' Polaris above and below pole. The 

 observation* are corrected for collimation and inclination, and for the 

 rate of the clock. If the times of passage thus corrected differ twelve 

 hours, there is no meridian error ; but if the difference U greater or 

 less than twelve hours, the deviation may be computed thus : Let < 

 and s be observed times of upper and lower culmination, corrected for 

 collimation, inclination, and rate: let a and a' be the apparent 

 right ascension of Polaris, taken from the ' Nautical Almanac ; x* the 



deviation as before ; p and p' the values of u for Polaris 



cos 8 



above and below pole, which have different signs. Then, exactly as 

 before, t-px and tf + p'x will be the corrected times of transit, and 



('+!>' x) -(t-px) -a' -a, 



When three consecutive transits are observed, there is no need of 

 taking any account of the rate of the clock (which is supposed to go 

 uniformly), or the change in right ascension of Polaris in the ini 

 only a mean is taken of the first and third transits, which is compared 

 with the second. The difference between these, divided by p' + p, gives 

 the value of x. The sign may be made out by seeing whether the 

 passage from upper to lower culmination is too small, and from lower to 

 upper culmination too large, when the deviation is to the west of north, 

 and therefore to the east of south, and the correction is to be added, 

 or rcrtS rice, when it U to be subtracted. t In well-regulated observa- 

 tories Polaris is always observed when it is convenient, and often when 

 it is not ; and the right ascension of the star, as well as the position of 

 the instrument, is deduced from these double transits. When the 

 right ascension is thus perfectly known, single transits of Polaris may 

 be safely used in combination with other stars, to determine the 

 azimuths! error for those times of the year when the star at one of its 

 culminations passes altogether unseasonably. It is assumed, in this 

 method, that the position of the instrument is the same for twenty- 

 four hours ; or that it changes uniformly. 



If the reader has fully understood what precedes, he will have no 

 difficulty in com|>n-hrmliiiK the mode of observing and reducing the 

 observations which is followed at Greenwich and our pn: 

 observatories ; but the private observer cannot always command a site 

 which is wholly to be relied upon, and very seldom can afford the time 

 for such an uninterrupted series of observations as is required to give 

 full effect to the system just described. The precautions to be taken 

 will depend mainly upon the objects he has in view, but generally 

 speaking, the private transit observer will do wisely to take the places 

 of the principal fixed stars from the lists furnished by Qreri 

 though he may subsequently modify the values inter tf by his own 

 observations. Having done this, it will be easy for him to fix any 

 other object with perfect accuracy, or to determine his time most 

 scrupulously, although the steadiness of his transit Is not beyond 

 suspicion, and his avocations or inclination should only allow him to 

 observe by fits and starts. We shall proceed to show how he may pro- 

 ceed under different hypotheses. 



If he has a distant mark or a collimating telescope [see COLLIHATOR] 



* HutMlltutinit for f' and f their ralurn, the denominator of thin fraction 



l (-*') am (->) "in (>-) 



.osf ~ " os (tan - tan ) or = 00.. _-_,_ > . 



Thin lut In th 1 ! usual form for computing the azimuth by a high and low atar. 



t To prevent mlitake, we nhull alwayn pcak of the quantity which ii to be 

 applied to the obMrTallon to make It correct. 



