micro
magnitude can be observed without difficulty. A delicate level and a filar micrometer constitute the zenith-telescope attachment. The level is affixed to one side of the tube, with its length at right angles to the horizontal axis, and revolves on a centre, its indications serving to give a measure of any small change of inclination of the telescope consequent on reversal. The value in arc of one division of this level, as determined at the Washington Naval Observatory, is 1.067. The meter-screw carries three parallel threads perpendicular to the ruled lines of the transit reticule, the middle one of the three being that which is commonly used for measuring differences of zenith distance. The value in arc of one revolution of the screw, as ascertained from numerous careful observations at the Washington Observatory, is 68."698; and, as the graduated screw-head is divided into 100 parts, each of which can be read to tenths by estimation, it affords a measure of .069 of a second of arc. The whole number of turns is read off on a comb at the side of the field of view, and these, for convenience, are numbered consecutively from left to right as viewed, the central tooth being Number 20. Parallactic motion is given to the eye-piece by a long-pitched screw, so that stars can be accurately observed either on the north or the south of the middle of the field.
P
Observing-hut. For shelter, I had a roomy and well-designed portable wooden observatory, with the requisite openings, which was given to me by Lieutenant-Commander GREEN.
Pier.—The instrument-pier is of Canton blue bricks set in Portland cement, and rests on a shallow footing of brickwork, but is not isolated below the ground-level, though it has no contact with any part of the hut. This was found to answer very well, no unsteadiness having been perceptible during ordinary movements in the hut.
Zenith-telescope method of determining the Latitude. In determining the latitude or declination of the zenith by the zenith-telescope method, the general procedure is as follows. Pairs of stars chosen for observation, such that their meridianal zenith-distances shall be nearly equal, but on opposite sides, north and south, of the zenith, and that they shall pass the meridian within a few minutes of one another. There is no fixed limit to the zenith-distance suitable for this method, but it is desirable not to pass much beyond 25°, and in this determination I have limited myself to within 20° while the mean for all the pairs observed is just 8°. For difference of zenith-distance of
any pair of stars the usual limit is 20′ of arc, in order that the stars may not be too near the edge of the field, and also that the resulting latitude may not be too largely affected by any error in the adopted value of the micrometer-screw. I have practically conformed to this limit on the present occasion, having in the case of two pairs only passed a little beyond it. The difference of Right Ascension should be not less than one minute, in order to allow time for reading and reversal, and it should not exceed twenty minutes, on account of the risk of instrumental changes in the interval, resulting from changes of temperature. About sixteen minutes is the greatest interval for any pair used in this determination.
two
Method of observation. For observing, the instrument, having been previously adjusted carefully to the meridian, and its cross-axis levelled, is clamped at the mean of the zenith-distances of the two stars, and then directed at the proper time to the star which will pass the meridian first of the pair—the bubble of the telescope-level being brought to the middle of its run by the screw for that purpose This star is bisected with the micrometer thread at or near the moment of its passage over the middle, transit thread, the time being noted and, immediately afterwards, the level and micrometer-screw readings. The instrument (telescope and level remaining clamped) is then reversed, and it will evidently now be inclined to the zenith at the same angle as before, but on the opposite side of it, so that the second star will at the proper time appear in the field. This star is bisected at or near transit, and the chronometer, level and micrometer read as before. The above constitutes one complete observation for latitude.
Advantages and disadvantages of the method. The chief advantages of this beautiful method over all others are that it substitutes for measurements of large arcs small micrometric measurements difference only of zenith-distance; that the time needs not be accurately known, an error of two or three seconds being of no moment; that possible errors arising from incorrect estimates of refraction are reduced to a minimum, inasmuch as the only term involving refraction which affects the result is the difference of refraction for stars at small and nearly equal zenith-distances; and that the effects of errors of flexure are similarly minimised. Its "weak point," at the present day, is the scarcity; for any station, of stars fulfilling the necessary conditions whose declinations have been determined with high precision. The stars available are mostly of small magnitudes, a circumstance which, though favourable to accurate bisection, presents the drawback that, as compared with the Clock and other Standard stars, they are seldom observed at the fixed observatories, except under special arrangements for this particular purpose.
Selection of Stars, and Computation of Mean Places.—It follows that, when possible, stars from the best catalogues only should be chosen; and, keeping this in view, I have limited myself to using none but stars that have been observed at the Royal Observatory, and whose places are given in the Greenwich Nine-Year Catalogue for 1872. In that catalogue, the Mean Places of the stars are given for the epoch 1872, January 1. Those places have been reduced to the epoch 1882, January 1 the formula,
Mean R.A., 1882, January 1 = A + (p + q
·
+
n
100
2
X) n + c
+
where A is the star's Mean Right Ascension for 1872, January 1, p its Annual Precession
* The method was invented in 1834, by the late Captain A. TALCOTT, U. S. Engineers.
micro
magnitude can be observed without difficulty. A delicate level and a filar micrometer constitute the zenith-telescope attachment. The level is affixed to one side of the tube, with its length at right angles to the horizontal axis, and revolves on a centre, its indications serving to give a measure of any small change of inclination of the telescope consequent on reversal. The value in arc of one division of this level, as determined at the Washington Naval Observatory, is 1.067. The meter-screw carries three parallel threads perpendicular to the ruled lines of the transit reticule, the middle one of the three being that which is commonly used for measuring differences of zenith distance. The value in arc of one revolution of the screw, as ascertained from numerous careful observations at the Washington Observatory, is 68."698; and, as the graduated screw-head is divided into 100 parts, each of which can be read to tenths by estimation, it affords a measure of .069 off second of arc. The whole number of turns is read off on a comb at the side of the field of view, and these, for convenience, are numbered consecutively from left to right as viewed, the central tooth being Number 20. Parallactic motion is given to the eye-piece by a long-pitched screw, so that stars can be accurately observed either on the north or the south of the middle of the field.
P
Observing-hut. For shelter, I had a roomy and well-designed portable wooden observatory, with the requisite openings, which was given to me by Lieutenant-Commander GREEN.
Pi.r.-The instrument-pier is of Canton blue bricks set in Portland cement, and rests on a shallow footing of brickwork, but is not isolated below the ground-level, though it has no contact with any part of the hut. This was found to answer very well, no unsteadiness having been perceptible during ordinary movements in the hut.
Zenith-telescope method of determining the Latitude. In determining the latitude or declination of the zenith by the zenith-telescope method, the general procedure is as follows. Pairs of stars chosen for observation, such that their meridianal zenith-distances shall be nearly equal, but on on posite sides, north and south, of the zenith, and that they shall pass the meridian within a few minutes of one another. There is no fixed limit to the zenith-distance suitable for this method, but it is desirable not to pass much beyond 25°, and in this determination I have limited myself to within 20° while the mean for all the pairs observe l is just 8°. For difference of zenith-distance of
any pair of stars the usual limit is 20′ of arc, in order that the stars may not be too near the edge of the field, and also that the resulting latitude may not be too largely affected by any error in the adopted value of the micrometer-screw. I have practically conformed to this limit on the present occasion, having in the case of two pairs only passed a little beyond it. The difference of Right Ascension should be not less than one minute, in order to allow time for reading and reversal, and it should not exceed twenty minutes, on account of the risk of instrumental changes in the interval, resulting from changes of temperature. About sixteen minutes is the greatest interval for any pair used in this determination.
two
Method of observation. For observing, the instrument, having been previously adjusted carefully to the meridian, and its cross-axis levelled, is clamped at the mean of the zenith-distances of the t stars, and then directed at the proper time to the star which will pass the meridian first of the pair- the bubble of the telescope-level being brought to the middle of its run by the screw for that purpose This star is bisected with the micrometer thread at or near the moment of its passage over the middle, transit thread, the time being noted and, immediately afterwards, the level and micrometer-screw readings. The instrument (telescope and level remaining clamped) is then reversed, and it will evidently now be inclined to the zenith at the same angle as before, but on the opposite side of it, s0 that the second star will at the proper time appear in the field. This star is bisected at or near transit, and the chronometer, level and micrometer read as before. The above constitutes one complete observation for latitude.
Advantages and disadvantages of the method. The chief advantages of this beautiful method over all others are that it substitutes for measurements of large arcs small micrometric measurements difference only of zenith-distance; that the time needs not be accurately known, an error of two or three seconds being of no moment; that possible errors arising from incorrect estimates of refraction are reduced to a minimum, inasmuch as the only term involving refraction which affects the result is the difference of refraction for stars at small and nearly equal zenith-distances; and that the effects of errors of flexure are similarly minimised. Its "weak point," at the present day, is the scarcity; for any station, of stars fulfilling the necessary conditions whose declinations have been determined with high precision. The stars available are mostly of small magnitudes, a circumstance which, though favourable to accurate bisection, presents the drawback that, as compared with the Clock and other Standard stars, they are seldom observed at the fixed observatories, except under special arrangements for this particular purpose.
Selection of Stars, and Computation of Mean Places.--It follows that, when possible, stars from the best catalogues only should be chosen; and, keeping this in view, I have limited myself to using none but stars that have been observed at the Royal Observatory, and whose places are given in the Greenwich Nine-Year Catalogue for 1872. In that catalogue, the Mean Places of the stars are given for the epoch 1872, January 1. Those places have been reduced to the epoch 1882, January 1 the formula,
Mean R.A., 1882, January 1= A + (p + q
· +
n
100 2
X ) n + c
+
where A is the star's Mean Right Ascension for 1872, January 1, p its Annual Precession
* The method was invented in 1834, by the late Captain A. TALCOTT, U. S. Engineers.
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