To adjust the equatorial for observation. Set the instrument on a firm support. Then to adjust the levels and the horizontal circle:—Turn the horizontal circle till the beginning O of the divisions coincides with the middle stroke of the nonius, or near it. In this situation one of the levels will be found to lie either in a right line joining the 2 foot screws which are nearest the nonius, or else parallel to such a right line. By means of the 2 last screws, cause the bubble in the level to become stationary in the middle of the glass; then turn the horizontal circle half round, by bringing the other O to the nonius; and if the bubble remains in the middle, as before, the level is well-adjusted; if it does not, correct the position of the level, by turning one or both of the screws which pass through its ends, till the bubble has moved half the distance it ought to come to reach the middle, and cause it to move the other half by turning the foot-screws already mentioned. Return the horizontal circle to its first position, and if the adjustments have been well made, the bubble will remain in the middle: if otherwise, the process must be repeated till it bears this proof of its accuracy. Then turn the horizontal circle till 90° stands opposite to the nonius; and by the foot-screw,immediately opposite the other 90°, cause the bubble of the same level to stand in the middle of the glass. Lastly, by its own proper screws set the other level so that its bubble may occupy the middle of its glass.
To adjust the line of sight. Set the nonius on the declination semicircle at O, the nonius on the horary circle at VI, and the nonius on the semicircle of altitude at 90. Look through the telescope towards some part of the horizon, where there is a diversity of remote objects. Level the horizontal circle, and then observe what object appears in the centre of the cross-wires, or in the centre of the field of view, if there be no wires. Reverse the semicircle of altitude, so that the other 90° may apply to the nonius; taking care, at the same time, that the other three noniuses continue at the same parts of their respective graduations as before. If the remote object continues to be seen on the centre of the cross-wires, the line of sight is truly adjusted.
To find the correction to be applied to observations by the semicircle of altitude. Set the nonius on the declination-semicircle to 0, and the nonius on the horary circle to XII; direct the telescope to any fixed and distant object, by moving the horizontal circle and semicircle of altitude, and nothing else; note the degree and minute of altitude or depression; reverse the declination-semicircle, by directing the nonius on the horary circle to the opposite XII; direct the telescope again to the same object, by means of the horizontal circle and semicircle of altitude, as before. If its altitude or depression be the same as was observed in the other position, no correction will be required; but, if otherwise, half the difference of the two angles is the correction to be added to all observations made with that quadrant, or half of the semicircle which shows the least angle, or to be subtracted from all the observations made with the other quadrant, or half of the semicircle. When the levels and other adjustments are once truly made, they will be preserved in order for a length of time, if not deranged by violence; and the correction to be applied to the semicircle of altitude is a constant quantity.
Description of the nonius. The nonius—sometimes called the vernier—is a name given to a device for subdividing the arcs of quadrants and other astronomical instruments. It depends on the simple circumstance, that if any line be divided into equal parts, the length of each part will be greater, the fewer the divisions; and contrariwise, it will be less in proportion as those divisions are more numerous. Thus, in the equatorial now described, the distance between the two extreme strokes on the nonius is exactly equal to 11 degrees on the limb, but that it is divided into 12 equal parts. Each of these last parts will therefore be shorter than the degree on the limb in the proportion of 11 to 12, that is to say, it will be 1/12th part, or 5 minutes shorter. Consequently, if the middle stroke be set precisely opposite to any degree, the relative positions of the nonius and the limb must be altered 5 minutes of a degree, before either of the two adjacent strokes next the middle on the nonius, can be brought to coincide with the nearest stroke of a degree; and so likewise the second stroke on the nonius will require a change of 10 minutes, the third of 15, and so on to 30, when the middle line of the nonius will be seen to be equi-distant between 2 of the strokes on the limb; after which the lines on the opposite side of the nonius will coincide in succession with the strokes on the limb. It is clear from this, that whenever the middle stroke of the nonius does not stand precisely opposite to any degree, the odd minutes—or distance between it and the degree immediately preceding—may be known by the number of the stroke marked on the nonius, which coincides with any of the strokes on the limb.[38] In some instruments the nonius-plate has its divisions fewer than the number of parts on the limb to which it is equal; but when once a clear idea of the principle of any nonius is obtained, it will be easy to transfer it to any other mode in which this instrument is contrived.
To find by this equatorial the MERIDIAN LINE, and the time, FROM ONE OBSERVATION OF THE SUN. In order to this it is requisite that the sun’s declination, and the latitude of the place be known. The declination of the sun may be found, for every day, in the Nautical Almanack, or any other astronomical Ephemeris; and the latitude of the place may be found by means of the semicircle of altitude, when the telescope is directed to the sun or a known fixed star. It is likewise requisite to make the observation when the azimuth and altitude of the sun alter quickly; and this is generally the case, the farther that luminary is from the meridian:—Therefore, at the distance of 3 or 4 hours, either before or after noon, (in summer) adjust the horizontal circle; set the semicircle of altitude, so that its nonius may stand at the co-latitude of the place; lay the plane of the last-mentioned semicircle in the meridian, by estimation, its 0 being directed towards the depressed pole; place the nonius of the declination semicircle to the declination, whether north or south. Then direct the telescope towards the sun, partly by moving the declination semicircle on the axis of the equatorial circle, and partly by moving the horizontal circle on its own axis. There is but one position of these which will admit of the sun being seen exactly in the middle of the field of view. When this position is obtained, the nonius on the equatorial circle shows the apparent time, and the circle of altitude is in the plane of the meridian. When this position is ascertained, the meridian may be settled by a land-mark at a distance.
With an equatorial instrument, nearly similar to that now described, I formerly made a series of ‘day observations on the celestial bodies,’ which were originally published in vol. 36 of ‘Nicholson’s Journal of Natural Philosophy,’ and which occupy twenty pages of that journal. Some of these observations I shall lay before the reader, after having explained the manner in which they are made.
The instrument was made by Messrs. W. and S. Jones, opticians, Holborn, London. The telescope which originally accompanied the instrument was an achromatic refractor, its object-glass being 8½ inches focal distance, and one inch diameter. This telescope, not admitting sufficiently high magnifying powers for the observations intended, was afterwards thrown aside for another telescope, having an object-glass 20 inches focal length, and 1¾ inch diameter, which was attached to the equatorial machinery in place of the small telescope. It was furnished with magnifying powers of 15, 30, 45, 60, and 100 times. The instrument was placed on a firm pedestal about three feet high. The feet of this pedestal had short iron pikes, which slipped into corresponding holes in the floor of the apartment adjacent to a south window, so that when the direction of the meridian was found, and the circles properly adjusted, the instrument was in no danger of being shifted from this position. Though this instrument generally stood fronting the southern part of the heavens, yet the equatorial part, along with the telescope, could occasionally be removed to another position fronting the north and north-west, for observing the stars in those quarters.
Manner of observing stars and planets in the day-time by the equatorial. Before such observations can be made, the semicircle of altitude must be placed in the meridian, and the degree and minute pointed out by the nonius on the horizontal circle, when in this position, noted down in a book, so that it may be placed again in the same position, should any derangement afterwards happen. The semicircle of altitude must be set to the co-latitude of the place; that is, to what the latitude wants of 90°. Suppose the latitude of the place of observation be 52° 30´ north, this latitude subtracted from 90°, leaves 37° 30´ for the co-latitude; and therefore, the semicircle of altitude—on which the equatorial circle is fixed—must be elevated to 37° 30´, and then the equatorial circle on the instrument coincides with the equator in the heavens. Lastly, the telescope must be adjusted on the declination semicircle, so as exactly to correspond with the declination of the heavenly body to be viewed. If the body is in the equator, the telescope is set by the index at 0 on the semicircle of declination, or at the middle point between the two quadrants, and then when the telescope, along with the semicircle of declination, is moved from right to left, or the contrary, it describes an arc of the equator. If the declination of the body be north, the telescope is elevated to the northern division of the semicircle; if south, to the southern part of it.
These adjustments being made, take the difference between the Right Ascension of the sun and the body to be observed; and if the Right Ascension of the body be greater than that of the sun, subtract the difference from the time of observation; if not, add to the time of observation.[39] The remainder in one case, or the sum in the other, will be the hour and minute to which the nonius on the equatorial circle is to be set; which being done, the telescope will point to the star or planet to whose declination the instrument is adjusted. When the heavenly body is thus found, it may be followed, in its diurnal course, for hours, or as long as it remains above the horizon. For as the diurnal motion of a star is parallel to the equator, the motion of the telescope on the equatorial circle, will always be in the star’s diurnal arc; and should it have left the field of the telescope for any considerable time, it may be again recovered, by moving the telescope onward according to the time which elapsed since it was visible in the field of view. We may illustrate what has been now stated by an example or two. Suppose on the 30th April, 1841, at 1 o’clock, P.M. we wished to see the star Aldebaran. The Right Ascension of this star is 4h 27m; and the sun’s Right Ascension for that day at noon, as found in ‘White’s Ephemeris,’ or the ‘Nautical Almanack,’ is 2h 30m. Subtract this last number from 4h 27m, and the remainder 1h 57m, shows that the star comes to the meridian on that day at 57 minutes past 1 o’clock, P.M. And as the time of observation is 1 P.M., the nonius which moves on the equatorial circle must be set to 3 minutes past XI, as the star is at that hour 57 minutes from the meridian. The declination of Aldebaran is 16° 11´ north, to which point on the semicircle of declination, the telescope must be adjusted, and then the star will be visible in the field of view. Again, suppose we wished to observe the planet Venus on the 1st January, 1842, at 12 o’clock noon. The sun’s Right Ascension on that day is 18h 46m, and that of Venus 17h 41m, from which the sun’s Right Ascension being subtracted, the remainder is 22h 55m, or 55 minutes past 10, A.M. Here, as the Right Ascension of Venus is too small to have the sun’s Right Ascension taken from it, we borrow 24 hours, and reckon the remainder from XII at noon. As the planet at 12 noon, is 1 hour 5 minutes past the meridian, the nonius on the equatorial circle must be set to that point, and the telescope adjusted to 23° 6´ of south declination, which is the declination of Venus for that day, when this planet will appear in the field of view.