One was to use a heliometer to measure the distance between the limbs of Venus and the sun during the whole time that the planet was seen projected on the solar disk, and the other was to take photographs of the sun during the period of the transit and subsequently measure the negatives.
The Germans laid the greatest stress on measures with the heliometer; the Americans, English, and French on the photographic method.
The German measures with the heliometer gave apparently concordant results, as follows: Transit of 1874: par.
HELIOMETER (from Gr.
His heliometer (described in a paper communi cated to the Royal Society in 1743, and FIG.
P. 639) mentions such a heliometer which had been in his possession from the year 1753, and of which he gives a representation on Plate XXVIII., fig.
Bouguer's heliometer was in fact similar to that of Savary's third model, with the important difference that, instead of both object-glasses being fixed, one of them is movable by a screw provided with a divided head.
In 1824 the great Konigsberg heliometer was commenced, and it was completed in 1829.
To Fraunhofer, some time not long previous to 1820, is due, so far as we can ascertain, the construction of the first heliometer with an achromatic divided object-glass, i.e.
The first heliometer of the modern type.
The Konigsberg heliometer is represented in fig.
There are three methods in which this heliometer can be used.
On the other hand it is not necessary to reset the telescope after each reversal of the segments.4 When Bessel ordered the Konigsberg heliometer, he was anxious to have the segments made to move in cylindrical slides, of which the radius should be equal to the focal length of the object-glass.
4) that Bessel had indicated, by notes in his handbooks, the following points which should be kept in mind in the construction of future heliometers: (I) The segments should move in cylindrical slides; b (2) the screw should be protected from dust; 6 (3) the zero of the position circle should not be so liable to change; 7 (4) the distance of the optical centres of the segments should not change in different position angles or otherwise; 8 (5) the points of the micrometer screws should rest on ivory plates; 9 (6) there should be an apparatus for changing the screen.'° Wilhelm Struve, in describing the Pulkowa heliometer,' 1 made The distances of the optical centres of the segments from the eye-piece are in this method as I; secant of the angle under measurement.
In Bessel's heliometer this would amount to a difference of o oat h of an inch when an angle of 1 ° is measured.
He actually introduced them in the Konigsberg heliometer in 1840, and they were renewed in 1848 and 1850.
11 This heliometer resembles Bessel's, except that its foot is a solid block of granite instead of the ill-conceived wooden structure that supported his instrument.
By Merz in 1839 on the model of Bessel's heliometer, submits the following suggestions for its improvement: 1 (I) to give automatically to the two segments simultaneous equal and opposite movement; 2 and (2) to make the tube of metal instead of wood; to attach the heliometer head firmly to this tube; to place the eye-piece permanently in the axis of the telescope; and to fix a strong cradle on the end of the declination axis, in which the tube, with the attached head and eye-piece, could rotate on its axis.
It is probable that the Bonn heliometer was in course of construction before these suggestions of Struve were published or discussed, since its construction resembles that of the Konigsberg and Pulkowa instruments.
As the transit of Venus of 1874 approached, prepara tions were set on foot by the German Government in good time; a commission of the most celebrated astronomers was appointed, and it was resolved that the heliometer should be the instrument chiefly relied on.
A similar heliometer was made by the Repsolds to the order of Lord Lindsay for his Mauritius expedition in 1874.
Still more recently the Repsolds have completed a new heliometer for Yale College, New Haven, United States.
Elkin found that the chief drawbacks to speed and convenience in working this heliometer were: (I) The loss of time involved in entering the corresponding readings of the micrometer pointings on two scales.
When the heliometer-part of Lord Lindsay's heliometer was acquired by Gill in 1879, he changed the manner of imparting the motion in question.
To remedy drawback (2) Repsolds provided for the Yale heliometer an additional handle for motion in position angle, intermediate in velocity between the original quick and slow motions.
Heliometer, completed in 1887 for the Royal Observatory at the Cape of Good Hope, Repsolds, on Gill's suggestion, introduced the following improvements: (a) Four different speeds of motion in position angle were provided.
This ring runs between friction wheels and is provided with teeth on its inner periphery, and these teeth transmit motion to a pinion on a spindle having at its other end another pinion which, through an intermediate wheel, rotates the heliometer tube.
The slowest speed is given by means of a tangent screw which is carried by a ball-bearing on the flange of the telescope sleeve, whilst its nut is double-jointed to a ring that encircles the flange of the heliometer-tube.
The reading of the positioncircle of the finder is then the reading to which the position-circle of the heliometer should be set, and from the readings of the micrometerscrew he finds, by a convenient table, the proper settings of the heliometer scales in distance.
When the scales and position-circle of the heliometer have been set to these readings, the comet and the selected comparison-star appear together in the field of view.
A series of observations can be easily and more accurately accomplished with the Cape heliometer in half an hour; with the Oxford heliometer it would occupy 2 hours, and with the 4-in.
Heliometer observations of distance in their most refined sense cannot be considered absolute measures of angles.
Thus, for refined purposes, it cannot be assumed with any certainty that the instantaneous scale-value of the heliometer is known, or that it is a function of the temperature.
Gill introduced a powerful auxiliary to the accuracy of heliometer measures in the shape of a reversing prism placed in front of the eye-piece, between the latter and the observer's eye.
- Various micrometers have been invented besides the heliometer for measuring by double image.
This lens is divided and mounted like a heliometer objectglass; the separation of the lenses produces the required double image, and is measured by a screw.
The present writer has successfully used a similar plan in measuring position angles of a Centauri with the heliometer, viz.
Again, in an ocular heliometer by Steinheil double image is similarly produced by a divided prism of total reflection placed in parallel rays.
Dr Chase's measures with the Yale heliometer indicated for it, in 1894, a parallax of about o" ï¿½ 035; 2 and it must, accordingly, be of nearly four times the total brightness of Sirius, while its aerial lustre exceeds seventyfold that of the solar photosphere.
Hitherto the heliometer has been most extensively used for this purpose, D.
Apart from the large scope of his activity, he introduced such important novelties as the effective use of the heliometer, the correction for personal equation (in 1823), and the systematic investigation of instrumental errors.
Now measures made by Auwers with the Cape heliometer showed no difference, amounting to o I ", and so far negative the idea that the rays reach us after issuing from a level where is sensibly different from unity.
The measures were made with the Cape heliometer and have never been superseded, for the latest results with the minor planet Eros exactly confirm Gill's result-8.80" - while they decidedly diminish the associated probable error.
Better determinations of the actual value came from the heliometer, and gave an angular diameter of 31' 59.26" =0.10", and the value of the polar diameter exceeded the equatorial by 0.038" =0.023".
Sir David Gill derived a highly satisfactory value of 8.78" for the long-sought constant from the opposition of Mars in 1877, and from combined heliometer observations at five observatories in 1888-1889 of the minor planets Iris, Victoria and Sappho, the apparently definitive value of 8.80" (equivalent distance, 92,874,000 m.).
He also invented a heliometer, afterwards perfected by Fraunhofer.