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Kennedy was talking rapidly of data which he had derived from his metric photograph, from plumb line, level, compass, and tape, astronomical triangle, vertices, zenith, pole and sun, declination, azimuth, solar time, parallactic angles, refraction, and a dozen bewildering terms. "In spherical trigonometry," he concluded, "to solve the problem three elements must be known. I knew four.
A sextant of the same materials and size. A quadrant of one and a half cubits radius, and an azimuth circle of three cubits. Ptolemy's parallactic rules, covered with brass, four cubits in the side. The sextant already described in page 134. Another quadrant, like No. 3. Zodiacal armillaries of melted brass, and turned out of the solid, of three cubits in diameter.
Nebular parallax has, up to the present, proved evanescent, and nebular parallactic drift, in response to the sun's advance through space, remains likewise imperceptible. It may hence be presumed that no nebulæ occur within the sphere occupied by the nearer stars. But the difficulty of accurately measuring such objects must also be taken into account.
In like manner he determined that the latitude of Konigsberg was 54° 43´, in place of 54° 17´, as adopted by Rheinhold. When Morsianus returned to Huen in July, he brought with him, as a present to Tycho, from John Hannovius, one of the Canons of Ermeland, the Ptolemaic Rules, or the Parallactic Instrument which Copernicus had used and made with his own hands.
Kennedy was talking rapidly of data which he had derived from his metric photograph, from plumb line, level, compass, and tape, astronomical triangle, vertices, zenith, pole and sun, declination, azimuth, solar time, parallactic angles, refraction, and a dozen bewildering terms. "In spherical trigonometry," he concluded, "to solve the problem three elements must be known. I knew four.
In order to determine the loss of energy attending the reflection of the rays by the diagonal mirrors, I have constructed a special apparatus, which, by means of a parallactic mechanism, faces the sun at right angles during observations.
Tobias Mayer of Göttingen, however, whose short career was fruitful of suggestions, struck out the right way to the same end; and Laplace, in the seventh book of the Mécanique Céleste, gave a solar parallax derived from the lunar "parallactic inequality" substantially identical with that issuing from Encke's subsequent discussion of the eighteenth-century transits.
From Ratisbon he passed to Saalfeld, and thence to Wittemburg, where he saw the parallactic instruments and the wooden quadrant which had been used by John Pratensis in determining the latitude of the city, and in measuring the altitudes of the new star.
But this angle of parallax or apparent displacement cannot be directly measured cannot even be perceived with the finest instruments. Not from its smallness. The parallactic shift of the nearest of the stars as seen from opposite sides of the earth's orbit, is many times smaller.
The existence of such apparent or "parallactic" displacements was accordingly regarded as the touchstone of the new views, and their detection became an object of earnest desire to those interested in maintaining them.
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