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This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1910 edition. Excerpt:...motion (chapter iv., 92), which was not actually detected till long afterwards (chapter xiii., 278), is called annual or stellar parallax (the adjective being frequently omitted); and the name is applied in particular to the greatest angle between the direction of the star as seen from the sun and as seen from the earth in the course of the year. If in fig. 69 we regard M as representing a star, o the sun, and the circle as being the earth's path round the sun, then the angle Omq is the annual parallax of M. In this particular case Cassini deduced from Richer's observations, by some rather doubtful processes, that the sun's parallax was about 9" '5, corresponding to a distance from the earth of about 87,000,000 miles, or about 360 times the distance of the moon, the most probable value, according to modern estimates (chapter xiii., 284), being a little less than 93,000,000. Though not really an accurate result, this was an enormous improvement on anything that had gone before, as Ptolemy's estimate of the sun's distance, corresponding to a parallax of 3', had survived up to the earlier part of the 17th century, and although it was generally believed to be seriously wrong, most corrections of it had been purely conjectural (chapter' vii., i45) 162. Another famous discovery associated with the early days of the Paris Observatory was that of the velocity of light. In 1671 Picard paid a visit to Denmark to examine what was left of Tycho Brahe's observatory at Hveen, and brought back a young Danish astronomer, Olaus Roemer (1644-1710), to help him at Paris. Roemer, in studying the motion of Jupiter's moons, observed (1675) that the intervals between successive eclipses of a moon (the eclipse being caused...