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Great Astronomers

Creator: Ball, Robert S. (Robert Stawell), Sir, 1840-1913
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was one known before Tycho's time, which we have represented in the adjoining figure. [PLATE: TYCHO'S CROSS STAFF.] Let us suppose that it be desired to measure the angle between two stars, then if the angle be not too large it can be determined in the following manner. Let the rod AB be divided into inches and parts of an inch, and let another rod, CD, slide up and down along AB in such a way that the two always remain perpendicular to each other. "Sights," like those on a rifle, are placed at A and C, and there is a pin at D. It will easily be seen that, by sliding the movable bar along the fixed one, it must always be possible when the stars are not too far apart to bring the sights into such positions that one star can be seen along DC and the other along DA. This having been accomplished, the length from A to the cross-bar is read off on the scale, and then, by means of a table previously prepared, the value of the required angular distance is obtained. If the angle between the two stars were greater than it would be possible to measure in the way already described, then there was a provision by which the pin at D might be moved along CD into some other position, so as to bring the angular distance of the stars within the range of the instrument. [PLATE: TYCHO'S "NEW STAR" SEXTANT OF 1572.
Bound to Rise

"Sit up to the table, children, breakfast's ready." The speaker was a woman of middle age, not good-looking in the ordinary acceptation of the term, but nevertheless she looked good. She was dressed with extreme plainness, in a cheap calico; but though cheap, the dress was neat. The children she addressed were six in number, varying in age from twelve to four. The oldest, Harry, the hero of the present story, was a broad-shouldered, sturdy boy, with a frank, open face, resolute, though good-natured. "Father isn't here," said Fanny, the second child. "He'll be in directly. He went to the store, and he may stop as he comes back to milk." The table was set in the center of the room, covered with a coarse tablecloth. The breakfast provided was hardly of a kind to tempt an epicure. There was a loaf of bread cut into slices, and a dish of boiled potatoes. There was no butter and no meat, for the family were very poor.
(The arms, of walnut wood, are about 5 1/2 ft. long.)] No doubt the cross-staff is a very primitive contrivance, but when handled by one so skilful as Tycho it afforded results of considerable accuracy. I would recommend any reader who may have a taste for such pursuits to construct a cross-staff for himself, and see what measurements he can accomplish with its aid. To employ this little instrument Tycho had to evade the vigilance of his conscientious tutor, who felt it his duty to interdict all such occupations as being a frivolous waste of time. It was when Vedel was asleep that Tycho managed to escape with his cross staff and measure the places of the heavenly bodies. Even at this early age Tycho used to conduct his observations on those thoroughly sound principles which lie at the foundation of all accurate modern astronomy. Recognising the inevitable errors of workmanship in his little instrument, he ascertained their amount and allowed for their influence on the results which he deduced. This principle, employed by the boy with his cross-staff in 1564, is employed at the present day by the Astronomer Royal at Greenwich with the most superb instruments that the skill of modern opticians has been able to construct. [PLATE: TYCHO'S TRIGONIC SEXTANT. (The arms, AB and AC, are about 5 1/2 ft. long.)]