"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.
(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