On the Use of this Book.

sighting hole of the movable slider, one sees simultaneously the horizon or sea-rim through the other sight hole of the slanted fixed slider, and holds that as straight as one can, or as one uses a small cross for it, as is drawn in figure N. 1 with S, so one holds the bottom edge V X straight on the horizon, just as with another common quadrant. With the hand, one then slides the long cross back and forth along the staff until the shadow of the ear on the top small cross falls into the white field, on the slanted fixed slider at the end of the staff, right between the two parallel lines above the sighting hole; to know, as follows: that one simultaneously sees the horizon through both sighting holes, and the shadow of the aforementioned ear right above there between the aforementioned parallel lines. Having obtained that simultaneously, the large cross then stands exactly by the degree of the true altitude of the Sun, to know, according to such markings as the top small cross (whether high or low) is set.

Having found the altitude of the Sun, if the declination is northerly, one subtracts it from the found altitude (properly understood, if one is north of the Sun, that is, if the Sun stands southward); if it is southerly, one adds it to it, and thus one finds the altitude of the Aequinoctiael Equinoctial/Celestial Equator, which subtracted from 90, one obtains the altitude of the Pole. As for example: Let in the figure here alongside the Sun be elevated in the south above the horizon B D 70 degrees, the declination toward the north E D 10 degrees; if one subtracts that from B D 70, there remains B E 60 degrees, the altitude of the Equinoctial, which

A circular astronomical diagram representing the celestial sphere. It features a horizontal line marked A and B representing the horizon, and a vertical axis. Various points are labeled with letters: S (the Sun), D, E, G, H, P (the Pole), and T at the center. Lines intersect to show angles of elevation and declination relative to the horizon and the equator.

subtracted from B Z, or E G 90, there remains E Z, or B G 30, equal to P A the altitude of the Pole.

But if one marks this staff with reversed numbers, to know, that instead of 90 one sets 0, for 80, 10; for 70, 20; and so forth, just as one commonly marks the common quadrants that one uses after the Sun, one shall then contrarily add the northerly declination, and subtract the southerly, and thus at once without further calculation obtain the altitude. As for example: If one, as in the foregoing, measured the Sun, being elevated at D 70 degrees, one shall in place of 70 find 20 on the staff. That is not the altitude of the Sun from the horizon B D, but the complement of the altitude to 90 degrees, as