CHAP. IX. How to use the Pole-Star's Declination and Table, and thereby to get the Latitude.
THe Pole-Star, being so very well known to all Sea-men, is therefore made the most use of by them: Therefore know, That this Table is made for the year 1660. the true Declination being 2 deg. 30 min. but will serve for many years after. This Table is made contrary to the two former Tables; for whereas the North Point of the Nocturnal is the first Point you reckon from, and was on the for∣mer Nocturnal reckoned from South: so of this Nocturnal you must take the Point at the Tooth for North, and so reckon forward North and by East, and so on to East and South-East, South and West, to North again.
And likewise in the Table, you must begin in like manner at that part of the Ta∣ble that lies directly under the Pole; which, as before-said, is properly called the North, and so proceed about the Pole, ascending from this lowest or North Point of the Me∣ridian, as was said before, to the North-East, East, and South-East, so to the South or highest Point of ascending, being directly over the Pole: From the South they descend again by the West, and so return to the North again.
Observe this, That the brightest of the Guards is the first of the little Bear, which is the Star you are to observe, and is almost in opposition to the Pole-Star.
Note, That when the Guard-Star is under the Pole, then the Pole-Star is above the Pole; and when the Guard-Star is above the Pole, then the Pole-Star is under the Pole so many Degrees and Minutes as the Table shews you.
The Use of this Table and Instrument is this: Look with the Nocturnal, and see what Point the Guards bears from the Pole, as before-directed; and if you find the Guard is not on a full Point, stay a little longer until he is just, and then observe the height of the Pole-Star exactly as you can; then knowing by your Dead Reckoning within a Degree or two what Latitude you are in, look for the nearest to that Lati∣tude in the top of the Table; and if you find the Point of the Compass which the Guard-Star is upon, in the first Column of the Table, and in that Line under the Co∣lumn of your Latitude, you shall find the number of Degrees and Minutes the Pole-Star is either above or below the Pole, according to the direction of the last Column of the Table, which you must thus make use of; If the Star be any thing above the Pole, substract the Number in the Table from the height of the Star observed: but if the Star be under the Pole, then add the Number found in the Table to the height observed, by which you shall have the height of the Pole.
For Example. Estimate the Latitude to be near 40 Degrees, and observe the Pole-Star. Suppose you find the Altitude 40 Degrees, and the Guard-Star bears N. N. E. from the Pole; therefore look for N. N. E. in the first Column, and right under your estimated Latitude 40, in the same Line with N. N. E. you will find the Declination to be 1 Degree 30 Minutes; substract that from 40 Deg. the Altitude observed leaves the true Latitude 38 Degrees 30 Minutes.
| d. | m. |
| 40 | 00 |
| 01 | 30 |
| 38 | 30 |
But if the Guard-Star had been S. S. W. then the Pole-Star had been 1 Degree 33 Minutes under the Pole; which being added to the Altitude observed 40 Deg. the Latitude would have been exactly 41 Degrees 30 Minutes by the Star. So the Star's Altitude by observation being 55 Deg. the Guard bears from the Pole-Star S. E. b. S. the Declination against that Point is 2 Degrees 30 Minutes, added to 55. had been 57 Degrees 30 Minutes for the Latitude: but if your estimated Latitude had been near 50, and the Guard bear from the Pole North-West