The mariners magazine, or, Sturmy's mathematical and practical arts containing the description and use of the scale of scales, it being a mathematical ruler, that resolves most mathematical conclusions, and likewise the making and use of the crostaff, quadrant, and the quadrat, nocturnals, and other most useful instruments for all artists and navigators : the art of navigation, resolved geometrically, instrumentally, and by calculation, and by that late excellent invention of logarithms, in the three principal kinds of sailing : with new tables of the longitude and latitude of the most eminent places ... : together with a discourse of the practick part of navigation ..., a new way of surveying land ..., the art of gauging all sorts of vessels ..., the art of dialling by a gnomical scale ... : whereunto is annexed, an abridgment of the penalties and forfeitures, by acts of parliaments appointed, relating to the customs and navigation : also a compendium of fortification, both geometrically and instrumentally / by Capt. Samuel Sturmy.

CHAP. VIII. The Projection of the Nocturnal.

IT consists, as you see, of three Parts. The greatest or handle-part hath on it two Circles divided: On the first or outmost is the Ecliptick, divided into 12 equal Parts, in which is put the 12 Signes; and each of those 12 Parts is divided into 30 equal Parts or Degrees, in each Signe, numbered 10, 20, 30, as you see the Figure plainly sheweth. The inward Circle is the 12 Months of the Year, set in by a Table of the Sun's place every day of the Year, accounting the Degrees of the outward Cir∣cle, and the number of Days in each Month equally divided and set down 10, 20, 30.

Note, Where to begin to divide the Months and Days is thus. Observe the bright∣est Guarde, or by Calculation or the Globes find when he comes to the Meridian just at 12 a Clock at Night. In the following Tables the Right Ascension of the brightest Guard is 223 Degrees 31 Minutes; from it substract 12 Hours, or 180 Degrees, the Remainder is 43 deg. 31 min. the Right Ascension of the Sun the 26th day of April, in 16 deg. of Taurus, which must be uppermost next the Zenith in the middle.

The other Part or middle Rundle equally divideth the outward Circle into twice 12 Hours; and within that is a Circle equally divided into 32 Parts, or Points of the Mariner's Compass projected thereon. The upper part is an Index, the length is from the Center to the Foot of the Instrument; all three being fastned with a piece of Brass, so Rivetted that the Center is an Hole through which you may see the North-Star. You may make it in Brass, or good dry Box.

The Ʋse of the Nocturnal.

THe Use of the Nocturnal is easie and ready. Let the Tooth or Index of the mid∣dle Circle be set to the Day of the Month, and it will cut in the outward Circle the Sun's Place in the Ecliptique. Then hold the Instrument on high, a pretty distance from you, with the Foot AB right with the Horizon level: Then look through the Hole of the Center, and see the North-Star, turning the long Index or Pointer up∣wards or downwards, untill you see the brightest of the Guards over or under the Edge that comes straight to the Center. Then look on the Hour-Circle by the Edge of the Pointer, and it shews the Hour of the Night, and likewise the Point of the Com∣pass the Guard beareth from the Pole; by the which you may have his Declination by the following Tables exactly.

The Hour of the Night may be also found by the Right Ascension of the Sun and Stars. Thus, When that you see any Stars in the South, whose Right Ascension is known, and also the Right Ascension of the Sun for that day, you shall substract the Sun's Ascension from the Star's; that which remaineth divide by 15, to bring it into

Hours;* 1.1 for 15 Degrees makes an Hour, and 4 Minutes make a Degree; thereby you have the Hour of the Night. If the Sun's Ascension be more than the Star's, in such case you shall add 360 Degrees to the Ascension of the Stars, and then substract the Sun's Ascension, as before directed, you have the time also.

Notes

• * 1.1

  Here place the North Nocturnal for the Pole Star and Guards