Magn. N Right Declina Latitude. Afcen, tion. Pole Star, or Nor. Star 2 26.05 66.30 N Whale's Jaw, Cetus-28 10.30 12.37 S 10.45 87.59. N. IX 10.55 59.55 S 21.15 58.55 S 42.02/03.04 N 5:58 5.30 S 65.25 15.58 N left Shoulder 18.02 22.52 N 74.36 45.43 N Orion's left Foot, Rigel III 13.00 31.10 S 75.39 08.30 S Orion's right. Shoulder 1 24.55 10.05 S 85.27 07.20 N 10.1939.32 S 98.34 16.23 S Little Dog, Procyon - I 22.00 15.58 S 111.3305.51 N 23.28 22.24 S 138.5107.36 S 26.0100.37 N 148.47 13.10 N 08.08 52.45 S 183.3561.31 S 20.01 02.02 S 198 0309.51 S 211.06 20.30 N Hydra's Heart, Alphard 1 I Foot of the Crofters 2 mg Problem 22. To find the Rifing, Setting, and Culmination of a The Rule. 1. Rectify the Globe and Hour Index, as in Problem 10. 2. Bring the Star (whofe Rifing you would know) to the Eaft Side of the Horizon, and then the Index will point to the Time of it's Rifing; alfo the Degree of the Horizon against the Star, is it's Amplitude at Rifing. 3. Bring the Star to the Brafs Meridian, and the Index, will fhew the Time of its Culminating, or coming to the Meridian; Alfo the Degree on the Brafs Meridian, contained from the Horizon to the Star, is it's Meridian Altitude. 4. Bring the Star to the Weft Side of the Horizon, and then the Index will fhew the Time of it's Setting; Alfo against the Star, (on the Horizon) is it's Amplitude at Setting, which is ever the fame Quantity as Rifing. This This is fo eafy to perform, and fo often done in the Problems concerning the Sun, it needs no Example. Problem 23. To know at any Time what Stars are above the Horizon, either rifing towards the Meridian, or declining from it, towards their Setting; as alfo what is their Latitude and Azimuth? The Rule. 1. To rectify the Globe, Hour Index, and Quadrant of Altitude, as before, in Problem 10. 2. Turn the Globe till the Hour Index points to the given. Time of the Day or Night; and there ftay the Globe. 3. Then observe what Stars are even with the East Side of the Horizon, those are then rifing; and all those that are between the Horizon and East Side of the Meridian, are rifen above the Horizon, and ascending towards the Meridian. 4. All thofe Stars near the Brafs Meridian, are then near the Meridian of that Place; and thofe at the Brafs Meridian, are then on the Meridian of that Place. 5. If the Quadrant of Altitude be put to any Star it will fhew it's Altitude at that Time and Place, and upon the Horizon, the Quadrant fhews it's Azimuth. 6. All thofe Stars on the Weft Side of the Meridian are defcending from it, towards their Setting; thofe near the Horizon are Setting, and those below the Horizon are Set. In a Word, let the Globe (by Help of a Magnetic Needle, or Compafs, or otherwife, (Variation allowed) be fet fo, as the North Pole thereof may point to the true North in the Heavens, and the South point to the South; then imagine your Eye placed within the Globe at its Center, and that the Globe was tranfparent; or fuppofing a fmall round Hole through the Center of any Star, now your Sight paffing through it, will direct to the Star in the Heavens, correfpondent to that on the Outfide of the Globe. This being mathematically confidered, will make the Ufe of the Globes eafy, and very much conduce to the Knowledge of the Stars, a Thing very neceffary in Navigation, but too much neglected in general by Mariners. Problem 24. To find the Hour of the Night by the Altitude of a known Star. The Rule. 1. Rectify the Globe, Hour Index, and Quadrant of Altitude, as before, in Problem 16. 2. Turn the Globe and Quadrant of Altitude till you bring the Star against it's given Altitude in the Quadrant, and there ftay them. 3. Then will the Index fhew the Hour required; and the Quadrant will cut the Horizon in the Star's Azimuth. Note; If the Star you obferve be on the East Side the Merididian, then turn the Quadrant of Altitude on the East Side of the Brafs Meridian of the Globe: And if on the Weft Side, turn the Quadrant alfo on the Weft Side. Example, At London, December the 23d, the Altitude of Regulus, or the Lyon's Heart, being 25d. 30m. Oriental, or on the East Side of the Meridian: I demand the Hour of the Night? Anfw. According to the aforefaid Rule, the Hour is near 30m. after 10 at Night, and the Star's Azimuth 78d. 30m. South Eafterly, or Eaft by South nearest. So much for the Ufe of the Globes, which is extended beyond it's first defigned Limit, occafioned by a conftant Resolution to be as explicit, and concise as poffible. And, by the Way, let me advise those who have not Conveniency for, or not able to purchase Globes, and yet would know the Stars, that they may attain it with a Pair of Hemispheres, wherein are all the Conftellations, and each Star according to it's Longitude and Latitude placed in them: fuch are made of near 20 Inches in Diameter, to fold in a Book, like a Sea Chart, in four Leaves: They are projected on the Plane of the Ecliptic, or, as fome fay, on the Poles of the Ecliptic: So that in one Hemifphere (which is one Leaf) you have all the Conftellations on the North Side of the Ecliptic, and in another all the Southern. Each Pole of the Ecliptic, is the Center of each Hemifphere, and the Margin going round them is the Ecliptic, being divided into 12 Signs, and each Sign into 30 Degrees; each Degree being fubdivided into Halves and Quarters; and Lines drawn from the Center (or Pole of the Ecliptic) to the beginning of each Sign. On one of thofe Lines are placed the Degrees of Latitude, and numbered from the Ecliptic with 10, 20, &c. to 90, at the Pole and Center of it. By these Hemispheres any of the former Problems (wrought on the Cœleftial Globe) may be folved: As for Inftance; To find the Longitude, and Latitude of a Star. The Rule. 1. Stretch out the Silk String (faftened in the Center for that Purpofe) over the Center of the given Star, and it fheweth, or cutteth the Star's Longitude in the Ecliptic. 2. With a pair of Compaffes, take the Distance from the Center of the Star to the Center of the Hemifphere; lay that Diftance on the Scale of Latitude from the Center of the Hemisphere, and it will fhew the Latitude required, reckoned from the Ecliptic. Example. The Foot of the Grofiers, a Star of the fecond Magnitude in the Southern Hemisphere, between Robur Carolinum, and Centaurus: I demand it's Latitude and Longitude? 1. Laying the faid Silk String over the Star's Center, it cuts the Ecliptic in Virgo, 8d. 8m. the Star's Longitude. 2. Take the Star's Distance from the Center of the Hemifphere, measure it on the Solftitial Colure, from the faid Center, and it reaches to 52d. 45m. the Star's Latitude South. There are drawn in thefe Hemispheres, the Equinoctial, Tropics, Polar Circles, and Poles of the World; all which are diftinguished by their Names fet to them; More Circles may be drawn, and are of Ufe in folving other Problems: which fhall be fhewed after the Problems in Aftronomy. CHAPTER. VII. Spheric Trigonometry, applied in Problems of Geography. Before Efore I treat of Great Circle Sailing, 'twill not be amifs to fay fomething concerning Geography; and for a more diftinét Knowledge thereof, take thefe following Definitions and Problems. The Definitions are much the fame as before, in Chapter 4. of Mercator's Sailing, in Page 82. I. Section I. Geographic Definitions. Plate 5. Fig. 1. THE Earth (on which we dwell) together with the Water, make one round Body or Globe, which is the Subject of Geography. 2. The Poles of the Earth, are two imaginary Points, directly oppofite, upon the Surface of it; that in the North called the North Pole; and that in the South, called the South Pole; as P and I. Plate 5. Fig. 1. 3. The Equator, or Line under the Equinoctial, is a Line drawn round the Globe, and lieth in the Middle between both Poles, cutting all Meridians at Right-Angles, and is a great Circle, from which Latitude taketh its Beginning, and in which Longitude is reckoned, as EAQ, Plate 5. Fig. 1. 4. Meridians, are great Circles drawn through both Poles cutting the Equator at Right-Angles, as PQI, PĂI, and PMI; M 4 anfwerable anfwerable to them, are the North and South Lines, drawn in any Chart. 5. Parallels of Latitude are lefs Circles drawn parallel to the Equator, through every Degree and Minute of the Meridian, between the Equator and each Pole, as alt and Z* lt, and are reprefented in any Chart, by the Eaft and Weft Lines therein. 6. Latitude is an Arc of a Meridian, contained between any Parallel and the Equator; from whence it is counted both Ways to cach Pole, where it ends in 90 Degrees, which is the greatest Latitude, 7. North Latitude is on that Side of the Equator towards the North Pole, and South Latitude towards the South Pole. 8. Difference of Latitude is an Arc of a Meridian, and the nearest Distance between any two Parallels, and fheweth how far any Place is to the Northward or Southward of another Place, and never exceedeth 180 Degrees. 9. Longitude is reckoned in the Equator, round which, increafing to the Eastward, it is counted (by fome) till it end (where it firft began) in 360 Degrees, which is the greatest Longitude: Or, according to Mr. Wakely, in his Mariner's Compass Rectified, it is counted from the Meridian of London, increafing on both Sides of it, Eastward and Westward, till it terminates in 180 Degrees, at the oppofite Meridian. 10. Longitude of a Place, is an Arc of the Equator contained between the Meridian of that Place, and the first Meridian where Longitude taketh its Beginning, and counted (by the old Way) to the Eastward of the firft Meridian, (but by the new Way in the Mariner's Compafs Rectified, and likewife in the Mariner's Calendar) it is counted both Eaftward and Weftward from the Meridian of London, which in both thefe Tables is the Meridian whence Longitude taketh its Beginning. II. Difference of Longitude is an Arc of the Equator, contained between the Meridians of any two Places, and never exceedeth 180 Degrees. 12. The Distance of any two Places, is an Arc of a great Circle, paffing through them, and never exceedeth 180 Degrees. 13. The Angle of Pofition, or the Angle of Situation of Places, is an Angle that the Arc of a great Circle paffing over two Places, makes with the Meridian of one of them, and is not the Courte leading from one to the other. In finding the Distance of Places, there are three Cafes: As (1.) when they differ only in Latitude; (2.) when they differ only in Longitude; (3.) when the two Places differ both in Latitude and Longitude; all which are performed by the fol |
