THE USE OF THE PLANISPHERE.

IN this Planisphere the Center re­presents the North Pole of the World, about which are these four Concentrick Circles.

1. The Artick Circle being a lesser Circle distant from the Pole 23 deg. 30 min.

2. The Tropick of Cancer distant from the North Pole 66 degrees 30 minutes, and from the Equator 23 degrees 30 minutes.

3. The Equator, which is distant from the Pole 90 degrees, numbred 10, 20, 30, &c. to 360.

4. The Tropick of Capricorn, distant from the Equator Southward 23 de­grees 30 minutes.

The Ecliptick makes an Angle of 23 degrees 30 minutes, with the E­quator at the two opposite Points ♈ and ♎, and is divided into 12 Signs, marked ♈, ♉, ♊, &c.

Any Right Line supposed to be drawn from the Pole or Center of the Planisphere is a Meridian, but that which is called the Graduated Meri­dian is numbred from the Equator both ways, with 10, 20, 30, &c.

By the help of this Planisphere, the following useful Problems may be re­solved.

1. The Day of the Month being given to find the Sun's Place in the Ecliptick.

2. To find the Hour of the Night by any known Star, being upon the Meridian.

3, To know what Month and Day any Star will South at Mid­night.

4. To find the Declination of any known Star.

5. To find the Right Ascension of any known Star.

6. To find the Time of the Moon's Southing any Day of her Age.

7. To find the Time of the Sun's Rising and Setting, any Day in the Year, in the Latitude of London.

For the Solution of those Seven Problems, you have Directions at the Corners of the Planisphere.

Prob. 8. Having the Sun's Place to find his Declination.

Extend your Compasses from the Center of the Planisphere, to the De­gree of the Ecliptick that the Sun is in; then keeping one Point in the Center, turn the other to the Gra­duated Meridian, and it will shew the Declination required.

Example. I desire to know the Sun's Declination when he is in 0 deg. 0 min. of Taurus.

Extend the Compasses from the Center to 0 deg. 0 min. of ♉ on the Ecliptick Circle, then turning them about as before directed, the move­able Point falls on 11 deg. 30 min. which (because it is numbred from the Equator towards the North Pole) is North Declination, which was re­quired.

Prob. 9. Having the Sun's Place given to find his Right Ascension.

Lay a Scale from the Center over that Point in the Ecliptick that the Sun is in, and observe where it cuts the Equator; the Degrees and Mi­nutes so found, is the Right Ascension of the Sun at that time.

Example. The Sun's Right Ascension when he is in 0 deg. 0 min. of ♉, is required.

Lay your Scale from the Center of the Planisphere over 0 deg. 0 min. of ♉ in the Ecliptick, and it will cut in the Equator 27 deg. 54 min. the Sun's Right Ascension sought.

Having thus found the Right As­cension of the Sun, and also the Right Ascension of any Star, by Prob. 5. you may find what time of the Day or Night the said Star will be upon the Meridian (besides the Method pro­posed in Prob. 2.) Thus,

Subtract the Right Ascension of the Sun from the Right Ascension of the Star (borrowing 360 deg. if Sub­traction cannot be made) the Re­mainder reduce into Time (allowing for every 15 degrees one hour, and for every degree 4 minutes) it gives the hour and minute Afternoon that the Star will be upon the Meridian; and if it amount to more than 12 hours, subtract 12 hours from it, the Remainder is the hour and minute that the Star comes to the Meridian next Morning.

Example. It is required to know what time the Lyon's Heart will be upon the Meridian on the Day that the Sun's Place is found to be in 0 deg. 0 min. of vs.

The Right Ascension of the Lyons Heart, is found (by Prob. 5.) to be 147 degrees 48 minutes, from which subtract the Right Ascension of the Sun (which is found by Prob. 9. to be) 270 degrees; the remainder 237 deg. 48 min. reduced to Time gives 15 hours 51 minutes Afternoon, from which abating 12 hours, the Re­mainder 3 hours 51 minutes is the hour of the Lyons Heart coming to the Meridian in the Morning.

Note, If you put a small Bead upon the String that is fixed at the Center of the Planisphere, the String and Bead so fitted will supply the Use of Scale and Compasses, the String ser­ving instead of a Scale, and the Bead slipping up and down upon the String at pleasure, gives any Extent from the Center upon the Graduated Me­ridian.

The Use of the Plani­sphere, upon the Plain of the Eclip­tick.

THESE two Plani­spheres upon the Plain of the Ecliptick, are one for the Northern Hemisphere, the other for the Southern; but their Use is the same, their Centers are the Poles of the Ecliptick, and their Periphery is the Ecliptick Circle, and the Poles of the World are 23deg. 30m. from the Center or Pole of the Ecliptick. In the Northern Hemisphere you have the Pole-Artick, or North Pole, and about it is Described the Artick Circle, the Tropick of Cancer, and one half of the Equator. In the Southern Hemisphere, you have the South Pole, (and respecting it for a Center, you have) the Antar­tick Circle, the Tropick of Ca­pricorn, and the other half of the Equator. The chief Use is,

1. To find the Longitude of any fixed Star.

Lay a Scale from the Cen­ter, over the Star whose Lon­gitude is required, and the Sign and Degree cut by the Scale upon the Ecliptick, or outermost Circle, is the Place of the Star in Longitude.

Example. I desire the Lon­gitude of Capella, (which is a bright Star in the Constellation called Auriga.)

I lay a Scale as before directed, and I find it cuts the Ecliptick in 17deg. 30m. of Gemini; the Place of the Star required.

2. To find the Latitude of any Star.

Extend your Compasses from the Center of the Plani­sphere, to the Star, then keep­ing one foot in the Center, turn the other to the Gradua­ted Line of Longitude; which (in both Hemispheres) is one of the Lines drawn from the Center to the Periphery and numbred towards the Center, with 10, 20, 30, &c. and you have the Latitude of the Star required.

Example. I desire to know the Latitude of the Star Capel­la, or the Goat, before mentioned.

Extend the Compasses from the Center to the Star, then turn it to the Graduated Line of Longitude, it falls upon 22deg. 50min the Latitude of the Star required.

Note, Always if the Star whose Latitude is sought, be found in the Northern Hemi­sphere, the Latitude is North; (as in the Example above) and and if it is in the Southern Hemisphere, the Latitude of the Star is South.