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equally distant from the poles. It divides the globe into two equal parts, called hemispheres :-the half which has the North Pole in its centre is named the northern hemisphere, and the other, the southern hemisphere.

4. The Equinoctial is a great circle in the heavens, every part of which is equally distant from the celestial poles, and corresponding with the equatorial circle on the earth :-hence, it may be considered as being the plane of the earth's equator, extended to the starry firmament.The equator, or equinoctial, as well as all the other great circles of the spheres, is divided into 360 equal parts, called degrees; each degree is divided into 60 equal parts, called minutes, and each minute into 60 equal parts, called seconds.

5. The Meridian of any place on the earth, is a semicircle which passes through that place, cuts the equator at right angles, and terminates at the poles of the world:-and, as every part of the globe has a meridian passing through it, there may be as many meridians as there are points in the equator. The meridians divide the great equatorial circle of 360 degrees into 24 equal parts, each consisting of 15 degrees; and since those equal parts correspond to the exact measure of a natural day of 24 hours, therefore, one hour is equal to 15 degrees of the equator, or of longitude.-When the plane of the meridian is extended to the sphere of the fixed stars, it then becomes a circle of right ascension, or an hour circle. And circles of right ascension divide the equinoctial into as many parts as the terrestrial meridians, or circles of longitude, divide the equator.

6. The First Meridian is an imaginary semicircle, passing through any remarkable place and the poles of the world: hence it is arbitrary. The English esteem that circle of longitude to be the first meridian, which passes through the Royal Observatory at Greenwich; the French esteem that to be the first meridian which passes through the Royal Observatory at Paris; the Spaniards, that which passes through Cadiz, &c. &c. &c.

7. The Zenith of any place is the point of the heavens which is directly above, or perpendicular to such place; and is, therefore, the elevated pole of the horizon. The Nadir is the depressed pole of the horizon, or the point diametrically opposite to the zenith.

8. A Vertical is a great circle passing through the zenith and nadir, intersecting the horizon at right angles.—A vertical is frequently called an azimuth circle; and when it cuts the horizon in the east or west points, it is then called the prime vertical:—it may also be called a circle of altitude.

9. The Horizon is a great circle which is equally distant from the zenith and nadir, and which divides the upper from the lower hemi

sphere; this is called the true, or the rational horizon, because it passes through the centre of the earth. The apparent or visible horizon is the utmost apparent view which the eye of an observer can take either at sea or on land. The sensible horizon is that which terminates our view; and it is represented by that great circle which we see in fine clear weather, when the sky, or the azure vault of heaven, seems to rest upon the earth or sea :-and as it may be represented by a plane, passing through the eye of an observer, perpendicular to a plumb-line hanging freely; it is therefore parallel to the plane of the true or rational horizon which passes through the earth's centre.

10. The Ecliptic is a great circle in the heavens, through which the sun appears to move amongst the fixed zodiacal stars; but, in reality, it is the great circle in which the annual revolution of the earth is performed hence it represents the earth's orbit. It is denominated the ecliptic, because the solar and lunar eclipses cannot take place unless the moon is either in or adjacent thereto; viz. except the moon be either in or very contiguous to the circle of the earth's annual path round the sun. The ecliptic cuts the equinoctial in an angle of 23:27:38%, which is called the obliquity of the ecliptic:-this angle is constantly diminishing, at the rate of about 50 seconds of a degree in a century. Some of the modern astronomers have endeavoured to set limits to the diminution of the angle of obliquity, by asserting that it will stop at 20:34; after which it will begin to increase at the rate of 50 seconds in a century, until it arrives at an angle of 27:48" :-this, however, is but a mere fanciful hypothesis.

The ecliptic, or the earth's orbit, like all other great circles, is divided into 360 degrees; and it is further divided into twelve equal parts, called signs; the names and characters of which are as follow, viz.

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The signs in the first, or left-hand division of the above, are called northern signs, because they are to the northward of the equinoctial:when the sun is in any of these, his declination is north.-Those in the second, or right-hand division, are denominated southern signs, because they lie on the south side of the equinoctial :-when the sun is in any of these, he is said to have south declination.

11. The points in which the ecliptic and the equinoctial intersect each other, are called the Equinoctial Points, or, the equinoxes ;-the sun-

enters one of these on the 21st March, and the other on the 23rd September. Those points of the ecliptic which are equidistant from the equinoxes, are called the solstitial points, or the solstices;-the sun enters one of these on the 21st June, and the other on the 21st December: hence, the first is named the summer, and the other the winter, solstice. The great circle which passes through the equinoctial points and the poles of the earth or heavens, is called the equinoctial colure; and that which passes through the solstitial points and the said poles, is called the solstitial colure.

12. The Declination of the sun, moon, star, or planet, is an arc of the celestial meridian intercepted between the equinoctial and the centre of the sun, moon, star, &c.; and it is called north or south, according as the sun, &c. &c. is situate with respect to the equinoctial. And the celestial meridian may be called a circle of declination; because it is a semicircle which passes through the centre of a heavenly body, cuts the equinoctial at right angles, and terminates at the poles.

13. The Tropics are two small circles, or parallels of declination; they are parallel to the equinoctial, from which they are distant 23:27:38%.-The tropic of Cancer is on the north side of the equinoctial, and the tropic of Capricorn on its south side.

14. Polar Circles are two small circles, parallel to the equinoctial; from which they are distant 66:32:22"; or, at the distance of 23:27:38" from each pole of the heavens or earth.

15. The Right Ascension of a heavenly body is an arc of the equinoctial, intercepted between the circle of declination passing through such body and the first point of Aries, measured according to the order of the signs or, it is the angle at the pole of the heavens which is contained between the vernal equinox and the celestial meridian, or circle of declination, which passes through the celestial body.

16. The Right Ascension of the Meridian, or mid-heaven, signifies that point of the equinoctial which comes to the meridian of any place at a given time.

17. The Culminating Point of a star, or planet, is that part of its orbit which, on any given day, is the most elevated. Hence, a star is said to culminate when it comes to the meridian of any given place; because then its altitude is the greatest. The culminating, time of transit, and meridional passage of a celestial object, are synonymous terms; each of which signifies the passage of a heavenly body over the meridian of any given place.

18. The Geocentric Latitudes and longitudes, right ascensions, and declinations of the planets, are their latitudes and longitudes, right ascensions, &c., as seen from the centre of the earth.

19. The Aphelion is that point in the orbit of the earth, or of any

other planet, which is most remote from the sun :-this point is called the higher Apsis.

20. The Perihelion is that point in the orbit of the earth, or of any other planet, which is most adjacent to the sun. This point is called the lower Apsis, being diametrically opposite to the aphelion or higher Apsis.

21. The Line of the Apsides is a straight line joining the higher and the lower apsis of the earth, or other planet ;-or, it is the imaginary straight line which is drawn between the centres of the sun and the earth :-hence, it may be supposed to attach the earth to the sun ;—it lies evenly between the aphelion and the perihelion points of the earth's orbit.

22. The Radius Vector of the earth is an imaginary right line, which extends from the centre of the earth to that focus in the transverse diameter of its orbit, in which the sun is posited at any given time ;—or, it may be considered as being the true line of distance between the centres of the earth and sun :-hence, it may be called the semi-line of the apsides.

23. An Occultation is the obscuration or hiding from our view, any fixed star or planet by the interposition of the moon, or by that of the body of some other planet.

24. The Transit of the first Point of Aries, signifies the instant in mean time when the true point of intersection, which is made by the ecliptic and the equinoctial at the vernal equinox, is on the meridian of the Royal Observatory at Greenwich.

25. The Nodes.-These are the two opposite points in the ecliptic, that are intersected by the orbit of the moon or of a planet. The point where the moon &c. appears to ascend from the south to the north side of the ecliptic, is called the ascending Node, and is marked thus and the opposite point, or that where the moon &c. appears to descend from the north to the south side of the ecliptic, is called the descending Node, and is marked 8.

26. The Aspect of the Stars or Planets.-This signifies their situation with respect to each other. There are five aspects, viz. 6, conjunction, when they are in the same sign and degree; ✶, sextile, when they are two signs, or 60 degrees distant; □, quartile, when they are three signs, or 90 degrees, viz. the fourth part of a circle, distant; A, trine, when they are four signs, or 120 degrees, or the third part of a circle, asunder; and 8, opposition, when they are six signs, or 180 degrees, or the measure of a complete semicircle from each other.

EXPLANATORY ARTICLES: for illustrating and reducing to familiarity all the essential points and expressions in the New Nautical Almanac.

Of Time.

1. Time is divided into periods, cycles, years, months, weeks, days, hours, minutes, and seconds. But as it is the year and the day that chiefly concern the navigator; it is of these that we shall now particu larly treat.

2. A Year is that period or portion of time, in which a revolution of the earth round its orbit, with respect to some fixed point in the heavens, is completed.

3. A Solar, or Tropical Year, is the interval of time between two consecutive returns of the earth to the same equinoctial, or to the same solstitial, point;-or, it signifies the time that the earth takes in moving round the ecliptic, from the first point of Aries, till its periodical return to the same point again :—this year consists of 365 days, 5 hours, 48 minutes, and 48 seconds.

4. A Sidereal Year is the interval of time between two consecutive returns of the earth to the imaginary straight line, which is conceived to pass through the centres of the earth and sun, and continued to a given fixed star in the firmament of heaven :-this year consists of 365 days, 6 hours, 9 minutes, 11 seconds; being 20 minutes and 231 seconds longer than the solar year.

5. As the length of the sidereal year cannot be clearly comprehended without a competent knowledge of the recession of the equinoctial points, commonly called "the precession of the equinoxes," I shall therefore endeavour to present the reader with a familiar view of the retrograde motion of the equinoctial points: for it is upon this that the absolute length of the sidereal year is entirely dependent.

In the following diagram let A B C D represent the orbit of the earth;, the place of the earth at any given time; S, sun, near the centre of the earth's orbit :-let the arc, F H, represent a portion of the firmament of heaven, beyond the sun, and * a, the place of a fixed star. Let the black line, or radius vector of the earth, ✪ S, be extended to the equinoctial colure, and thence to the fixed star at a; and let the three objects, viz. the earth, sun, and star, be in this line or colure on the 21st of March in any year. Now, while the earth is progressing round its orbit from west to east, viz. from the point ✪ according to the direction of the letters and characters EwAb, &c., &c., the equinoctial points and are moving slowly in a contrary direction, viz. from east to west; and thus, by the time that the earth has arrived at the point of the ecliptic, ', from which it set out; the equinoctial point will have fallen back to b, and to e; which

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