1. ASTRONOMY is the science, which treats of the

stars.

It is a branch of mixed Mathematics, or Natural Philosophy.

2. The stars are the heavenly bodies, that shine by emitting or reflecting light, namely, the sun, planets, comets and fixed stars.

3. Plane or pure Astronomy is the part, which determines the magnitudes, distances, motions and orbits of the said. bodies, and deduces them from observations on the appearances.

4. Physical Astronomy is the part, which investigates the causes of the celestial motions, and reasons analogically from the principles and laws of motion, which are found to govern terrestrial bodies.

5. By spherical Astronomy is here meant that part, which consists in the application of the principles of Spherics to Astronomy.

An observer on the earth conceives himself to be in the centre of a vast concave sphere, and all the celestial bodies to be situated on its surface. And it is evident, that, were his observatory in any other part of the Universe, the same spherical appearance of situation would be exhibited to his view. This sphere is represented by the celestial globe, and on its surface the various motions are apparently performed. It is evident then, that Spherics are extensively applicable to the explanation and determination of celestial phenomena.

6. The apparent celestial sphere, being supposed concentric with the earth, has corresponding circles. Thus, the planes of the terrestrial equator, meridians, horizon, polar circles, and tropics, being extended to the heavens, mark there the corresponding circles of the celestial sphere.

7. The Ecliptic is the circle, which the sun apparently describes in a year. It cuts the celestial equator or cquinoctial in two opposite points, called the equinoctial points; one of which is the first point of Aries, or vernal equinox ; and the other the first of Libra, or autumnal equinox. And the acute angle at this intersection, which is about 23° 28', is called the obliquity of the ecliptic. The ecliptic is divided into twelve equal parts, called signs, which are v Aries, 8 Taurus, Gemini, Cancer, & Leo, my Virgo, Libra, m Scorpio, Sagittarius, vs Capricornus, Aquarius, Pisces.

3. The tropic of Cancer touches the ecliptic in the first point of Cancer, and that of Capricorn touches it in the first point of that sign. These points of contact are called the solstitial points; the former being the summer, and the

latter the winter solstice.

9. The signs on the north side of the equinoctial are called northern, and those on the south side the southern signs. Also six of the signs in order, beginning with Capricorn, are called ascending, and the other six descending signs.

10. Motion according to the order of the signs is said to be forward, direct, or in consequentia; but, when contrary to this order, backward, retrograde, or in antecedentia.

11. The highest point of the ecliptic is called the Nonagesimal degree, being 90° from each of the intersections of this circle with the horizon,

12. The point of the ecliptic, which is on the meridian, is called the culminating point, or medium cœli; and is the distance of the meridian from the vernal equinox.

13. The zodiac is a zone extending about 8° on each side of the ecliptic, and bounded by two small parallel circles. Within this zone all the motions of the planets are performed,

14. A secondary to a great circle is a great circle passing through its poles, and consequently perpendicular to it.

15. Meridians, which with respect to the earth are circles of terrestrial longitude, called also circles of right ascen sion and hour circles, are secondaries to the equator or equinoctial.

16. Circles of celestial longitude are secondaries to the ecliptic.

17. The zenith and madir are the poles of the rational horizon; the former being in the direction of a perpendicular above it, and the latter in the opposite direction below it.

18. The vertical or azimuth circles are secondaries to the horizon.

19. The

19. The altitude of a star is the arc of a vertical circle, contained between the star and the horizon.

nith distance is the complement of the altitude.

And the ze

20. Almacanter is a small circle parallel to the horizon. The sensible horizon is an almacanter, whose plane touches the earth at the station of the spectator, and terminates his view.

21. Longitude of a star is the arc of the ecliptic, contained between the vernal equinox and the star or its circle of longitude, reckoned in the order of the signs. If the body be in our system, the longitude, viewed from the earth, is called the geocentric longitude; viewed from the sun, the heliocentric longitude.

22. Latitude of a star is the arc of its circle of longitude, contained between the star and the ecliptic. If the body be in our system, the latitude, viewed from the earth, is called the geocentric latitude; viewed from the sun, the heliocentric latitude,

23. Parallels of celestial latitude are small circles parallel to the ecliptic.

24. Declination of a star is the arc of its meridian, contained between the star and the equinoctial. Declination and parallels of declination correspond to terrestrial latitude and parallels of latitude.

25. Right ascension of a star is the arc of the equinoctial, contained between the vernal equinox and the star or its meridian, reckoned in the order of the signs.

26. Ascensional difference is the arc of the equinoctial, contained between the star's meridian and the point, that rises with the star. Or, with respect to the sun, it is the angle at the pole, formed by the sun's meridian and the 6 o'clock meridian.

27. Oblique

27. Oblique ascension is the sum or difference of the right ▾ ascension and ascensional difference. Or, it is the arc of the equinoctial, contained between the vernal equinox and the point, that rises with the star. And the oblique descension is the arc of the equinoctial, contained between the vernal equinox and the point, that sets with the star.

28. Azimuth of a star is the angle at the zenith, formed by the star's vertical or azimuth circle and the meridian of the place. Or, it is the arc of the horizon, contained between the meridian and the star's vertical circle.

29. Amplitude of a star is the angle at the zenith, formed by the prime vertical and the star's vertical circle. Or, it is the arc of the horizon, contained between the cast or west point and that, where the star's vertical circle intersects it.

30. Angle of position is the angle at a star, formed by two great circles, one passing through the pole of the equi noctial and the other through the pole of the ecliptic.

31. Hour of the day, or hour angle, is the arc of the equinoctial, contained between the meridian of the place and the star's meridian.

32. Nodes are the points, where the orbits of the pri mary planets cut the ecliptic, and where the orbits of the secondaries cut the orbits of their primaries. The ascending node is the point, where the planet crosses, moving northward and the descending the point, where it crosses moving southward.

33. Two heavenly bodies are in conjunction, when they have the same longitude; in opposition, when the difference of their longitudes is 180°; in quadrature, when the difference is 90°. And syzigy denotes either conjunction or opposition.

NOTE.