in the circumpolar constellation of the Lesser Bear, and may be found by its direction from two remarkable stars, called the Pointers, in the constellation of the Great Bear.

This constellation is always above our horizon, and is commonly known as the "Plough" or "Charles' Wain" (Waggon).

12. Q. Which are the stars called the Pointers?

A. The four stars forming a quadrangle in the constellation of the Great Bear are popularly taken to represent the wheels or body of the waggon. Of these the two stars forming the supposed back of the waggon are the Pointersone (Beta in the Figure) being called Merak, and the other (Alpha) Dubhe.

The pole-star is at present about 1° 21' from the celestial pole, and is approaching still nearer. It has not always been, nor will it always continue to be, the pole-star. The five stars to the left in the figure are the chief stars in the constellation of Cassiopeia-"The Lady in the Chair." They are in form not unlike a W, and are situated on the opposite side of the pole-star from the "Plough" (Lesson 14).

13. Q. How is the Pole-Star situated with respect to the Pointers?

A. A straight line drawn through the Pointers

(from Beta to Alpha) and continued about 29° beyond Dubhe would pass near the Pole-Star. The distance between the Pointers is about 5°, or nearly 10 times the moon's apparent diameter, and therefore Dubhe is about 6 times farther from the polestar than from Merak (Lesson 8).

LESSON 2.-ASTRONOMICAL TERMS.

1. Q. What is the Horizon of an observer ?

A. The Sensible Horizon is the circle in which the earth and heavens seem to meet; or that circle which would be marked out in the heavens by a plane extending all round from the observer's station to the region of the stars. The Rational Horizon would be marked out by such a plane passing through the earth's centre. The two opposite points in the heavens, called the Zenith and the Nadir, are the Poles of the horizon.

2. Q. In what cases is the distinction between the Sensible and Rational Horizons of importance?

A. The distinction is important only for comparatively near objects. With regard to the stars the sensible and rational horizons are the same, because, from the centre and from the surface of the earth, the stars would be seen in the same relative positions and, from the two stations, the same portion of the heavens would be visible.

3. Q. What is meant by the Equator?

A. The Earth's Equator is an imaginary line round the earth midway between the poles; or it is that circle of the earth whose plane passes

through the centre and is perpendicular to the earth's axis. This plane, produced indefinitely in all directions, would mark out a great circle of the heavens which is called the Celestial Equator or the Equinoctial.

4. Q. How is the position of any place on the earth indicated?

A. The position of any place is determined by its Latitude and Longitude. By Latitude is usually meant the angular distance North or South from the equator as measured on the meridian of the place; but, since the earth is not a perfect sphere, the altitude of the elevated pole is more correctly the latitude (Lesson 3).

5. Q. What is the Meridian of a place?

A. The Terrestrial Meridian of any place is a great circle of the globe passing through both its poles and through the place itself. A spectator's Celestial Meridian is that great circle which would be marked out on the sphere of the heavens by the extension of the plane of the terrestrial meridian, on all sides, to the region of the stars.

6. Q. What is meant by a Great Circle of the Sphere? A. A Great Circle is one whose plane passes through the centre of the sphere, or it is the greatest circle which can be described on the sphere. If an orange be divided into two halves by a straight cut through the centre, the cut will be bounded by a Great Circle, and the flat surface of either half may be considered the Plane of that circle.

7. Q. How is Terrestrial Longitude measured?

A. Longitude is measured in degrees and parts of a degree East and West from the meridian of

some fixed station. The meridian chosen is called the First Meridian, and is different for different countries-the First Meridian for England passing through Greenwich, and that for France through Paris.

8. Q. How is the position of a star indicated?

A. The situation of any celestial object may be described by giving its Declination and Right Ascension which correspond to terrestrial latitude and longitude, or by giving its Latitude and Longitude. Declination is the angular distance of an object, north or south, from the Equinoctial; Celestial Latitude is the distance north or south from the Ecliptic?

9. Q. What is the Ecliptic?

A. The Ecliptic is the sun's apparent path, or that great circle. in the heavens which the sun appears to describe once a year. Part of this circle lies above and part below the Equinoctial to which the Ecliptic is inclined at an angle of about 23° 28' (Lesson 31).

Just as the position of a point in a plane is defined by its distance from two other points properly situated, so the position of a body on the surface of a sphere-such as that of the heavens-is defined by its distance from two great circles of Q the sphere at right angles to one another. If the Figure represent the sphere of the heavens, Pp the poles of the equinoctial, and P' p' the poles of the ecliptic -P being nearly 23 degrees from P'-then the circle EQ will be the Equinoctial, and E C the ecliptic. P is a quarter of a circle (90°) distant from every point of the equinoctial E Q, and P'is the same distance from

every point of the ecliptic E C. The points e and 1, where the ecliptic and equinoctial cut each other, are the Equinoctial Points, and the line joining these points is the Line of the Equinoxes.

A great circle drawn through the pole P and the star 8 will cut the equinoctial in the point R. From the Vernal Equinox e to the point R is the Right Ascension of S, and R S is its Declination. The arc P S, the Polar Distance of S, is the complement of the Declination. Thus if the Declination R S be 62° the Polar Distance P S will be 28° (=90°-62°). Similarly if a great circle through P' and 8 cut the ecliptic in L, the arc e L will be the longitude and LS the latitude of S.

10. Q. How are Right Ascension and Celestial Longitude reckoned?

A. Both are reckoned from the Vernal Equinox, but Celestial Longitude is measured on the Ecliptic, and Right Ascension is reckoned on the Equinoctial. By all astronomers Right Ascension is reckoned from the Vernal Equinox, eastwards, ali round the circle from 0° to 360°, or it expressed in time from 0 hours to 24 hours.

may be

11. Q. Why may Right Ascension be expressed either in angular measure (degrees, minutes, &c.), or in time (hours, minutes, &c.) ?

A. In 24 hours the whole circle (360°) of the heavens passes over the meridian, and therefore 15° must pass over it in 1 hour, and 1° in 4 minutes. Hence degrees, minutes, and seconds of Right Ascension may be converted into hours, minutes, and seconds of time; thus 35° 12′ Right Ascension equal 2h 20m 48a, and 290° 11′ 15′′ R. A. equal 19h 20m 458.

12. Q. What is the Vernal Equinox ?

A. The Ecliptic being partly above and partly below the Equinoctial the two circles cut one another at two opposite points, and these points are called the Equinoxes or Equinoctial Points. The Vernal Equinox is the point at which the sun crosses the Equinoctial (in March) when passing from south to north, and the opposite point is called the Autumnal Equinox.