It (Fig. 4). A brilliant bluish-coloured star called Vega attracts the attention of the most casual observer. forms a triangle with the Pole-Star and Arcturus. Two fairly bright stars will be seen forming a small obtuse triangle with Vega, and the three represent the chief objects in the constellation Lyra. Cygnus, the Swan, has the form of a cross, and lies close to Lyra. The brightest star in this constellation marks the top of the cross, and makes a rightangled triangle with the Pole-Star and Vega, itself being situated in the right angle. The bright star Altair, in the constellation Aquila, can be seen on the meridian about ten o'clock in the beginning of September. A line from the Pole-Star, through the cross of Cygnus, if continued for about the same distance towards the south, leads to Altair-the middle of three stars in a line-all of which belong to the constellation of the Eagle. An extremely well-marked group, covering an enormous area of sky, arrests the attention in October. Four stars in the form of a square, known as the Great Square of Pegasus, will be visible when looking. towards the south about ten o'clock (Fig. 5). The group is so conspicuous that a directing line is hardly required to point it out. Should there be any difficulty, however, carry a line from the Pole-Star past the brightest side of the W in Cassiopeia, and the square will be found. By connecting the two stars which form the western edge of the square, and producing the line southwards for about three times the distance, Fomalhaut, a solitary bright star in the constellation of the Southern Fish, is located. The four stars which make up the Great Square of Pegasus do not all belong to this constellation. That in the north-east corner and nearest Cassiopeia is really Alpha Andromedae. A line from the Pole FIG. 5.-Relative positions of the constellations Pegasus, Star, through the side of the W in Cassiopeia most distant from the Pole, indicates the position of this star. And it is worth remark that the stars on this line appear due south at the same time. The line connecting them happens to lie very close to what may be termed the "Greenwich meridian" of the sky, for it is used in much the same way as the meridian which passes through Greenwich Observatory. One represents the starting line from which terrestrial longitudes are counted, the other roughly indicates the position of the zero from which astronomers reckon celestial "Right Ascensions." From a Andromedae, and extending to the northeast under Cassiopeia, a curved line of stars can be made out. The first three stars belong to Andromeda, but that with which the line is terminated is the chief star in the constellation Perseus. Perseus and Andromeda are best seen about the end of November. The brightest star in the former constellation is situated between two fainter stars, one to the north-west, the other to the south-east of it. A remarkable object in this constellation is the variable star Algol. It is in the right angle of a right-angled triangle which it forms with a Persei and y Andromedae. In an interval of a little less than three days Algol periodically blazes out and then fades into comparative insignificance. A line from y Andromedae, if imagined to pass midway between Algol and a Persei, points out Capella in the constellation Auriga. Capella and Algol are at the top of a V-shaped configuration, having Aldebaran in the angle. We have now made a journey round the celestial world, and pointed out some of the chief stars and groups of stars to be seen in the northern sky. It is as if you had been guided through the streets of a great city and shown the remarkable buildings which adorned it. In a superficial survey, many objects of interest are passed in silence, but a framework is erected whereby the student will be able to find his way about the heavens. The forms of the Great Bear, or of Cassiopeia, or of Orion, appear to be the same from year to year. Indeed, the positions of stars relatively to one another remain unchanged to the unaided vision, though each star is really in motion through space. A movement which can be seen, however, and one which must have been noticed by every intelligent observer, is that of the heavens from east to west. Face the north on any clear night and note the positions of the Great Bear and Cassiopeia at an interval of about two hours. The sky will appear to have rotated as a whole in the opposite direction to that in which the hands of a watch move. If the observation is made in the autumn, when the Great Bear is below the Pole-Star and Cassiopeia above it, the former group will be found to have moved towards the east, whilst the latter has experienced a shift westward. Six hours after making the first observation, the constellations will appear to have rotated through a right angle, and twelve hours after they will be seen to have described a semicircle in the sky. The fierce rays of the sun obliterate the light of the stars during the day, but if this were not so, we should see the heavens perform a complete rotation round a point near the Pole-Star in a little less than twenty-four hours. The points about which the rotation appears to take place are called the North and South Celestial Poles. If the Pole-Star were situated exactly at one of these points in the sky, it would remain motionless while all other stars were in apparent rotation. But since this star is at a short distance from the North Celestial Pole, it traverses a minute circle round the stationary point. All other stars behave in a similar manner. If, when facing the north, you imagine one of the legs of a pair of compasses to be fixed in the position of the pole, while the other leg touched any star, then the circle which the compasses can be supposed to trace out in the sky is the circle along which that particular star always appears to travel. In other words, the radius of the circle of travel is determined by the distance of a star from the pole. Now let the legs of the compasses be opened to such an extent that when one extremity touches the pole the other is on the horizon. All the stars embraced by the circle described with this radius are always visible on a clear night, thus earning for it the title of "The Circle of Perpetual Apparition." A circle of the same size drawn round the South Celestial Pole includes within it stars which are perpetually invisible in England. Objects embraced by the former circle do not rise and set like other stars, in fact, they never sink beneath the horizon. Sometimes below the pole, at other times above it, sometimes to the east, and sometimes to the west of it, these "circumpolar" stars and constellations shine throughout the year, and are thereby distinguished from those only visible at certain seasons. So far only the diurnal motions of circumpolar stars have been described. The facts thus obtained must now be extended to the whole sky. Again using a pair of compasses for the measurement of dimensions |