upon the celestial sphere, let one leg be fixed pointing towards the pole of the heavens while the other is directed towards a star more distant from the pole than the pole is from the horizon. This star will describe a part of its daily or diurnal circle above the horizon, and part below it. Increase the angle between the legs, still keeping one extremity fixed at the pole, and a point will be reached when one leg is perpendicular to the other. Stars at this angular distance from the pole are on the celestial equator. One half of their daily circle of travel is performed above the horizon, the other half below it. They are the only bodies which rise due east and set due west all the year round. We have, in fact, stars north of the equator which rise north of east and set north of west, and stars south of the equator which rise south of east and set south of west. The first class are longer above than below the horizon, when observed in England; in the second class the reverse in the case, for stars belonging to it are longer below the horizon than above it. A third class, included in the circle of perpetual invisibility, never appear above our horizon. The saving clause "in England" frequently used in the foregoing description of the apparent daily motions of the stars, needs some explanation. It is natural to assume that the movements would appear the same when viewed from any place on the earth, but, as a matter of fact, this is not the case. The changes in the sky first noticed by an observant traveller journeying southwards for a considerable distance, is that the Pole-Star sinks towards the northern horizon, while new stars make their appearance in the south. The oblique paths familiar to all who watch the heavens become more and more upright until, when at the equator, all the stars are seen to rise straight up on the eastern horizon, and set in a similar manner on the western. Before this FIG. 6. To explain why the altitude of the Pole-Star varies with latitude, and also to illustrate the apparent paths described by stars when viewed at the poles, at London, and at the equator. point is reached, however, the Pole-Star will have sunk into the mists of the northern horizon. If the journey is made towards the North Pole of the earth, instead of being in the direction of the equator, a different appearance is again noted. Night after C night, the Pole-Star would appear higher above the horizon. The circle of perpetual apparition would thus daily increase in size. Stars which rose and set in the southern sky would disappear, while others which described part of their daily circle below the horizon would rise to the position of those which never set. And, could the North Pole be reached, the observer would see the Pole-Star overhead. In concentric circles all the stars would appear to travel round the celestial pole, the diameters of the circles increasing in size from this stationary point down to the horizon (Fig. 6). It appears higher The North Pole of the heavens is on the horizon of an observer at the equator. and higher in the sky as a journey is made northwards, and, when the north pole is reached, it will be exactly overhead. There is, in fact, a close connection between latitude and the angular distance of the celestial pole from the horizon. The latitude, or distance from the equator, of any spot on the earth is equal to the altitude of the celestial pole at that place. Hence, wherever you may be, a rough determination of the latitude of the place can be made by noticing the height of the Pole-Star above the horizon. SYNOPSIS OF CHAPTER I The number of stars visible to the naked eye at any one time is about 2500. With the largest telescopes 100,000,000 can be seen. Constellations are groups into which astronomers have arranged the stars according to their place on the sky. Ptolemy (140 A.D.) named forty-eight groups, and about twenty names since added are now accepted. The names of the brightest stars visible from England are as follows: a Canis Majoris or Sirius a Bootes or Arcturus. B Orionis or Rigel a Aurigae or Capella a Lyrae or Vega a Canis Minoris or Procyon a Orionis or Betelgeuse a Eridanus or Acharnar a Tauri or Aldebaran a Scorpii or Antares a Piscis Australis or Fomalhaut B Geminorum or Pollux a Leonis or Regulus a Geminorum or Castor The relative positions of stars remain seemingly unaltered throughout the year, but the configurations change in centuries owing to the fact that each star is rapidly moving through space. The heavens appear to rotate once in a little less than twentyfour hours round two points known as the celestial poles. The Pole-Star is near the north celestial pole, but the position of the south celestial pole is not similarly distinguished. The celestial poles are the two points in the sky possessing no diurnal motion; the celestial equator is midway between them. The apparent motions of the stars differ in character when observed from different places on the earth. To a person at the equator the stars seem to rise straight up and set straight down; at the poles the stars describe circles parallel to the horizon. In middle latitudes there are (1) stars which never set; (2) stars which never rise; (3) stars which both rise and set. A determination of latitude on the earth can be made by observing the altitude, or angular distance, of the pole above the horizon. Right Ascension is analogous to terrestrial longitude, and is reckoned along the celestial equator from a certain point in the sky. CHAPTER II THE EARTH AS A SPINNING GLOBE To primitive Man, as to a child, nothing could appear more convincing than that the earth was a vast plain upon which a solid black vault was resting. FIG. 7.-Old idea as to the shape and foundations of the earth. His world was bounded by his horizon, and the immense proportions of starry space were reduced to the proportions of a dome like that which topped a mighty house. But, with the march of time, men's ideas became enlarged. Travellers found that, however long their journeyings, "the end of the world" and |