Francisco. 8,000 miles, is very minute in comparison with the distance from the earth to any fixed star, for the latter distance is expressed by many millions of millions of miles. In consequence of this, the angular distance between any two stars always appears the same, wherever the observer may be on our planet. If an astronomer in Boston were to measure the Boston and San angular distance between Polaris and one of the Pointers, with the most perfect instrument ever devised for such work, and another astronomer in San Francisco were to make a similar measurement, the two results would agree if the observations were free from error. This remark applies only to the fixed stars, and is not true of the moon or the planets, which are much nearer to us. For all our naked-eye observations we may therefore assume that the eye of the observer is located in the center of the celestial sphere, and that all of the fixed stars are fastened to the sphere, turning with it as it turns. We are thus taken back to the crystal spheres, studded with golden nails, with which the ancient Greeks dealt. Center of the sphere. Rotation of the sphere. A sleeping car. We may imagine the moon, the planets, and comets to be likewise located on the inner surface of the sphere, but to be endowed with powers of locomotion, so that they can move about among the golden nails. Remembering then that we are in the center of the celestial sphere, we ask the question, "How does the star-sphere appear to turn?" In answering this we have recourse to the cause of the apparent turning, which is the spinning of the earth upon its axis with such evenness of motion that we experience no jar or shock. Every reader has had similar experiences with motions on the earth's surface. A sleeping-car passenger awakes suddenly in the middle of the night, and concludes by the comparative silence and the absence of noticeable jarring that his train is stopping at some station. Looking out of the window he sees a freight train apparently slowly backing on the next track. The truth is that the freight train is at rest, while his own train is just starting up. A passenger steamer leaves Chicago at night; having a steamer. gotten fairly out of the harbor, it turns in order to head in a certain direction. While it is turning the lights of the city and the stars in the sky appear to the passengers to be revolving in the opposite direction to that in which they themselves are turning. Conceive the axis of the earth to be prolonged till it strikes the celestial sphere. The north end of the axis strikes near Polaris, at a point called the north celestial pole. The south end strikes at the opposite point of the celestial sphere, called the south celestial pole. A straight line joining these two points is the axis of the celestial sphere, about which it appears to rotate. If there were a bright star at each pole, and we could see both of them at the same time, we should have little difficulty in getting an accurate idea of just how the heavens rotate. The poles. rotation. A line drawn from the eye of the observer parallel to the earth's axis, and prolonged to the celestial sphere, The axis of would strike so near the centers of the stars, which we have imagined to be at the celestial poles, that no astronomer could measure the deviation. We are therefore entirely justified in laying down the following principle to guide our thinking in this matter of the apparent daily rotation of the star-sphere : The star-sphere appears to turn once a day about an axis drawn from the observer's eye to the north celestial pole, which is in the vicinity of Polaris. We may now locate the north celestial pole more acThe north pole. curately than by saying that it is in the vicinity of Polaris. The star which is situated at the bend of the handle of the Great Dipper is called Mizar. Let the eye travel slowly from Polaris directly toward Mizar; when it has gone a distance equal to one fourth of the distance between the Pointers, it has reached the north celestial pole. Observations and records. The first drawing. Polaris and the But the explanation which has just been given does not suffice for our needs. The motion of rotation can be well grasped only by repeated observations of the heavens. Since we now purpose to get acquainted with the heavens, gaining knowledge which will be a source of delight throughout life, we must not only observe, but also record some of our observations, that they may be the better fixed in mind. A common blank book will answer our needs. A picture of the Great Dipper is first to be drawn. We get it and Polaris well in mind by looking at them a minute or two. Polaris is considerably brighter than any other star within fifteen degrees of it, and is almost directly north of us, about half way from the horizon to the zenith. It is also at the end of the handle of the Little Dipper, which is shown in the figure. The distance from Polaris to the furthest corner of the bowl of the Little Dipper is nearly twenty degrees, and the curved handle is about twelve degrees in length. We first locate Polaris on a page of the blank book, and then draw a faint line directly down from it, to represent a vertical line; we also draw a horizontal line similarly. These are only to assist in getting the Dipper correctly located. The Pointers are next drawn, care being taken that the distance from Polaris to the nearest Pointer shall be five times the distance between the Pointers. Then come the other two stars of the bowl in their proper relative positions, and lastly the handle. After this the Little Dipper may be drawn. The picture now resembles Fig. 11, except that the vertical and horizontal lines may not lie in the same positions with reference to the stars as in the diagram, and that the dotted lines have not been drawn. The date of observation and the time (within five minutes) when the drawing was finished are recorded. If the drawing was made early in the evening, another similar one should be made just before retiring for the night. A comparison of the two will show that the Dippers have shifted their positions with reference to the vertical and horizontal lines. After watching the Dip L FIG. II.-THE TWO DIPPERS. pers Is Polaris as bright as either of the Pointers? Is any star in the bowl of the Little Dipper brighter than the Queries. faintest in the bowl of the Great Dipper? How many stars can be seen within the bowl of the Great Dipper? There is a faint star, called Alcor, which is within a degree of Mizar; what is its color? The distance from Alcor to Mizar is what fraction of a degree? What is the color of each of the Pointers (white, yellowish, reddish, bluish)? Is the Great Dipper higher up late in the evening than early? At some time during the night How the Motion of other stars. would the bowl of the Great Dipper be near the zenith ? If so, would the handle be east or west of the bowl at that time? At about what time on the day of observation was the bowl underneath Polaris? Where was the bowl of the Little Dipper with reference to Polaris, when the large bowl was underneath? If a watch were held between your eye and Polaris in such a position that you looked squarely at its face, would the extremity of the minute hand travel around the face in the same direction in which the Dippers go around the pole-star, or in the opposite direction? Twelve hours after the time of your first observation where would the Great Dipper be with reference to Polaris? Does Mizar keep at the same distance from Polaris? Does the bowl of the Great Dipper ever disappear below your horizon? Does this bowl move downward, when at the left of the pole-star as you face it? If it were below Polaris would it appear to be moving toward your right as you face it? Is there any time during the twenty-four hours which are consumed by a revolution of the star-sphere when Alcor appears to be exactly in line between Mizar and Polaris? Did you ever see the moon close to either Dipper? If you turn your back on the pole-star and face southward will a star off in the south appear to be traveling toward your right? If you face westward and look up at a star near the zenith, will that star be moving westward down the vault of the sky? Will its distance from Polaris apparently alter as the hours of the night roll on? Will the star slide straight down the sky, as if endeavoring to reach the horizon by the shortest path, or will it veer off toward the north? A star has just risen close by the east point of the horizon; as it climbs the sky will it go straight toward the zenith, or will it veer off toward the south? Are there any stars except Polaris and those |