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26 How the Heavens Appear to Us. The heavens appear to us like an immense dome that turns slowly over our heads. The stars seem to. be tiny lights that are fastened on the inside of this dome. Tonight we might make a map of all the stars that appear to us at a certain hour and tomorrow night we would find these same stars back again. But they would be one degree farther to the westward each night. But they would keep the same form--the dipper would always resemble a dipper, etc. Occasionally, however, we see a very bright star that doesn't seem to stay in any particular place. It wanders around. This is a planet and its movement among the stars is caused by its revolution around the -sun. We can't stand off in the distance and watch the actual movements of these worlds and therefore we can't make them seem as they really are. We are passengers on one of these moving planets and we can't see our earth move. The daily whirl of the earth makes everything in the heav- ens seem to rise in the east and set in the west. And the yearly revo- lution of the earth around the sun makes these heavenly bodies _ rise and set at a different time each night. The stars all come in regular time-- four minutes earlier each night--but the rising time of the sun, planets and moon is different every night. 'The -~sun remains. fixed; of: course, but the revolution of the earth around the sun makes the sun appear to move just as a planet does, There- fore the sun changes its position among the stars just as a planet does, How Astronomers Locate Heavenly Bodies, A navigator locates a place on the earth by its latitude and longitude. An astronomer locates a_ heavenly body by its declination and right as- ccension. For purposes of measure- ment and location of heavenly bodies the sky is divided up with lines of latitude and longitude just the same as the earth is. But the terms dec- lination and right ascension are used instead of latitude and longitude. All calculations are made on. the under- standing that the sky is just as it looks to us--an immense dome over our heads. In a former chapter we THe Marine REVIEW "Modern Methods of Lake Navigation for the - Beginner. bo BY CAPT, GEORGE TRIMBLE. explained how the lines of latitude and longitude could be drawn on a ball. We will now take this ball, with all the lines of latitude and longitude on it, and stick a knitting needle through it at the poles. See Fig. 35, FIG, Oo; In order to make our model com- plete we must enclose this ball with- in another larger ball or globe. A perfectly round glass globe would do splendidly for the experiment. It must be made to turn on the knitting needle so that the north pole in the heavens will be directty over the earthly north pole. The inside of this glass globe will represent the heavens, as they appear to us. Spots to represent stars can be painted on ie inside of the slobe, In . Fig. 36 we show our model as it now appears but with only half the outer globe in view, You will notice that the heavenly north pole is directly over the earthly north pole--the knitting needle shows that. Then the heavenly equator js drawn directly over the earthly equa. tor, It you. were to. stand Gp the earth's equator and look straight up you would be looking at the place in the sky where astronomers have drawn the celestial equator, We told you before that all places on the earth that are north of the equator are in north latitude. You can now under. stand that all heavenly bodies that are north of the celestial equator are in north declination. Declination simply means heavenly latitude, We have told you before that the earth is generally divided up by 24 standard meridians of longitude, These meridians are lines drawn from the north to the south on the earth. We will now divide the heavens in the same manner, You can call these lines by several names--either merid- ians of right ascension, meridians of heavenly longitude or hour-circles. They are generally called hour-circles. There are 24 of them and as the sun takes 24 hours to traver around the earth then she reaches one of these hour circles every hour. Now, for measuring longitude on the earth we had to have a first meridian, or start- ing point, and this was placed at Greenwich, Eng, Now we must also have a first hour-circle in the heavens. This heavenly first meridian is placed at the spot where the sun crosses the heavenly equator when she comes north on the 21st of March, each year. We all know that the sun. comes north in the summer and goes back south in the winter. Or, correctly speaking, she appears to do so and all calculations of heavenly bodies are made with the understanding that they do as they appear to. The rush of the earth around the sun makes the sun appear to move because it is not possible for us to realize that the earth is moving. The place where the sun crosses the line on the 21st of March is called the First Point of Aries. The first heavenly meridian or hour-circle cuts through here and goes north and south, to each pole. All measufe= ments in the heavens start here. Hf any heavenly body is on this line its right. ascension is 0 hours, 0 minutes, O seconds. Right ascension is figured