Astronomy in a nutshell, the chief facts and principles explained in popular language for the general reader and for schools . ne of the ecliptic. But,in consequence of its spinning motion, theearth resists this pull, and tries, so to speak,to keep the inclination of its axis result is that the axis swings slowly roundwhile maintaining nearly the same inclinationto the plane of the ecliptic. Here, again, we may employ the illustrationof a top. If the peg of the top is tipped alittle aside, so that the attraction of gravi-tation would cause the top to fall flat on thetable if it w
Astronomy in a nutshell, the chief facts and principles explained in popular language for the general reader and for schools . ne of the ecliptic. But,in consequence of its spinning motion, theearth resists this pull, and tries, so to speak,to keep the inclination of its axis result is that the axis swings slowly roundwhile maintaining nearly the same inclinationto the plane of the ecliptic. Here, again, we may employ the illustrationof a top. If the peg of the top is tipped alittle aside, so that the attraction of gravi-tation would cause the top to fall flat on thetable if it were not spinning, it will, as longas it continues to spin, swing round and roundin a circle instead of falling. We cannotenter into a mathematical explanation ofthis phenomenon here, but the reader willfind a clear popular account of the wholematter in Prof. John Perrys little bookon Spinning Tops. It is sufficient hereto say that the attraction of gravitation,tending to make the top fall, but reallycausing the peg to turn round and round,resembles, in its effect, the attraction of thesun and the moon upon the equatorial. The Milky Way about Chi Cygni Photographed at the Lick Observatory by E. E. Barnard, with the six-inch Willard lens. Observe the cloud-like forms. THe Zodiac and Equinoxes 59 protuberance of the earth, which makes theearths axis turn round in space. Now, as we have said, this slow swinginground of the axis of the earth produces theso-called precession of the equinoxes. In aperiod of about 25,800 years, the axis makesone complete swing round, so that in thatspace of time the celestial poles describea revolution about the poles of the ecliptic,which remain fixed. But since the equatoris a circle situated half-way between the poles,it is evident that it must turn also. Toillustrate this, take a round flat disk of tin,or pasteboard, to represent the equator andits plane, and perpendicularly through itscentre run a straight rod to represent theaxis. Put one end of
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