ASTRONOMY. 



towards the poles, because the circles to be 

 described become less ; but in Great Britain it is 

 still nearly 600 miles an hour. We are not 

 sensible of this motion, because we and all our 

 surroundings are carried along with it. It is in 

 this way that the swift, but smooth and noiseless 

 whirl which carries the earth's surface and its 

 inhabitants eastward, makes the starry vault seem 

 to flit past the eyes of these inhabitants towards 

 the west. 



It is the earth's rotation that causes the vicissi- 

 tude of day and night. The earth being a globe, 

 only one-half of it can be in the sun's light at 

 once ; to that half it is day, while the other half is 

 in its own shadow, or in night. But by the earth's 

 rotation, the several portions of the surface have 

 each their turn of light and of darkness. 



Length of a Day. One complete rotation of the 

 -earth does not make a day, in the usual sense. 

 If the time is noted when a particular fixed star is 

 exactly south or on the meridian, when the same 

 star comes again to the meridian the next day, 

 the earth has made exactly one rotation, and the 

 time that has elapsed is called a sidereal day. 

 This portion of time is always of the same length. 

 Sidereal time, or star-time, from its unvarying 

 uniformity, is much used by astronomers. But 

 the passage of a star across the meridian is not 

 a conspicuous enough event for regulating the 

 movements of men in general. It is not a 

 complete rotation of the earth, but a complete 

 alternation of light and darkness that consti- 

 tutes their day. This, which is called the natural 

 or the solar day, is measured between two meri- 

 dian passages of the sun, and is about four 

 minutes longer than the sidereal day. The 

 cause of the greater length is this : When the 

 earth has made one complete turn, so as to bring 

 the meridian of the place to the same position 

 among the fixed stars as when it was noon the 

 day before, the sun has in the meantime moved 

 eastward nearly one degree among the stars, and 

 it takes the earth about four minutes more to 

 move round so as to overtake him. If this east- 

 ward motion of the sun were uniform, the length 

 of the solar day would be as simple and as easily 



determined as that of the sidereal But the 

 ecliptic or sun's path crosses the earth's equator, 

 and is therefore, more oblique to the direction of 

 the earth's rotation at one time than another ; 

 and besides, as the earth moves in her orbit with 

 varying speed, the rate of the sun's apparent 

 motion in the ecliptic, which is caused by that of 

 the earth, must also vary. The consequence is, 

 that the length of the solar day is constantly 

 fluctuating ; and to get a fixed measure of solar 

 time, astronomers have to imagine a sun moving 

 uniformly in the celestial equator, and completing 

 its circuit in the same time as the real sun. The 

 time marked by this imaginary sun is called mean 

 solar tintc; when the imaginary sun is on the 

 meridian, it is mean noon; when the real sun is 

 on the meridian, it is apparent noon. It is obvious 

 that a sun-dial must shew apparent time, while 

 clocks and watches keep mean time. Only in 

 four days of the year do these two kinds of time 

 coincide. In the intervals, the sun is always 

 either too fast or too slow ; and the difference is 

 called the equation of time, because, when added 

 to or subtracted from apparent time, it makes it 

 equal to mean time. The mean solar or civil day 

 is divided into twenty-four hours, the hours into 

 minutes and seconds. A sidereal day, we have 

 seen, is shorter ; its exact length is 23 hours, 56 

 minutes, 4 seconds of mean solar or common 

 time. Astronomers divide the sidereal day also 

 into twenty-four hours, which are, of course, 

 shorter than common hours. In the course of a 

 civil year of 365 days, the earth turns on its axis 

 366 times, or there are 366 sidereal days. 



The earth, then, gliding noiselessly and steadily 

 round on its axis, is the great time-keeper, and 

 the heavenly bodies are the pointers or indices by 

 which we note its progress. The art of reading 

 off the hour of the day from the heavenly bodies 

 is necessary in the important problem of finding 

 the longitude at sea. 



The Seasons. The way in which the earth's 

 annual motion round the sun produces the alterna- 

 tions of the seasons will be understood from the 

 accompanying sketch, which represents a bird's- 

 eye view of the earth's orbit as it would be seen 



from the north side. At the left of the figure, the 

 earth is seen in the position it has at the winter 

 solstice (2 ist December), the upper end of the 



axis, or north pole, leaning directly away from the 

 sun at an angle of 23^ from the perpendicular to 

 the plane. Thus, the sun's rays, which can only 



