2l6 



THE GUIDE TO NATURE 



volve about us. Not many, however, 

 realize that their times of revolution 

 differ, and yet it is this difference that 

 gives the key to the puzzling- appear- 

 ances noted. Its conventional explana- 

 tion is readily provided, but a simple ob- 

 servation helps wonderfully in putting 

 meaning into it. But why should we 

 have all our astronomy in pictures and 

 in easy reading with thrills, and make 

 not the slightest effort on our own part ! 



Let us go out of doors and look at the 

 heavens for ourselves. Choose a con- 

 spicuous star in the south, not far 

 above the horizon, and keep watch till 

 it passes a fixed line of reference. Ap- 

 parently the star moves westward, but 

 this is in nowise due to any motion of 

 its own ; for we, unconscious of the 

 earth's turning below, refer the motion 

 to the stars above. The stars are so 

 distant that they are for us practically 

 fixed points of reference, so when our 

 chosen star again reaches, on the next 

 night, the same reference line, the earth 

 has turned once on its axis. The inter- 

 val recpiired for this single complete 

 turn is, by common agreement, taken 

 as the sidereal or star day ; but it is 

 found to fall short of the sun day by 

 about four minutes ; that is, the interval 

 measured by a common watch or clock- 

 lacks four minutes of twenty-four 

 hours. 



Among hundreds who have, under 

 the writer's direction, made proof of 

 this by direct observation, one of the 

 first was a student at Carleton College 

 thirty years ago, and the record then 

 written reads as follows: 



"Northfield, Minn., E. Hall, north 

 window. Monday, Feb. 28, 1887. 



"The star chosen was Beta LTrsae 

 Majoris, and the middle bar of the win- 

 dow was taken as the reference line. 

 T began watching at /h. 15m. P- M., and 

 at yh. 22m. the star pas c ed behind the 

 window bar. On the following even- 

 ing, Beta was observed from the same 

 place [this is essentiall and at /h. 

 17.5m. it passed again behind the same 

 bar. 



Hence, according to these observa- 

 tions, the sidereal day is 4-5 minutes 

 shorter than the mean solar day." 



If instead of a star in the north, as 

 here, a southern star like Fomalhaut or 

 Sirius is taken, the line of direction 

 fixed by two plumb lines, and heed 



given to other details, results are often 

 obtained differing only a second or 

 two from the rigorously correct value 

 of astronomers which makes the side- 

 real day shorter by 3m. 56s. 



Then since one turn of the earth 

 on its axis gives the star day, it must 

 turn a little further to fill the full 

 measure of the apparent solar day. 

 Here again there is transfer of motion, 

 for no one realizes directly that the 

 earth is making an annual journey 

 around the sun ; so, as we move onward 

 a little each day, the sun appears to ad- 

 vance among the stars, and in order 

 to bring it a second time to the refer- 

 ence line, there is needed, in addition, 

 the small fraction of a turn, taking 

 nearly four minutes. 



Both expressions, apparent solar day 

 and mean solar day, have been used 

 above, but unfortunately they are not 

 the same, though why and how they 

 differ is "another story." It suffices to 

 say here that the average apparent so- 

 lar day equals the unvarying mean day, 

 and so either may be used in making 

 comparison with the star day. 



This comparison is the essential 

 thing; for once the idea is really 

 grasped that the stars revolve above 

 us in the shorter day, it follows as a 

 matter of course that any and every 

 star crosses the fixed line of reference 

 four minutes earlier, by watch time, on 

 each succeeding night. Let the horizon 

 be the reference line, then stars that 

 appeared on the line at about ten o'clock 

 on the first of the week, are at its close 

 rising at half past nine- Watch, for 

 example, the stars in the sword and 

 belt of Orion. Early in October, in 

 our latitude, they are, at eleven, just 

 visible above the eastern horizon ; but 

 in the first week in November, they are 

 rising at nine. And their rising and 

 setting comes earlier and earlier till 

 an evening is reached in the spring 

 when Orion sets as the sun sets, and 

 there follow nights with no Orion in 

 the sky. 



It is seen then that the same aspect 

 of the heavens is found, not by observ- 

 ing at the same time but at a different 

 time on different nights. The notion 

 should be dispelled that capricious 

 changes are taking place in the star 

 world overhead. 



