J 24 EQUATION OF TIME. 



well-regulated clock and a true sun-dial is called the equa* 

 tiofi of time. 



Since the stars are found to gain three minutes and fifty-six 

 seconds upon the sun every day, amounting in a year to one 

 diurnal revolution, it follows that, in three hundred and six- 

 ty-five days as measured by the sun, there are three hundred 

 and sixty-six days as measured by the stars. This regular 

 return of the fixed stars to the meridian affords an easy 

 method of determining vi'hether our clocks and watches keep 

 true time. For if through a small hole in a window-shutter, 

 or in a thin plate of metal fixed for that purpose, it be ob- 

 served at what time any star disappears behind a chimney 

 or the corner of a building at a small distance ; then if the 

 star disappears the next night three minutes and fifty-six 

 seconds sooner by the clock or watch than it did the night 

 before, on the second night seven minutes fifty-two seconds 

 sooner, and so on, it is a certain sign that the machine goes 

 right ; but if it does not observe this rule, it is evidently not 

 accurate, and as the disappearing of a star is instantaneous, 

 we may depend upon this information to half a second at 

 most. 



Questions. — 1. What is said of the form and commencement of 

 the year among different nations ? 2. What is an astronomical month ? 

 3. Lunar month ? 4. Solar month ? 5. Civil month ? 6. Solar day ? 

 7. Sidereal day ? 8. How does it appear that sidereal days are all of 

 the same length ? 9. Why is there a difference between the lengths 

 of a solar and sidereal day ? 10. When is the sun said to be slow of 

 the clock ? 11. Fast of the clock ? 12. What is mean time ? 13. 

 Apparent time ? 14. Equation of time .^ 15. What follows incon- 

 sequence of the stars gaining upon the sun ? 16. What is an easy 

 method of determining whether clocks and watches keep true time ? 

 [Note. The inequality of solar days, as caused by the eccentricity 

 of the earth's orbit, and the obliquity of the ecliptic, is clearly illustrat- 

 ed in Wilkins' Elements of Astronomy : the work has been recom- 

 mended as containing a judicious selection and concise statement of 

 the leading facts and principles of the science.] 



