PRACTICAL ASTRONOMY 141 



TIME 



The passing of a heavenly body across the meridian of a 

 place is called its culmination, or transit. It is upper or lower 

 culmination, according as it is then occupying the highest or 

 the lowest position with regard to the horizon. 



The interval of time that elapses between two successive 

 upper or lower transits of a star over the same meridian is 

 called a sidereal day. It begins, for any place, when the vernal 

 equinox crosses the meridian above the pole. This instant is 

 called sidereal noon. Sidereal hours, minutes, and seconds are 

 reckoned from to 24 hr., starting from sidereal noon. Time 

 expressed in sidereal days and fractions (hours, minutes, sec- 

 onds) is called sidereal time. 



From this, it follows that sidereal time is the hour angle of the 

 vernal equinox; also, that the right ascension of a star is equal 

 to the sidereal time of its transit, or culmination. For any 

 other position of the star, the sidereal time equals the algebraic 

 sum of the right ascension and the hour angle of the star. 



The interval between two successive upper transits of the sun 

 is called a true solar day, or an apparent day. Owing to the 

 fact that the motion of the sun is not uniform and that the 

 solar days are not of equal duration, apparent time is not used 

 for the ordinary affairs of life. 



The mean sun is an imaginary body supposed to start from 

 the vernal equinox at the same time as the true sun, and to 

 move uniformly on the equator, returning to the vernal equinox 

 with the true sun. The time between two successive upper 

 transits of the mean sun is called a mean solar day, and time 

 expressed in mean solar days is called mean solar time, or simply 

 mean time. This is the time shown by ordinary clocks and 

 watches. 



A mean solar day is the mean of the duration of all the true 

 solar days in a year (a year being the time in which either the 

 true or the mean sun makes a complete circuit of the heavens). 

 As there are 365.2422 true solar days and 366.2422 sidereal 

 days in a year, 



366.2422 

 1 mean solar day = = 1.0027379 sidereal dp^s 



= 24 h 3 m 56.55 s , sidereal time 



