ASTRONOMICAL DETERMINATIONS. 17 



methods known to modern science, in order to reduce all avoidable errors 

 to a minimum. 



Whatever determinations have been made by the U. S. Coast and 

 Geodetic Survey, the United States Lake Survey, or the Mississippi River 

 Commission, whether by astronomical work or by triangulation, these posi- 

 tions may be utilized for the above purposes. 



DEFINITIONS. 



Sidereal time is the time indicated by the stars, a sidereal day being 

 the time which elapses between two passages of the vernal equinox across 

 the meridian. Solar or apparent time is the time measured by the sun's 

 apparent movement or the revolution of the earth with reference to the sun, 

 and since the earth revolves at a differing rate in different portions of its 

 orbit, the solar days are not of equal length. A mean day is the average 

 solar day; -mean time differs from solar time by an amount which varies 

 with the time of year, and which, under the name of " equation of time," is 

 given in the Nautical Almanac. Mean time differs from sidereal time by 

 a 1 lout a day in the course of a year, or about four minutes in each day; 

 the mean day being longer than the sidereal day. To convert a given date 

 of mean time into sidereal time it is necessary to obtain, from the Nautical 

 Almanac, the sidereal time at noon immediately preceding the date in ques- 

 tion. Then the interval after noon, expressed in mean time, is converted 

 into sidereal time by table xxxn in this volume, and the result added to the 

 sidereal time of mean noon. Local time, whether sidereal, solar, or mean, 

 is the time of the locality as distinguished from the time of any other 

 locality. It must be distinguished from railroad time, which is the local 

 time only of certain meridians. 



The right ascension of the sun or a star is the sidereal time which has 

 elapsed between the passage of the venial equinox and the star across the 

 meridian. It is commonly expressed in hours, minutes, and seconds. 



Declination is the angular distance of a heavenly body north or south 

 of the equator. It is plus when north and minus when south of the equator. 



The zenith distance of a heavenly body equals its declination, minus 

 the latitude of the place of observation. 



Latitude is determined by what is known as Talcott's method, by 

 MON xxii 2 



