ON IPRAcrlCAh ASTRONOJir. 541 



If we subtract 1 from the golden number, then multiply by 11, and 

 divide by 30, the remainder Avill be the epact, which is the moon's age on 

 the first of January, without any material error; thus, for 1806, the epact 

 is 11, and the moon is actually 11 days old on the first of January. 



From a combination of chronological periods of various kinds, Scaliger 

 imagined the Julian period, as an epoch to which all past events might with 

 convenience be referred, beginning 4713 years before the birth of Christ. 

 Laplace proposes, as a universal epoch, the time when the earth's apogee was 

 at right angles with its nodes, in the year 1250, calling the vernal equinox 

 of that year the first day of the first year. But the fewer changes of this 

 kind that we make, the less confusion we introduce into our chronology. 

 The astronomical year begins at no'on on the 31st of December, and the date of 

 an observation expresses the days and hours actually elapsed from that time. 

 Thus, the first of January 1806, at 10 in the morning, would be called, in as- 

 tronomical language, 1805 December 31 days 22 hours, or more properly 

 1806 January day 22 hours. 



For ascertaining, by immediate measurement, the position of any of the 

 heavenly bodies, it is usual to determine its meridian altitude by quadrants, 

 and the time of its passing the meridian by transit instruments. The large qua- 

 drants, generally used for this purpose in observatories, are fixed to vertical walls, 

 in order to give them greater stability, and are thence called mural quadrants; 

 sometimes a smaller portion of an arc only is adapted for observations near 

 the zenith, under the name of a zenith sector. A transit instrument is a 

 telescope so fixed on an axis as to remain always in the plane of the meridian; 

 the axis being perpendicular to this plane, and consequently in a horizontal 

 position, and directed east and west. Those who are in the constant habit of 

 observing with attention, can estimate, in this manner, the precise time of the 

 passage of a celestial object over the meridian ; without an error of the tenth 

 of a second, so that its angular right ascension may be thus determined 

 Avithin about a second of the truth. A very convenient mode of adjusting a 

 transit instrument is to direct it to the north polar star, at the same time that 

 the last of the three horses in the wain is perpendicularly above or below it: 

 this process, in 1751, gave precisely the true meridian; but since that time. 



