ON PRACTICAL ASTRONOMY. 429 



bration of the Olympic games, to be engraved in letters of gold on a pillar 

 of marble. At present, if we add 1 to the number of the year, and divide 

 it by 19, the remainder will be the golden number ; thus, for 1806, the 

 golden number is 2. If we subtract 1 from the golden number, then mul- 

 tiply by 11, and divide by 30, the remainder will 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 noon on the 31st of Decem- 

 ber, 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 astronomical 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 

 quadrants, 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 esti- 

 mate, 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 within 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, the pre- 

 cession of the equinoxes, which produces a slight change in the places 

 of the stars, has made it necessary to wait 1 minute 13 seconds for 

 every ten years that have elapsed. Thus, in 1806, if we wait 6 

 minutes, the pole star will then be precisely in the meridian, and will 

 serve for the correct adjustment of the instrument. (Plate XXXV. Fig. 

 507-.. 510.) 



* On the Calendar, consult Sauveur, Hist, et Mem. 1732, H. 94. Lord Mac- 

 ' clesfield, Ph. Tr. 1750, p. 417. Lalande, Hist, et Mem. 1789, p. 95. Halma, Sur 

 la Reduction des Annees des Anciens a la Forme des Notres, 4to, 1819. Sur les 

 Mois Macedoniens, 4to, 1820. Encyc. Brit. art. Calendar. 



