C 15 ] 



V. On a Method of observing Solar Eclipses by means of the 

 Altitude and Azimuth Instrument. By A Correspondent. 



To the Editors qf^ the Philosophical Magazine and Journal. 



A DAY or two ago, after perusing Mr. Troughton's able 

 ■^^^ defence of the altitude and azimuth instrument, it struck 

 me that it might be advantageously employed at the end (and 

 probably at the begimiing) of the next solar eclipse to ascer- 

 tain the moon's declination and right ascension, as well as the 

 longitude of the place of observation. 



The instrument being carefully oriente, observe the azimuth 

 (from the north) and the zenith distance of the point of con- 

 tact, and correct the latter for refraction. 



With this datum, the calculated zenith distance of the sun's 

 centre affected by parallax, and his semi-diameter (diminished 

 by irradiation ?), find the angle formed at the sun's centre be- 

 tween the zenith and the point of contact. This angle, toge- 

 ther with the {parallaxed) Z. D. of the sun, and the sum of 

 the apparent semi-diameters, will give the moon's azimuth and 

 {parallaxed) zenith distance. 



Admitting the earth to be a spheroid, the difference of the 

 true and apparent zeniths, together with the azimudis and 

 zenith distances, afford data to diminish* the former and in- 

 crease the latter to the quantities due to the reduced latitude. 



The parallaxes being subtracted from these transposed 

 zenith distances, we find (as in lunars) the distances of the cen- 

 tres of the sun and moon, and consequently the longitude 

 of the observer. The method of deducing the N. P. di- 

 stance of the moon, as well as the right ascension, is sufficiently 

 obvious. 



The apparent time of the end of the eclipse being known to 

 great accuracy, it might serve to calculate the azimuth of the 

 sun's centre; and consequently the semi-diameter. By com- 

 paring the two methods, we might learn the value of the irra- 

 diation. 



An observer in possession of a well regulated chronometer 

 furnished with a micrometer (or merely with vertical and 

 liorizontal wires) might arrive at the same results by com- 

 paring the point of contact with the proper parts of the sun's 

 disk. 



June 21, 1823. X. X. 



* The objects being to the north of cast. 



VII. 7-; 



