94 



Art. XIV. ASTRONOMICAL AND NAUTICALi 

 COLLECTIONS.— No. XXIII. 



i, A Method of Computing the Sun's Horizontal Pa r a llax from 

 Observations of the Transits of Venus. By Thomas Hender- 

 son, Esq. 



The method of computing an occultation of a fixed star by the 

 moon, explained in No. XX. of these Collections, Art. III., may be 

 applied with advantage to solar eclipses, occultations of the pla- 

 nets, and transits of Vertus and Mercury. In each of these pheno- 

 mena, the sun or planet occulted is to be substituted for the star, 

 in the precepts given for occultations, and in transits, the planet is 

 to be substituted for the moon. The orbital angle, in place of 

 being constant as in occultations of stars, will (owing to the mo- 

 tion of the sun or planet) undergo a small variation equal to the 

 change in the sun or planet's angle of position, which, when great 

 precision is requisite, must be allowed for ; and in transits, the 

 complement of the orbital angle, and the side of the right-angled 

 triangle, mentioned in Precept III., will have the contrary signs to 

 those prescribed for occultations, by reason of the planet's retrograde 

 motion. The difference of the horizontal parallaxes of the two 

 bodies is to be employed in place of the horizontal parallax of the 

 moon ; and, while the semidiameter of the moon, or occulting 

 body, remains without augmentation, the semidiameter of the sun 

 or planet undergoing occultation, is to be diminished by a small 

 quantity, obtained from this formula, 

 s sin p sin A, 

 where s denotes the semidiameter to be diminished, p the hori- 

 zontal parallax of the other body, and A the altitude of the sun 

 or planet. The sum or difference of the semidiameters will be 

 employed according to the particular phenomenon to be investi- 

 gated. 



These modifications of the rules for occultations being observed, 



