6l2 



TRANS. ST. LOUIS ACAD. SCIENCE. 



double-micrometer observations will ditVer but little from that ot 

 Bessel. At the time of transit the angular semi-diameter, accord- 

 ing to Bessel, was 6".02, which, compared with the measured 

 semi-diameter, would indicate a diminution of the apparent semi- 

 diameter of i".04 on account of the irradiation, a result quite fair- 

 ly in accord with the preceding. 



The irradiation effect would undoubtedly vary with the ob- 

 server and telescope, and probably with the tint of shade-glass 

 employed. 



The result of this discussion would seem to show : — 



1. That the discrepancies in different observations is due to actual 



differences in the apparent disc of the planet, due to irradiation. 

 That the phases of geometric contact and actual separation of the 

 discs are distinct and separate phenomena. 



2. That the observation of the moment of bisection of the planet's disc 



is free from this effect. 



In this connection, it may not be out of place to call attention 

 to the fact that the values of the angular diameters of the planets 

 Venus, Mars, Jupiter, Saturn, and Uranus, now in use in the 

 computations of the American Ephemeris, are derived from the 

 mural circle observations made at Washington in 1S45-46. This 

 instrument is small, and, when it is remembered that the obser- 

 vations were not made for the purpose of determining diameters, 

 but for determinations of declination, it is not surprising that they 

 should differ considerably from more modern observations. 



The following comparison between the values used in the 

 American Ephemeris, and those of the Berlin Jahrbuch, shows 

 that there is a systematic difference in the assumed diameters of 

 the bright planets Mars, Jupiter, and Saturn. I have reduced 

 both values to the average distance of the planet from the Earth 

 at opposition. 



Planet. 



Log-. Dist. 



Ang. Diameter, 

 Bar. Jahr. 



Ang. Diameter, „ t • Ti- 

 Ara. Ephm. \ «.J.— A.±i. 



Mars j 0.25 



Jupiter (Polar) 1 0.70 



Jupiter (Eq.) 0.70 



Saturn (Polar) . 

 Saturn (Eq.) 



0-95 

 0-95 



5".26 

 36 .94 

 39 -82 

 16 .48 

 iS .26 



5"-7o 

 37 -56 

 40 .00 



17 -54 



18 .76 



-0 .44 

 -0 .62 

 -0 .iS 

 - 1 .06 

 -0 .50 



