33 ° 
Mr. Schroeters Observations 
planet be assumed = 834, geographical miles, then will the ex- 
tent of the twilight, in the perpendicular direction from the 
terminating border, amount to of those miles : and this ex- 
tent of twilight on Venus, which appears to us to measure 
4° sS'-j* may, with great propriety, be compared with what 
Lambert calls the common twilight on our globe, when its 
border passes immediately through the zenith, at a time when 
he estimates the place of the sun at 6 ° 23'x below our ho- 
rizon.* 
This computation affords also the following conclusion : ac- 
cording to the measurement of the 12th of March, 1790, the 
projection of the twilight, or f c, fig. 7, amounted in its whole 
length to 8" ; whence sin. /cx sin. 17 0 49' 45", gives for the 
true extent of the twilight/c only 2,45 seconds of a degree ; 
and hence it appears, 
1. Why this twilight appears only at the points of the cusps 
and not at other parts of the terminating border, its extent, 
with its proportionably faint light, being too small ; and this 
narrow faint light being, moreover, eclipsed by the superior 
brightness of the luminous hemisphere. 
2. Why this twilight is distinctly seen only a few days be- 
fore and after the inferior conjunction ; the visible light of the 
luminous hemisphere being then only 2" of a degree in 
breadth, and only the faint, declining, less luminous part of it 
apparent to the eye ; whereas the projection of the twilight 
f c, fig. 7, will in this position be most extensive, and hence 
the more perceptible. 
3. But should Venus at this time not be in the sign Aries, 
nor at so high a dedication as when I saw it in March, 1790, 
* Lambert’s Photom. §. 998, sqq . Berl. Ephem. for 1 776. 
