Mr. Schroeter’s Observations 
326 
at which I observed the planet on the 12th of March, 1792, 
when I measured the extent of its twilight. 
Let also A B G C fig. 7, be the hemisphere of the pla- 
net, turned towards our earth T, and A F D E C the other 
hemisphere turned towards the sun S, at the time of an inferior 
conjunction. And let A C represent the terminating surface 
perpendicular to the plane A F D E C G B extending from 
the centre of the sun through that of the planet Venus, and 
the eye of the observer on our earth. A f c C will then be the 
mutual edge of the two hemispheres, and the whole of the 
dark one A B G C will be directed towards the eye of the 
observer at T, 
But whenever Venus, before or after an inferior conjunc- 
tion, comes into such a position, relatively with the sun and 
our earth, as to be illuminated by the former in the direction 
b E c, fig. 7, which position, on the 20th of March, 1790, 
must have been nearly at F fig. 6 ; then will F gcG, 
7, be the mutual terminating surface of the dark and light 
hemispheres of the luminous circumference; of which surface, 
however, only one half, c G, can be seen from the earth ; the 
other half, F g c, lying in the hemisphere that is turned away 
from the eye: c will be the point of the cusp seen from T. 
This being premised, let the twilight of Venus be so con- 
siderable as to extend, in a gradually-fading progression, from 
the luminous terminating border F g c G, to the line hf k i, 
which let us assume as the real border of the twilight. Then 
will a part of it, K /, terminating at /in the direction a g, pa- 
rallel to b E c, the central ray from the sun, be seen projecting 
into the dark hemisphere of the planet, and this prolongation, 
as it was seen on the 12th of March, 1790, will be the appa- 
