My goodness , it hardly seems like two years since the issue of inner planetary phases dictated the position of a planet to both the Earth and the Sun using the old language of oppositions,elongations and quadratures .

This is right in terms of elongation -

http://astronomy.swin.edu.au/cms/cpg...longationB.jpg

This isn't as phases dictate the symmetry provided between a portion of the planet in light and in darkness -

http://www.1728.org/synodic3.png

The orbital input of the Earth using a pivotal angle off the center of the Sun causes a change in position of the background stars to the central Sun which in turn effects the observation where Mercury and Venus reach their widest point from the Sun as seen from our orbit. The geocentric framework which used the motion of the Sun through the constellation along with all the planets doesn't permit the accurate picture to emerge where the circle of illumination of the Earth and that of Venus are at right angles to each other where their orbits are at 90 degrees with the central angle being the center of the Sun, again -

http://astronomy.swin.edu.au/cms/cpg...longationB.jpg


It is a talent to work with light and shadow and use these features to create a perspective and the geocentric astronomers and the original heliocentric astronomers didn't have the equipment to deal with these wonderful clues..