It's not just Mercury from Earth, it happens with all outer planets looking in
The inner planet stays almost full throughout it's orbit
For example
10/28/2011 - Earth from Jupiter - Earth in Inferior conjunction, none of Earth should be illuminated but it's almost full
11/07/2011 - Mars From Jupiter - Mars' Greatest Elongation, Mars should be ~50% illuminated but it's full
12/23/2011 - Earth from Uranus - Earth's GE, should be 50% illumination it's full
Just pick any outer planet, go to it and pick an inner planet and follow it through it's orbit, the inner planet will stay almost full
It's not just Mercury from Earth, it happens with all outer planets looking in
The inner planet stays almost full throughout it's orbit
For example
10/28/2011 - Earth from Jupiter - Earth in Inferior conjunction, none of Earth should be illuminated but it's almost full
11/07/2011 - Mars From Jupiter - Mars' Greatest Elongation, Mars should be ~50% illuminated but it's full
12/23/2011 - Earth from Uranus - Earth's GE, should be 50% illumination it's full
Just pick any outer planet, go to it and pick an inner planet and follow it through it's orbit, the inner planet will stay almost full