* Venus's Phases: Like the Moon, Venus goes through phases as it orbits the Sun. This is because we see different amounts of the sunlit side of Venus depending on its position relative to the Sun and Earth.
* Full Phase Invisibility: A "full" Venus would mean we're seeing the entire sunlit side of the planet. However, this can't happen because Venus is an interior planet – it orbits closer to the Sun than Earth. When Venus is on the opposite side of the Sun from Earth (a position called "superior conjunction"), it's lost in the Sun's glare and cannot be seen.
* Crescent Phases: When Venus is closest to Earth and showing us a crescent phase, it appears larger because it's closer to us. As it moves further away, it appears smaller, even though it's showing us a larger portion of its sunlit side.
In summary: Venus doesn't appear full because it's an interior planet. It's larger when it's closer to Earth and in a crescent phase, not when it's full.
