As I understand it Edmund Halley postulated that transits of Mercury or Venus could be used to determine the true distances to objects in our solar system.

For the life of me I can't understand why Mercury wasn't used and Venus was. I know there must be a simple reason for this, it might related to its size but since Mercury's transits are far more common I can't figure out what this reason is.

Again, any help would be appreciated. Thanks in advance.

Richard Merriman