Stars are so distant that they appear as points of light even through powerful telescopes. Reflective objects like asteroids and planets, on the other hand, display some size, even if it's not evident to the unaided eye. That little bit of dimension makes their images resistant to distortion from our churning atmosphere. Stars twinkle, then, but planets do not. A planet's reflectivity determines its appearance and gives instant clues to its surface composition.
Reflectivity is described in terms of albedo, which is the percentage of light that's reflected.
Thick gases, droplets, and ices reflect more light than, say, pools of water, so objects rich in these substances have a high albedo. Let's see for ourselves how it works this month. Their being equidistant provides a perfect opportunity to compare them in every other way. For instance, Venus is twice as close to the sun as the Red Planet is, and that alone would make it four times brighter. It's also twice the size of Mars, which gives it four times the surface area and so another fourfold brightness boost.