Consider the light in December's night sky: everything you see either radiates its own light (sun and stars) or merely bounces light away, like the planets. The difference between emitters and reflectors, between true stars and starlike objects, isn't immediately obvious--but an old trick helps you tell them apart.
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. For an Earthly example, think of snow and charcoal: snow is 18 times more reflective than charcoal, and that's why it appears white, while charcoal looks black. 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. Rocky or especially powdery surfaces reflect the least light; our powdery, rocky moon has the lowest albedo (9) of any object in the solar system (proximity compensates for the murkiness).
To arrive at a good estimate of how bright a planet, asteroid, or moon should appear, astronomers combine albedo with three other parameters: size, distance from the light source, and distance from your eyes.
Let's see for ourselves how it works this month. In December, Venus and Mars sit at exactly the same distance from Earth, 155 million miles (though they're in radically different directions: Venus challengingly hugs the southwest at evening twilight; Mars rises in the east at 1 a.m.). 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.
Now comes albedo: Mars' dull reddish soil helps give that world a reflectivity of just 24 percent. But Venus, shrouded by shiny clouds of sulfuric acid droplets, has the highest planetary albedo--76. Its ability to bounce light straight back (called the visual geometric albedo) is even greater, and so the brilliant evening star has four times the luminosity of Mars. Multiply the factors--4 (for size) times 4 (for distance from the sun) times 4 (for reflectivity)--and we find that Venus (this month) should appear about 64 times brighter than Mars, or almost five magnitudes on the standard brightness scale. A check of the tables reveals that, sure enough, Venus registers magnitude -3.9 while Mars is +1. Bingo.
Up for another comparison? Try Jupiter and Saturn. When you add in Saturn's rings, the two gassy giants have about the same reflecting area and the same shiny albedo of 45. But Jupiter is nearly twice as close both to the sun and to Earth, so it ought to appear eight times brighter than Saturn. That's just what we see to the south at nightfall, where the two sit side by side.
Interestingly, Earth's albedo varies more than that of any other planet. Our changing cloud cover takes it from about 35 to 48. But that's no problem, since we, of course, never see Earth in the sky. It's enough that the rest of the splendid December dome measures up so perfectly.