Venus: Why So Bright?
I’ve noticed, as most folks have, that Venus is very bright. Why is Venus so bright?
Venus is one of the brightest objects in the night sky. Venus can often be seen within a few hours after sunset or before sunrise as the brightest object in the sky (other than the moon). It looks like a very bright star. It is for this reason that it is often called the Morning Star or Evening Star.
When you “Google” the question, “Why is Venus so bright?” The answer you always seem to get is: Venus is the brightest planet because it has the highest surface reflectivity. That is to say, its thick clouds reflect most of the sunlight that reaches it, about 70% reflects back into space. The other factor that is often mentioned is that Venus is the closest planet to Earth. If Venus were deep in our solar system, then the reflectivity would be overcome by the remote distance.
As I ponder this question, I wonder if the size of the planet and the distance it is from the Sun and Earth may also have an effect on its brightness in our night sky. The answer is, of course they do. If the reflectivity of all of the planets were identical then the distance from the Sun and Earth and their relative size would definitely affect the planet’s relative brightness.
I created a spreadsheet which listed each planet and its relative distance to the Sun and its relative diameter (see attached). We know that the surface area of a circle with a diameter of “d” is equal to Pi times ½ d squared: (area = π ½ d2). Therefore, everything else being equal (reflectivity and distance), the larger the planet, the brighter it would appear. In fact, if it had twice the diameter, it would reflect four times the light or 2 squared.
Now we have the relative size of the planets computed for brightness, we need to figure in the relative distances from the Sun to the planet and then the planet to the Earth. This is where we must consider the way light radiates out from an object. In Physics we learn the light radiates out in all directions, in a sphere.  Calculating the area of a sphere at a certain radius “r” we find that it is equal to four times pi times the radius squared (area = 4 π r2). This appears similar to our surface area formula but this time, greater the radius (distance from the sun) the dimmer the planet will be by squared the radius ratio. That is to say, if a planet (A) is three times further away than planet (B), planet A will be one-ninth (1/9) as bright as planet B.
Given this notion, I took the relative distances from the planet to the sun and calculated the relative sunlight that struck the planet’s surface. Taking into consideration the planets relative size, I calculated the amount of sunlight that would be reflected from its surface; the further out the planet, less sunlight that would strike it by the relative distance squared. Next I took the amount of sunlight reflected from the planet and calculated the amount that would head to Earth, using a mean relative distance from the planet to the Earth. This relative distance will also follow the inverse squared law with respect to the amount of light that would reach Earth.
When all was said and done, I found that Venus is the brightest planet when only considering distance and disc diameter, ignoring relative reflectivity. Roughly figuring, I calculated that the brightest would be Venus by 8 times brighter than Mercury. Mars would be third and Jupiter would be fourth. But as we know, the reflectivity of the planet’s surface or atmosphere has a huge effect as well. In conclusion, Venus is the brightest planet because of its reflectivity, size and relative distance from the sun and earth.
Comment: If anyone thinks they see an error in my logic or analysis, I would love to hear your comments and thoughts – please be specific.