Why is Venus so Bright?

Venus is so bright and luminous that it outshines all other objects in the night sky except for the Moon. How so?

Every other star and planet pales in comparison to Venus as viewed from Earth, and that’s irrespective of whether Venus is at its closest to or farthest from Earth in its orbit. Seen next to Mars - a bright planet in its own right - as it appeared during conjunction on 12 July, Venus appeared about 200 times brighter than Mars, or almost six full astronomical magnitudes.

Every object in the Solar System has what’s known as an albedo: a measure of how reflective its surface is. There are two types of albedo that scientists talk about:

  • Bond albedo, which is the ratio of the total reflected radiation compared to the total incoming (solar) radiation, and

  • Geometric albedo, which is how much light actually gets reflected compared to a flat, ideally reflective surface.

By both measures, Venus is by far the most reflective planet in the Solar System, with albedos that are each more than double the next closest planet. Whereas airless worlds like Mercury or the Moon reflect only about 11 - 14 percent of the total incoming light, similar to what Earth would reflect if it were airless and free of icecaps, Venus reflects between 75-84 percent of the total light, dependent upon how it's measured. This high level of reflectivity makes it appear intrinsically brighter than any other planet in the Solar System, with only a few ice-rich moons, like Saturn's Enceladus, possessing a higher total albedo.

Apart from its extraordinary reflectivity, Venus also has a couple of other tricks up its sleeve. One is that Venus is relatively large (almost the same size as Earth) for a rocky planet as well as relatively close to the Sun; in terms of the total amount of solar radiation incident on its surface, only Jupiter receives more. The other is Venus’s proximity to Earth. At its closest, Venus comes within a 'mere' 41 million km (25 million miles) of Earth, closer than any other planet.



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