Venus is bright because it’s nearby, because it’s blanketed in highly
reflective clouds and because of its location in orbit with respect to
Earth now
Venus is much brighter than any other planet viewed in Earth’s
sky. It’s the third-brightest object in the sky, after the sun and
moon, and right now it’s at another time of greatest brilliancy
on July 9-10, 2015. Look for Venus now in the west after sunset. It’s
near another very bright planet – but not as bright as Venus. The
other planet is Jupiter.
Why is Venus bright? As the planet next-inward from Earth in orbit around the sun, Venus is relatively nearby. But it’s nearness isn’t the only reason Venus is bright. Consider that Mars orbits one step outward from Earth. And Mars waxes and wanes in brightness in our sky. It’s only exceptionally bright around the time Earth passes between Mars and the sun, when the Red Planet is at its closest to us.
Why is Venus bright? As the planet next-inward from Earth in orbit around the sun, Venus is relatively nearby. But it’s nearness isn’t the only reason Venus is bright. Consider that Mars orbits one step outward from Earth. And Mars waxes and wanes in brightness in our sky. It’s only exceptionally bright around the time Earth passes between Mars and the sun, when the Red Planet is at its closest to us.
With Venus, something else is going on. Astronomers use the term albedo
to describe how bright a planet is in absolute terms. When sunlight
strikes a planet, some of the light is absorbed by the planet’s surface
or atmosphere – and some is reflected. Albedo is a comparison between
how much light strikes an object – and how much is reflected.
Venus has the highest albedo of any major
planet in our solar system. Its albedo is close to .7, meaning it
reflects about 70 percent of the sunlight striking it. When the moon is
close to full in Earth’s sky, it can look a lot brighter than Venus, but
the moon reflects only about 10 percent of the light that hits it. The
moon’s low albedo is due to the fact that our companion world is made
of dark volcanic rock. It appears bright to us only because of its
nearness to Earth. It’s only about a light-second away, in contrast for
several light-minutes for Venus.
Venus is bright (it has a high albedo)
because it’s blanketed by highly reflective clouds. The clouds in the
atmosphere of Venus contain droplets of sulfuric acid, as well as acidic
crystals suspended in a mixture of gases. Light bounces easily off the
smooth surfaces of these spheres and crystals. Sunlight bouncing from
these clouds is a big part of the reason that Venus is so bright.
By the way, Venus isn’t the most reflective
body in our solar system. That honor goes to Enceladus, a moon of
Saturn. Its icy surface reflects some 90% of the sunlight striking it.
