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. Click the links below to learn more about why Venus is so bright and when to see it as its brightest:
Why is Venus so 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 so 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.
When will Venus be at its brightest for 2015? We mentioned above that Mars is brightest when Earth passes between the Red Planet and the sun. At such times, Mars is closest to us, and so it appears brightest in our sky. A similar situation occurs for Venus: the planet is brightest around the time Venus passes between us and the sun, but not exactly at that time.
Because Venus orbits the sun inside Earth’s orbit, when it goes between us and the sun it’s lighted hemisphere, or day side, is facing away from us. At such times, it’s difficult or impossible to see Venus at all.
Because it’s an inner planet, as Venus approaches its time of passing between the Earth and sun, we see the planet exhibit phases, like a tiny moon. As Venus draws up behind Earth in orbit – and prepares to “lap” us in the race of the planets – observers on Earth can watch as the phase of Venus wanes. Meanwhile, as the crescent Venus in waning in phase, the overall size of the disk of Venus gets larger in Earth’s sky, as Venus draws closer to us and prepares to go between us and the sun.
Venus is brightest when those two factors combine – waning crescent, plus largest overall size of Venus’ disk – so that the greatest amount of surface area of Venus shows in our sky. Astronomers call this greatest illuminated extent.
Venus greatest illuminated extent in evening sky: July 10, 2015
Venus greatest illuminated extent (in morning sky) September 21, 2015
Bottom line: Venus is very bright. It’s the third-brightest object in the sky, after the sun and moon. That’s partly because sunlight is easily reflected by acidic clouds in the atmosphere of Venus. This planet is brightest around the time it passes between us and the sun. In 2015, its times of greatest brilliancy for Venus will occur around July 10 and September 21.