Astronomy Essentials

Why is Venus so bright in Earth’s sky?

Why is Venus so bright? Bright dot in dark sky with scattered stars including group of stars to right.
View at EarthSky Community Photos. | In April 2023, Venus is a dazzling light in the west after sunset. Have you ever wondered why is Venus so bright? We answer that question here. Charlie Favret of Round Rock, Texas, captured this image on April 11, 2023, and wrote: “A photograph of Venus visiting the Seven Sisters on the night of April 11. It is always a pleasure to see the two of these in the sky, and even better when they come close together!” Thank you, Charlie!

Why is Venus so bright?

Jupiter is a bright planet, and Mars is sometimes bright, too. But neither Jupiter nor Mars at its brightest can outshine Venus. Why is Venus so bright?

Our neighboring world – orbiting one step inward from Earth around the sun – is the 3rd-brightest natural object in the sky, after the sun and moon. It’s currently a brilliant light in the evening sky shining at magnitude -4.1.

As the planet next inward from Earth in orbit around the sun, Venus is relatively nearby. But its 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 over about a two-year cycle. It’s only exceptionally bright around the time Earth passes between Mars and the sun, at the same time Mars is closest to the sun. The last time that happened was in 2018. And the next time will be in 2035.

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, the planet’s surface absorbs some of the light and reflects the rest. Albedo is a comparison between how much light strikes an object – and how much the objects reflects.

As you might have guessed, Venus has the highest albedo of any major planet in our solar system.

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The reflectivity of Venus makes it bright

The albedo of Venus is close to .7, meaning it reflects about 70% 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% 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.

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A fuzzy white crescent, pointing downward, on a black background.
View at EarthSky Community Photos. | When Venus passed between us and the sun in late October 2018, its lighted half – or day side – was still facing mostly away. It’s an interesting fact that this crescent Venus can appear brighter than the fuller Venus we see at other times. Read more about why. This image was captured by Prabhakaran A on November 20, 2018.

Apparent size of Venus

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 in our sky around the time Venus passes between us and the sun, although not exactly at that time.

Because Venus orbits the sun inside Earth’s orbit, when it goes between us and the sun its 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 inferior planet (one that orbits the sun inside Earth’s orbit), we see Venus exhibit phases like a tiny moon as it approaches its time of passing between the Earth and sun. 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 at greatest brilliancy in 2023

This year, Venus will be brightest in the evening sky on July 7, 2023, when it reaches magnitude -4.7. And it’ll be brightest again in the morning sky on September 19, 2023, when it will shine at magnitude -4.8.

Read about Venus at greatest brilliancy

Thin, fuzzy crescent pointing toward upper right, on deep blue background.
Steven Bellavia wrote: “October 21, 2018, crescent Venus – 5 days before the planet’s inferior conjunction, when it will go between the Earth and sun – at 1.6% illumination. At inferior conjunction, on October 26, Venus will be 0.6% illuminated.” If it’s between us and the sun, why won’t its night side be entirely facing us? It’s because, at this inferior conjunction, it passed to one side of the sun in our sky so that very skilled observers might be able to record a very slim crescent Venus at the moment of inferior conjunction! At such times, you can see Venus in the daytime. Read more about how. Thanks, Steven!
Deep blue sky with scattered stars, small dot circled, labeled Uranus, and larger dot labeled Venus.
View at EarthSky Community Photos. | Jim Bruzek of Dayton, Maryland, captured this image on March 23, 2023, and wrote: “Venus and Uranus at dusk from Dayton, Maryland.” Thank you, Jim!

Bottom line: Venus is the 3rd-brightest natural 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.

Read more: Venus after sunset: Greatest elongation on June 4, 2023

Read more: Venus before sunrise: Greatest elongation October 23, 2023

April 23, 2023
Astronomy Essentials

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