Astronomy Essentials

Why is Venus so bright in our Earth’s sky?

Two thin white vertical crescents on a dark background, the right one smaller and fuzzy.
View at EarthSky Community Photos. | Karthik Easvur in Delhi, India, captured these 2 images on September 12, 2023. They show a crescent moon – and a crescent planet Venus – on the same evening. “It was just amazing,” he wrote. You need a telescope to see Venus as a crescent. But the planet and the moon appear as crescents to us for the same reason. It happens when they are located nearly along our line of sight to the sun, so that their lighted portions – or day sides – are turned mostly out of our view. On the evening Karthik captured Venus, it was just a week away from its greatest brilliancy on September 19. Why is Venus so bright in our sky, when it’s showing us a crescent phase? Read on to find out.

Venus brightest in morning sky, September 19, 2023

Why is Venus so bright?

Jupiter is a bright planet, and Mars is sometimes bright, too. But neither Jupiter nor Mars at their 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 the moon. It’s currently a brilliant light in the morning sky, shining at magnitude -4.4. Greatest brilliancy for Venus for this 2023 morning apparition will happen on September 19.

Look at the photo above. Venus now appears as a crescent – through telescopes – as seen from Earth. How can a crescent Venus appear brighter to us than the fuller Venus we see at other times?

Albedo = reflectivity

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

White dots for Venus and Procyon plus a nearly vertical green ecliptic line.
After passing between the sun and Earth on August 13, 2023, brilliant Venus is up in the September eastern sky before sunrise. Venus will be brightest in the morning sky around September 19, 2023, and will reach 44 degrees elongation from the sun by the end of the month. The other inner planet, Mercury, will shine near Venus in the sky in late September 2023. Chart via John Jardine Goss/ EarthSky.

Reflectivity makes Venus bright

The albedo of Venus is close to 0.7, 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 – whose surface is dark volcanic rock – reflects only about 10% of the light that hits it. The moon appears bright to us because it’s close to Earth. It’s only about a light-second away, in contrast to the several light-minutes distance of 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 why Venus is so bright.

By the way, Venus is the brightest major planet. But it isn’t the most reflective body in our solar system. That honor goes to Enceladus, a moon of Saturn. The little moon’s icy surface reflects some 90% of the sunlight striking it.

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When and why is Venus brightest?

Venus is brightest when two factors combine – the phase of its 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.

Why does it happen? Because Venus orbits the sun inside Earth’s orbit, it sometimes goes between us and the sun. At such times, its lighted hemisphere, or day side, is facing away from us. Then it’s difficult to see Venus at all (though experienced astrophotographers sometimes catch it).

Also, around the time it passes between us and the sun, we see Venus 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. When it comes from behind the sun, racing ahead of us in orbit again, we see Venus wax in phase.

Meanwhile, as the crescent Venus waxes in Earth’s sky, the overall size of the disk of Venus gets smaller in our sky, as Venus speeds ahead of us. That’s what’s happening now.

Venus went between us and the sun on August 13, 2023. Now it’s rushing ahead of us in orbit. Its phase is increasing. But its disk size is decreasing. Greatest brilliancy happens when we see the greatest illuminated surface area of Venus: a combination of phase size and disk size. Astronomers call this a “greatest illuminated extent” of Venus.

It’s happening now in the east before sunrise … and it’s a sight to see! Don’t miss Venus blazing before dawn in September 2023.

Read about Venus at greatest brilliancy

Circle with sun at center, planets around, and zodiac names on outer edge.
View larger. | Heliocentric view of solar system, September 2023. Chart via Guy Ottewell.

More photos from our community

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 was a dazzling light in the west after sunset. Charlie Favret of Round Rock, Texas, captured this image on April 11, 2023, and wrote: “A photograph of Venus visiting the Seven Sisters. It is always a pleasure to see the 2 of these in the sky, and even better when they come close together!” Thank you, Charlie!
Thin crescent Venus in a slate blue sky.
View at EarthSky Community Photos. | Steven Bellavia of Mattituck, New York, captured this image on August 13, 2023, at 3:28 p.m. He wrote: “Venus, 0.9% illuminated, at (or very close to) inferior conjunction.” Thank you, Steven!
Line of crescents starting small and getting larger but thinner.
View at EarthSky Community Photos. | Vedant Pandey wrote: “I am Vedant Pandey, a 17 year old amateur astrophotographer from Varanasi, Uttar Pradesh, India. I photographed Venus since it appeared in the evening sky in February 2023. And here are the phases of Venus, from waning gibbous in February to its crescent phase in August, as seen by my telescope.” Wow! Thank you, Vedant!
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 30, 2023, and wrote: “Venus and Uranus at dusk from Dayton, Maryland.” Thank you, Jim!

Bottom line: You can’t miss dazzling Venus in the east before sunup now. It’s the 3rd brightest object in the sky, after the sun and moon. But why is Venus so bright?

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

September 17, 2023
Astronomy Essentials

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