Why is Venus so bright?

If you peer at the sky during a cloudless dawn or dusk, you'd immediately spot Venus. Appearing as a brilliant, steadily shining speck, it's the second-brightest object in the night sky after the moon.

"The planet is about 100 times brighter than a first magnitude star," Anthony Mallama, a researcher at the IAU's Centre for Protection of the Dark and Quiet Sky, told Live Science in an email. First magnitude stars are the brightest stars visible in the night sky. For example, when looking at average brightness, the first magnitude star Sirius is at -1.47, and Venus is at -4.14 (on the scale astronomers use, dimmer objects have a more positive magnitude).

But what makes Venus super-bright? Astronomical research suggests several factors can change how luminous Venus appears from Earth.

Reflective cloud cover

Venus' shininess is largely due to the planet's high albedo, or the amount of light reflected off its surface. Venus has an albedo of 0.76, meaning it scatters about 76% of the sunlight it receives back into space, according to Sanjay Limaye, a distinguished scientist in the Space Science and Engineering Center at the University of Wisconsin-Madison. In contrast, a perfect mirror would bounce off 100%, Earth bounces 30% and the moon has a low albedo, reflecting just 7% of the light that hits it.

Venus' high albedo arises from a thick, all-swaddling cloak of clouds. Extending from 30 miles to 43.5 miles (48 to 70 kilometers) above the Venusian surface, these decks of clouds are cushioned between haze layers, and are mostly suspended droplets of sulfuric acid, according to a 2018 review of data from 1970s and 1980s space missions to Venus. Limaye noted that such droplets are tiny, mostly about the size of a bacterium. Together, the droplets and haze layers scatter sunlight extremely efficiently.

But Venus isn't the solar system's shiniest object. Saturn's ice-covered moon Enceladus has a high albedo of around 0.8, a 2010 study noted. From Earth, though, this cosmic object appears much dimmer than Venus. That's because it's much farther from the sun. While Earth's "morning star" is 67 million miles (108 million km) from the sun, Enceladus is at least 13 times as distant. The inverse square law shows that Venus consequently receives 176 times more intense light compared to Enceladus, giving it a significant edge.

Distance from Earth

Being close to Earth also influences Venus' brightness. The average Venus-Earth distance is 105.6 million miles (170 million km). Sometimes, Mercury is the closest planet to Earth at an average distance of 96.6 million miles (155.5 million km), but Venus' larger size (of 7,521 miles (12,104 km) compared to Mercury causes it to look brighter.

But Venus' distance from our planet — and consequently, its apparent luminosity — aren't fixed. At its closest, when Venus lies directly between Earth and the sun, it's a mere 24 million miles (about 38 million km) away, according to NASA. Yet at this point — called the inferior conjunction — it's actually extremely dim, according to the National Astronomical Observatory of Japan.

This arises because the inner planets show moon-like phases when viewed from Earth, Limaye said. At inferior conjunction, Venus' illuminated surface is completely invisible from Earth. In contrast, most of Venus' illuminated surface can be seen only when Earth and Venus are on opposite sides of the sun, a position called the superior conjunction. At this point, though, Venus is at its smallest and is very dim because it is extremely far from Earth.

A rainbow-like phenomenon

Venus is at its brightest when only a crescent-like sliver of its sunlit surface can be seen. Termed the point of greatest brilliancy, this typically occurs a month before and after the inferior conjunction. A 2006 study co-authored by Mallama suggested that, at this phase, Venus' suspended sulfuric acid droplets scatter sunlight toward Earth. "This phenomenon is called a glory and it is in the same family of optical effects that includes rainbows," Mallama explained.

Together, variations in the albedo, its distance from the Earth and sun, and its phases seen from Earth can all cause the brightness of Venus to swing from -4.92 to -2.98, according to a 2018 study. However this is still luminous enough to make Venus viewable most of the year, even from urban areas.

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