Venus radiates brilliantly adjacent to a lunar crescent in the night’s darkness.(Image credit: Jordan Lye/Getty Images)ShareShare by:
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If you examine the heavens during a clear sunrise or twilight, you can’t miss Venus. Exhibiting a bright, consistently gleaming spot, it is the second most luminous object in the night sky, after the moon.
“The planet is roughly 100 times more radiant than a star of the first class,” Anthony Mallama, a researcher affiliated with the IAU’s Centre for Protection of the Dark and Quiet Sky, communicated to Live Science via email. Stars of the first class are the most radiant stars observable in the night sky. For example, considering average luminescence, Sirius, a star of the first class, registers at -1.47, while Venus comes in at -4.14 (using the scale employed by astronomers, less luminous objects show a more positive magnitude).
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Venus’ luminosity is greatly attributed to the planet’s elevated albedo, that is, the proportion of light mirrored from its surface. Venus possesses an albedo figure of 0.76, indicating it diverts nearly 76% of the sunlight it absorbs back into space, according to Sanjay Limaye, a noted scientist within the Space Science and Engineering Center at the University of Wisconsin-Madison. Conversely, an ideal mirror would send back 100%, Earth returns 30%, and the moon presents a diminished albedo, reflecting merely 7% of the light impinging on it.
Venus’ heightened albedo stems from a dense, fully enveloping shroud of clouds. Stretching from 30 miles up to 43.5 miles (48 to 70 kilometers) above the Venusian terrain, these tiers of clouds are cushioned by layers of haze and are chiefly suspended droplets of sulfuric acid, as per a 2018 analysis of data obtained from space expeditions to Venus in the 1970s and 1980s. Limaye pointed out that these droplets are minute, typically around the dimension of a bacterium. Collectively, the droplets and haze strata scatter sunlight in a profoundly efficient manner.
Venusian phases, resembling our moon’s phases as viewed from Earth. They were initially noted by Galileo during the 17th century.
However, Venus isn’t the most reflective celestial body in our solar system. Enceladus, one of Saturn’s icy moons, boasts an elevated albedo of approximately 0.8, as indicated in a 2010 study. Nevertheless, from Earth, this astral entity presents as significantly dimmer than Venus. This occurs because its separation from the sun is considerably greater. Whereas the “morning star” is positioned 67 million miles (108 million km) from the sun, Enceladus resides at a distance at least 13 times greater. The inverse square law elucidates that Venus, in effect, experiences 176 times more concentrated light compared to Enceladus, bestowing it with a noteworthy advantage.
Distance from Earth
Proximity to Earth also impacts Venus’ radiance. The mean distance between Venus and Earth is 105.6 million miles (170 million km). Occasionally, Mercury establishes itself as the closest planet to Earth with an average separation of 96.6 million miles (155.5 million km), but Venus’ greater dimensions (measuring 7,521 miles (12,104 km) against Mercury’s) prompt it to seem more brilliant.
However, Venus’ distance from our world — and by extension, its detectable luminosity — are not constant. At its minimal separation, when Venus positions itself squarely between Earth and the sun, it’s a mere 24 million miles (roughly 38 million km) distant, according to NASA. Yet at this juncture — labeled as the inferior conjunction — it presents as exceptionally faint, according to the National Astronomical Observatory of Japan.
A graph illustrating the fluctuation of Venus’ distance and luminosity from Earth across a complete orbit.
This arises owing to the inner planets displaying moon-like phases when observed from Earth, Limaye explained. At the point of inferior conjunction, Venus’ illuminated face is entirely obscured from Earth’s perspective. On the contrary, the bulk of Venus’ illuminated facade is observable solely when Earth and Venus are located on opposing aspects of the sun, a configuration referred to as the superior conjunction. During this alignment, however, Venus is at its most diminutive and significantly subdued due to its substantial distance from Earth.
A rainbow-like phenomenon
Venus shines at its absolute brightest when only a sliver of its sunlit surface is discernible, analogous to a crescent. Designated as the point of peak brilliance, this generally materializes a month prior to and following the inferior conjunction. A 2006 study, co-authored by Mallama, proposed that at this stage, Venus’ sulfuric acid droplets hanging in suspension reflect sunlight toward Earth. “This phenomenon is known as a glory, and it corresponds to the same class of optical occurrences encompassing rainbows,” Mallama specified.
When Venus attains sufficient radiance and illumination, it can be observed even during daylight hours — as demonstrated in this image, where it manifests as a shining speck near the 10 o’clock position relative to the crescent moon. RELATED MYSTERIES
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In summation, variations in the albedo, separation from the Earth and sun, coupled with phases observed from Earth, all contribute to fluctuations in Venus’ luminosity, spanning from -4.92 to -2.98, according to findings in a 2018 study. Nonetheless, this remains adequately radiant for Venus to remain discernible for a substantial portion of the year, inclusive of urban areas.
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Deepa JainLive Science contributor
Deepa Jain is a freelance science communicator residing in Bengaluru, India. Her academic background includes a master’s level education in biology from the Indian Institute of Science, Bengaluru, alongside a near completion of a bachelor’s degree in the field of archaeology from the University of Leicester, UK. She has a passion for composing content regarding astronomy, the physical world, and archaeological discoveries.
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