Stargazing: Venus, Diamond-Bright
Photograph of the Moon, Venus and Jupiter by Sally J Smith © Copyright.
Today on the autumnal equinox we have a special sight in the night sky if the clouds clear - Venus at her most brilliant. Look for the planet in the western heavens shortly after sunset when she reaches her 'greatest illuminated extent'. Venus always shines as the third brightest celestial body (after the sun and moon) but at her most illuminated she is about two and a half times brighter than at her dimmest. Far more dazzling than any of the true stars in the sky, Venus does not appear to twinkle, but instead glows with a steady, silvery light
PHOSPHORUS AND HESPERUS
Venus is known as 'wandering', being one of the first celestial bodies to appear in the evening and last to disappear in the morning, but never visible late at night owing to her position between Earth and the Sun. At the moment Venus is brighter in the evening sky, but on 26 October she will become more luminous in the morning sky instead.
This pattern led the Greeks and Egyptians to believe Venus was two separate celestial objects. The early ancient Greeks named the morning star Phosphorus, 'the bringer of light', and the evening star Hesperus, 'the star of the evening'. A few hundred years later, the Hellenistic Greeks realised Venus was a single planet - a fact already deduced by the Babylonians in the 8th century BC.
WHAT MAKES VENUS SO BRIGHT?
As our immediate neighbour, Venus is relatively close to Earth but this isn’t the only reason the planet appears so bright. Compare and contrast with Mars, which waxes and wanes considerably in its luminosity. With Venus, something else is going on. Astronomers use the term albedo to describe a planet's brightness 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 expresses how much light strikes an object and how much is reflected back.
It's not surprising to learn that Venus has the highest albedo of any major planet in our solar system, reflecting back about 70 percent of the sunlight striking it.
Despite the moon appearing to be so bright, it reflects only about 10 percent of the light that hits it. The moon’s low albedo is due to its surface of dark, volcanic rock. It appears bright only because of its proximity to Earth (about a light-second away, in contrast to several light-minutes for Venus).
Venus has such a high albedo because it is blanketed in highly-reflective clouds. These clouds contain droplets of sulphuric acid, as well as acidic crystals suspended in a mixture of gases. Sunlight bounces easily off the smooth surfaces of the droplets and crystals, creating the brightness we associate with the planet.
Even so, Venus isn’t the most reflective body in our solar system - that honour goes to Enceladus, a moon of Saturn. Its icy surface reflects some 90% of the sunlight striking it, but unlike Venus there is no chance of spotting Enceladus with the naked eye.
TO LEARN MORE
Visit www.earthsky.org and www.solarsystem.nasa.gov
Copyright © Karen Meadows 2018