Physics Photo of the Week

March 27, 2009

Venus - Discussion by David Penketh
Venus is the second planet in our solar system, and is commonly referred to as Earth's 'sister planet' because, like Earth, it is a terrestrial planet, and it is similar in size and mass to Earth.  In fact, Venus's diameter is only about 650 kilometers smaller than Earth's.  Venus's atmosphere is about 96.5% carbon dioxide and close to 3.5% nitrogen.  It is believed that Venus's internal structure is similar to Earth with a liquid core, a mantle, and a crust.  The principal difference between the planets is that Venus lacks plate tectonics.  This prevents the planet from losing heat, helping to keep Venus hot.  Actually, despite being approximately two times farther from the sun than Mercury, and receiving less solar radiance, Venus is the much hotter planet.  A likely factor in this is Venus’s atmosphere, which is 92 times denser than Earth’s.

In September of 1610, Galileo showed that, like our moon, Venus displayed phases.  Galileo’s findings went against the long-held Ptolemaic (Earth-centered) beliefs that the crescent could only exist because Venus's epicycle always put it between the sun and the Earth.  Instead, Galileo's observations gave credence to the Copernican (or sun-centered model).  The photo above (taken on March 17, 2009) shows Venus at about its brightest when it is between the Sun and the Earth.  Galileo discovered that Venus - when not so bright - is small and nearly full like a full moon.  Logically, if Venus was always between the sun and the Earth, we could  never see it completely lit up, like a full moon.  The planet presents a small full image when it is on the opposite side of the sun.  It shows a larger 'quarter phase' when it is at its maximum elongation from the Sun.  And, in the night sky, Venus is at its brightest when it presents a larger crescent shape.  This occurs when it comes around to the near side between the Earth and the Sun.


Venus this weekend is between the Earth and the Sun and is no longer visible in the evening at sunset.  Soon Venus will be visible before sunrise in the eastern sky. 

The photo at the right was taken with a DSLR camera mounted only on a tripod while using a 300 mm telephoto lens.  The photo at the top of the page was taken with a DSLR camera mounted at the focus of an 8-inch Schmidt-Cassegrain telescope.  The relatively large size of the crescent-shaped Venus when it relatively close to the Earth permits easy photography.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to 

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