Physics Photo of the Week
Venus - Discussion by David Penketh
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.
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
Photo of the
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