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SYFY WIRE Bad Astronomy

Astrophoto: Venus Rises in the Ashes of Comets

By Phil Plait

There are so many things to love about astronomy, but one of my favorites is when science and beauty merge and become one. The beauty is what first captures your attention, and the science is what makes you look closer and stay enraptured.

May I present to you my evidence: Venus rising into a beam of zodiacal light:

This photo was taken by astrophotographer Rudi Dobesberger, who has dozens upon dozens of similarly stunning pictures on his site (and more jaw-droppers on 500px). He took this photo on Nov. 16, 2012, in the early morning at Kalkalpen National Park in Austria. The sky was very dark; the glow on the left is Vienna (which was 150 km (90 miles) away), and Graz on the right (110 km/70 miles distant). Fog blankets lights from the town in the foreground, further damping light pollution.

Which was a wonderful happenstance, because it allowed the normally-invisible light of zodiacal light to shine through. This glow is due to sunlight reflected by dust shed by countless comets over the eons. These comets all had short-period orbits, most taking only 20 years or less to circle the Sun. Over time, itâs inevitable that such comets would have their orbits heavily influenced by the massive presence of Jupiter; in fact, itâs likely they started as comets on much, much larger orbits, but a close pass by Jupiter bent their paths inward toward the Sun. This warmer environment disintegrated the ice holding the comets together, so as they slowly died they sloughed off megatons of dust.

The orbits of all the planets are aligned fairly well, forming a flat disk: the plane of the solar system. The Earth does too, so weâre in that plane and we see it edge-on, a line across the sky. It passes through a series of constellations, which means the Sun and planets always travel through those constellations. As a group we call them the zodiac.

Those âJupiter family cometsâ orbited the Sun in the same plane as the planets, too, so their dust does as well. Put it all together, and you get the lovely term zodiacal light.

So think on this as you look at the picture: That lovely glowing beam apparently spotlighting Venus is actually a vast, flat cloud of dust, circling the Sun out by Jupiter; all that remains of a billion long-dead comets feebly reflecting the fierce light of the Sun so very far away.

See? Certainly the immediate beauty of the picture catches your eye, but itâs the reality behind it that catches your brain.

Tip oâ the lens cap to the Earth Science Picture of the Day.


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