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The transit of Venus

December 2012: Our sister planet's trip across the Sun produced some spectacular effects.
stephen_james_o_meara_new
On June 5, my wife, Donna, and I watched Venus sail across the Sun’s face from Volcano, Hawaii. From beginning to end, the nearly seven-hour event thrilled us with a series of remarkable phenomena. Earth’s atmosphere caused most of the effects, but Venus’ atmosphere created one, and another occurred in our minds. We observed them all at 100x through a 3-inch f/7 Tele Vue refractor with a full-aperture Astrozap Baader AstroSolar Filter.

The aureole of Venus
One of the most memorable moments came as Venus’ disk neared second contact, just as it completely slipped into the Sun’s disk. This is when we spied the “aureole effect,” a whisker-thin arc of white light surrounding the following limb, or “rear edge,” of Venus. It results from sunlight refracting through the planet’s dense atmosphere.
Venus-across-Sun
As Venus moved across the Sun's face during the June 5 transit, Earth's atmosphere dispersed some of the disk's color at far right (near sunset). The author created this composite image from photos taken through his 3-inch f/7 refractor; he and his wife, Donna, also viewed the event through safe Mylar filters without optical aid.
The aureole persisted throughout the “black-drop effect” — when Venus appeared to stick like gum to the Sun’s limb, before “peeling” off at second contact. At times, the aureole divided the black spot of Venus from a mirror-reversed “mirage” at the limb; it fully resembled the miraged Sun when it flattens into an omega (Ω) shape at the horizon. As Venus pulled away, its black disk turned more teardrop-shaped, but we could still see the aureole clinging to its tapered tip like a tiny dewdrop.

Mind, color, and form
Perhaps the most enjoyable aspect of the passage from second to third contact (when Venus started exiting the Sun’s face) was using our mind’s eye to “see” the three-dimensional clockwork motions of our solar system in action: Earth rotating as Venus revolved. We also imagined ourselves as inhabitants of a cloud-free Venus, gazing from the bright side at the monstrous Sun.
Black-drop effect
The "black-drop effect" is visible in this sequence, along with further atmospheric turbulence just before sunset.
From Volcano, the transit ended at third contact with another, much more prolonged and dramatic black drop, just before the Sun set behind the long slope of Mauna Loa volcano. About 45 minutes before sunset, we saw the so-called “reverse flash” of sunspots and of Venus’ silhouette.

George J. Gibbs first described the phenomenon for sunspots in the September 1918 Journal of the British Astronomical Association. As the Sun neared setting April 2, 1918, he wrote, its upper limb turned blue-green while its lower limb appeared red; at the same time, the largest sunspot showed the opposite effects. In 2004, Astronomy Contributing Editor Alister Ling saw these “reverse” red and green flashes originate from Venus at sunrise during its June 8 transit.
 
To understand the phenomenon, imagine the “top” of Venus’ silhouette forming an artificial lower limb to the Sun at that spot. When the Sun’s upper limb turns blue-green as it experiences the normal effects of atmospheric dispersion near the horizon, its artificial lower limb (at the top of Venus) appears orange-red, and vice versa.
Venus-took-on
Venus took on unusual shapes throughout its transit, including these two remarkable shapes witnessed in high atmospheric turbulence. The image at left shows an elongated silhouette, while at right Venus has a distinct polygon form.
Turbulent views
In concert with the dispersal of colors, Earth’s atmosphere became viciously turbulent as the Sun neared setting. For 45 minutes, we witnessed all manner of wild distortions to Venus’ silhouette. Most remarkably, the black spot erratically shape-shifted, turning into a polygon (looking like the facets cut into a black diamond), then into a blimp, and sometimes into a teardrop, among other shapes.
 
The Zeppelin-like form appeared most dramatic in the moments leading up to third contact and the final black-drop effect. At times, the black spot would elongate with opposing colors (red and blue) appearing to blow out of the tips in erratic puffs. The Sun’s setting limb itself became animated with moving ripples that led to rapid distortions in the black drop, before the mountain “swallowed” Venus — ending the show, but not its impact.

As always, let me know of your observations at someara@interpac.net.
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