During transits of Venus, a bright arc about 0.1 second of arc wide has often been reported, seen around the circumference of Venus’ disk which is partially outside the solar limb. It was first described by Russian scientist Mikhail Lomonosov when he observed the transit of Venus in 1761. Just after interior contact at egress, this aureole effect starts with the appearance of a bright spot of light near one of the poles of Venus. Generally, this spot gradually grows into a thin arc as Venus moves further off the sun. The ring of light may then either reduce to a segment close to one of the planet’s poles or completely encircle the planet near exterior contact, showing brighter spots or zones. Sometimes, the annular ring of light can be distinguished, in continuation, around the limb on the sun, but this continuation is rendered visble chiefly by the movements taking place in it. At ingress, the phases occur in reverse order. The brightness of the aureole is close to that of the solar photosphere, making it visible through a solar filter. Though, it can only be seen under good observing conditions, using an excellent telescope.

The elusive aureole effect during egress on 2004 June 8, captured by the Swedish 1-m Solar Telescope on La Palma. Click the image to see the movie. (Movie by courtesy of Dan Kiselman, Royal Swedish Academy of Sciences)

The following table gives an overview of the different reported aspects of the aureole effect.

Venus leaves the sun’s disk during egress on 2004 June 8. The aureole effect became very bright according to this drawing by Mario Frassati from Crescentino, Italy. Note the asymmetry of the aureole’s appearance.

The ring of light is caused by refraction – not scattering – of sun light in the dense upper atmosphere of Venus. The Cytherean atmospheric conditions determine the appearance of the aureole. If the refraction index of the atmosphere is small, the aureole already breaks into bright spots when the disk of Venus is just off the solar disk. But if the refraction index of the atmosphere is high, the aureole will extend all around the planet’s limb as a complete arc as long as Venus is less than half off the solar disk. If Venus is further off the sun, the arc will detache from the solar limb at both ends, and reduce to a spot of light near one of the planet’s poles.

Enhanced movie of the aureole effect during egress on 2004 June 8, observed by André Rondi using a home made H-alpha coronograph. Note that the limb brightens close to the south pole of Venus. To the right of the south pole, there is a second bright spot, which may be caused by variations of cloud density in the Cytherean atmosphere.

Numerous people observed and recorded the aureole effect during the 2004 transit of Venus, at both ingress and egress. Most of these observers saw the aureole when Venus was at least halfway onto the sun, indicating a dense Cytherean atmosphere. Belgian observer Tonny Vanmunster saw the aureole at ingress: “Shortly before the very black disk of Venus completely slided into the sun’s bright edge, it grew a small, easy to spot halo (aureole) of light around its dark edge. The halo lasted a few minutes and then vanished.” At egress the aureole was seen again: “Shortly after the start of egress, I switched back to visual observing, and immediately noticed Venus’ atmosphere again,” says Tonny. “My wife Kathleen also easily spotted it, although I didn’t mention where to look. Again, the halo disappeared after a few minutes, shortly before half of Venus’ disk had left the sun’s limb.”

Orientation of Venus’ disk at ingress (left) and egress (right) at the 2012 transit. When Venus is halfway or less off the solar disk, you might expect a bright polar spot near the north pole of Venus.

Because Venus will pass in front of the northern half of the sun’s disk during the 2012 transit of Venus, the aureole will then form near the north pole of Venus. Be on the lookout for this remarkable phenomenon!

Photograph by Lorenzo Comolli from Tradate in Italy, depicting the aureole at ingress on 2004 June 8, 5.34 UT.