Although invisible to the eye, Venus - along with Mercury - show up nicely in the coronagraph of the orbiting Solar and Heliospheric Observatory this morning. The lines are electronic artifacts caused by the bright planets. Credit: NASA/SOHO
What should you care that Venus is in inferior conjunction with the sun tomorrow? I mean, you can’t see the planet with the naked eye – it’s too close to the sun in the daytime sky. But there’s at least one reason why this conjunction brings good news for Venus watchers. It’s a sign the planet will soon return for a fine appearance in the morning sky. If we’re patient, we’ll be face to face with the goddess of beauty again in just a couple weeks.
As Venus orbits the sun, we see it pass through phases just like the moon. They're caused by the changing angle the planet makes with respect to the sun and Earth during a Venusian year. As seen by an observer (orange figure), Venus' phase wanes from half to crescent (left side) until reaching inferior conjunction (center). After tomorrow, the planet will move off to the right and appear as a thin crescent in the morning sky. Illustration: Bob King
While it may sound politically incorrect, inferior conjunction simply means that Venus is exactly between the Earth and the sun. As we gaze it its direction, we see the planet very close to the sun in the sky, because it’s in the same line of sight.
Venus' orbit is tilted about 3.5 degrees in relation to the Earth's, causing it to swing north of south of the sun during most conjunctions. Credit: Theresa Knott
You might think that Venus would cross directly over the sun’s face during conjunction, but its tilted orbit typically causes it to pass several degrees north or south of our star. That’s the case this time when the planet will lie only 6-7 degrees (about three fingers held together at arm’s length against the sky) due south of the sun today and tomorrow. Only experienced amateur astronomers exercising great care with the proper solar filters might attempt to tackle the task of spotting the planet. Should they succeed, the view would be unforgettable. Because Venus’ unlit backside is presented toward us, only an ultra-thin rind of crescent along its topside will be visible. While exceedingly delicate, the crescent will appear its largest since March 2009, with a diameter of one minute of arc or 1/30 the size of the lunar crescent. That’s theoretically big enough to resolve its shape with the naked eye. Too bad the searing sun will make Venus unviewable!
The thinnest edge of Venus will catch a bit of grazing sunlight today and tomorrow. Only 1/20 of 1% of the planet facing Earth is illuminated by sunlit during this inferior conjunction.
Venus’ large size is due to its proximity to Earth. Venus gets closer to us than any other planet around the time of inferior conjunction – today it’s just 25 million miles away. Even Mars, the solar system’s publicity hound, can only pull within 34.6 million miles of Earth at best. Since Venus is closest to us, it also moves fastest. During the upcoming two weeks, it will swing to the right of the sun – as seen in the diagram above – and pop out into the morning sky for easy viewing. We’ll start to notice its return in mid-November, when the planet will still be a very thin crescent. Maybe you’d like to see how sharp your vision is and try to see its shape. I’ve tried but never accomplished this feat of eyesight. Instead I use binoculars. Through them, the Venusian crescent stands out crisply – a wonderful sight.
Venus looks like a perfectly black, circular sunspot when it transits the sun as it did in June 8, 2004.
On rare occasions, Venus passes directly in front of the sun at inferior conjunction. They’re called transits, and they come in pairs separated by eight years that occur every 122 and 106 years. The last one took place on June 8, 2004 and the next will occur on June 6, 2012. The previous pair was in December 1874 and December 1882. My daughter Maria and I drove two hours through a thunderstorm in 2004 to catch the transit just after sunrise outside the city of Tower in northern Minnesota. She remembers the trip well, because we had to turn around as soon as Venus exited the sun’s face, so she could get back to Duluth in time for school. I love astronomical adventures.
Through November and into winter, Venus will pull away from the sun and shine brilliantly at dawn. It remains in the morning sky until August of 2011, when it reaches superior conjunction and once again appears too close to the sun to observe. In astronomy, superior means ‘outside’ or ‘beyond’ as opposed to inferior which means ‘below’ or ‘nearby’. During a superior conjunction with the sun, Venus will on the opposite side of its orbit from Earth (top position in the diagram) and farthest from our planet.
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