Venus Approaching Inferior Conjunction

Based on a number of comments to this blog, many people have noticed Venus. Shining high in the evening sky over the past few months, Venus is the brightest thing in the sky after the Sun and Moon. All this is rapidly changing. Those of you who have been watching Venus night after night have probably already noticed that it is not as high in the sky as it was a month ago. Careful observers may also have noticed that Venus is not as bright as well (though still much brighter than any other planet or star).

Venus’ reign over the evening sky is about to end. Over the next few weeks, Venus will be seen lower and lower in the sky. By the end of the month, it will no longer be visible as Venus shifts to the morning sky.

So what’s going on?

Being the 2nd planet from the Sun, Venus orbits inside the orbit of Earth. Like a race car passing on the inside of a turn, Venus is passing the Earth as they orbit the Sun. On March 27, Venus catches up with the Earth and passes between the Earth and Sun, an event known as “inferior conjunction“.

Just like the Moon, Venus displays different phases. It was these changing phases that Galileo observed ~400 years ago and convinced him that Venus was orbiting the Sun rather than the Earth. When it is located on the other side of the Sun, we see a small Venus with a near-full phase. As Venus comes closer to Earth, its phase becomes gibbous (between half and full), then half and finally it appears as a crescent. The whole time Venus increases in size  it comes closer to Earth. Its crescent shape is because as it approaches inferior conjunction, we view more and more of its nightside. After inferior conjunction, Venus vaults into the morning sky and the progression of phases reverses (crescent to half to gibbous to full) while its distance from Earth increases. An example of Venus’ changing phases can be seen in the collection of images taken by Statis Kalyvas of Thessalonica, Greece in2004.

The changes are obvious in images taken with my 12″ telescope over the past ~10 days. In that time, the distance to Venus has decreased by 16% meaning Venus appears 16% larger (from 43″ to 50″ in diameter). The percentage of its disk illuminated by the Sun has also decreased from 22% to 13%.

At inferior conjunction on the 27th, Venus will be almost an arc minute in diameter (59″) but only 1% illuminated. On that date, Venus will be a difficult observation since it will be located only 8 degrees from the Sun. Observers with an unobstructed view of the western and eastern horizons may be able to catch a view of Venus both in the evening and morning sky. At that time, a telescope would see Venus as a ring like the bottom right photo in the images above from 2004. What we are seeing, is the light from the Sun passing through the atmosphere of Venus.