Because of the different timing of the orbits of the solar system's planets and their different trajectories, lining up is a pretty rare event. From our vantage point on Earth, Venus passing in front of the sun is very rare indeed. It happens just twice every 115 years, with 8 years between transits.
The most recent transit of Venus across the sun occurred in June 2012, and, in a paper published in June in the journal Nature Communications, NASA has explained how it used images of the event to examine the planet's atmosphere.
The previous transit occurred in June 2004. This means that researchers were able to get better photos of the transit than we've ever seen before, capturing the entire event in several wavelengths.
The high resolution of the images, combined with the sun providing backlighting, allowed a team of researchers to learn about the composition of Venus' atmosphere. Just like Earth and Mars, the layers of Venus' atmosphere absorb light differently from each other.
Since the sun emits light in almost every wavelength of the electromagnetic spectrum, scientists were able to analyse the images to determine how light filters through each layer of Venus' atmosphere and more accurately ascertain the molecules and atoms it contains.
For instance, the thermosphere layer of the upper atmosphere absorbs some high-energy wavelengths, which will make the layer seem opaque when viewed in one of these wavelengths. In the ionosphere, a layer created by radiation being absorbed by the atmosphere and creating ions that capture light.
When viewed against the sun in these wavelengths, the atmosphere then appears opaque, which means the size of Venus appears to change as the atmosphere blocks light.
By measuring the size change of Venus across various wavelengths a team of scientists led by Fabio Reale of the University of Palermo in Italy was able to calculate the composition of Venus' atmosphere. Because various atoms absorb light differently, the wavelengths blocked by each atmospheric layer allowed the team to tell which atoms are inside it and from there construct a molecular profile of the atmosphere.
This information will be vitally important for NASA's proposed mission to Venus.
"Learning more about the composition of the atmosphere is very important for understanding the braking process for spacecraft when they enter the upper atmosphere of the planet, a process called aerobraking," Reale said in a statement.
The research also allowed the team to take a look at whether the atmosphere changed at sunrise and sunset. Because Venus was between Earth and the sun, we could only see the night side of the planet -- but around the edges, where the team was observing the atmosphere, is the edge between night and day -- sunrise on one side and sunset on the other.
At these transitional periods from night to day and day to night, interesting effects can occur in the ionosphere. If the atmosphere was different at sunrise and sunset, that would have resulted in an asymmetry -- one side of the planet would appear bulged in certain wavelengths as the atmosphere blocked light that wasn't blocked on the other side.
This bulge never eventuated, which means the atmosphere remains the same at sunrise and sunset.
Studying the atmosphere of Venus adds to the database of information we have about atmospheres, which in turn could help study exoplanet atmospheres.
"In the future, there might be missions that have enough sensitivity to detect the difference in radius in different wavelengths," said Reale. "In particular, if there are exoplanets with an extremely thick thermosphere, the size difference in different wavelengths will be larger and there will be a better chance of detecting the change."
The next time Venus is due to pass between the Earth and the sun will be in December 2117.