The transit of mercury

Guest post by Alex Diaz

Whenever 2 objects get really close together, you can bet an Astronomer will know about it and use everything that they have at their disposal to study such wondrous events. The transit of Mercury occurred last week on May 9 and it was one of the 13 transits it will make each century. A rare event indeed, these transits are known as planetary transits. Only Mercury and Venus are able to go across the Sun, from our perspective, as they are the inner most planets after Earth. Despite Mercury zooming around the sun every 88 days, Earth, the sun, and Mercury actually rarely align. As well, Mercury orbits in a plane that is tilted from Earth’s orbit, it usually moves above or below our line of sight to the sun. It’s because of these 2 things that we only see these planetary transit events occurring 13 times every century.

What’s the big deal about these transit events though? If you’ve had a chance to see one in a telescope with proper filters that allow you to safely view the Sun, then you’ll know that you simply see a black dot moving across the Sun very slowly. They are actually very useful to us for many reasons. They provide a great opportunity to study the way planets and stars move in space – information that has been used throughout the ages to better understand the solar system and which still helps scientists today to calibrate their instruments. Three of NASA’s solar telescopes watched the transit for just that reason.

Mercury Transits, as well as Venus Transits, allow instruments to be calibrated to detect dips in in brightness in Stars. These dips in brightness signal to us that there is some object orbiting the Star. With how small Mercury is compared to our Sun, these spacecraft can calibrate their instruments to a level of sensitivity of finding small planets like Earth, Mars, Venus or Mercury. We can get data on how big a planet is, what the chemical composition of the atmosphere is (if one was detected), how fast its orbiting its Star, and its distance from it.

The search for these exoplanets, as they are called, has become synonymous in our search for life in other far away solar systems. Take the Kepler spacecraft mission for example. NASA announced last week that the Kepler spacecraft mission had discovered 1,284 more planets in addition to what it has already discovered. This brings Kepler’s total to more than 2,000 planets discovered in a narrow patch of the sky that’s just about the size of your fist if you hold it up in the sky. This remarkable feat has used planetary transits in many of these discoveries to find other planets outside of our solar system.

Mercury transits have been key to helping astronomers throughout history: In 1631, astronomers first observed a Mercury transit. Those observations allowed astronomers to measure the apparent size of Mercury’s disk, as well as help them estimate the distance from Earth to the Sun, known as an Astronomical Unit (AU).

Since then, technological advancements have allowed us to study the sun and planetary transits in greater detail. In return, transits allow us to test our spacecraft and instruments.

Regardless of how simple some astronomical events may seem, we can always mine them for an incredible amount of information that can help us better understand the universe around us. The Transit of Mercury is such an event that allows us to do that and will continue to do so for centuries to come!