What is a Transit?
More About the 2012 Venus Transit
A transit occurs when a planet passes directly between the Earth and the sun, so when viewed from the Earth, the planet appears to pass across the face of the sun. Transits occur with planets whose orbit is between the Earth and the sun -- or, specifically, Mercury and Venus. The transit of a planet is similar to a solar eclipse. The planet appears to be much smaller than the moon, so it cannot cover the sun. It looks like a small black dot slowly crossing the sun.
How Often Do Transits Occur?
Transits of Mercury occur quite regularly, but they are difficult to observe due to the small size of Mercury. Transits of Venus are more rare and interesting due to the larger size of Venus. Since the phenomena was first recognized, there have only been six transits of Venus: 1639, 1761, 1769, 1874, 1882 and most recently in 2004. The June 6 transit is the last opportunity to observe one of Venus until 2117.
History of Venus Transits
The first recorded sighting of a Venus transit was by British cleric Jeremiah Horrocks and William Crabtree, on Dec. 4, 1639. Scientists discovered the transit could be used to figure out the size of the solar system. In 1663, mathematician Rev. James Gregory suggested that a more accurate calculation of the Earth-sun distance could be made during the transit of Venus.
The Benefit of Observing Transits
Jeremiah Horrocks, a young English astronomer, recorded the first observation of a transit in 1639. In 1769, survey crews, including Captain James Cook, gathered transit data from various locations around the world that were used later to calculate the distance between Earth and the sun and the size of the solar system. Today, transit events are used to detect planets beyond the solar system. NASA's Kepler space telescope
measures the change in brightness from distant stars when a planet passes in front of the star. From the transit data, scientists can determine the size of the planet, the length of its year, and calculate the distance the planet is from its star. Kepler has confirmed 61 planets and more than 2,300 planet candidates using the transit technique.
More Frequently Asked Questions
Are there transits seen on other planets?
Yes. For any planet, all other planets orbiting closer to the sun are potential transit candidates. The best example is that there will be an Earth (and moon!) transit on November 10, 2084 as seen by an observer on Mars.
Why couldn’t you use Mercury Transits to gauge distances in the solar system?
You can! The problem is that Mercury is much further away so the angle of parallax is much smaller and harder to measure accurately. It is also a much smaller planet than Venus and so being further from Earth appears a lot smaller. The good news is that Mercury transits occur much more frequently than Venus transits. There are about 10 Mercury transits per century. The next one is in 2016.
Why would Venus not travel in a straight line across the sun?
That depends on how you view the transit. Using only binoculars (with filters!) or an alt-az mounted telescope, as the sun rises, moves across the sky, and sets, its orientation to you changes, swinging its pole one way on one side of the meridian (line connecting north and south across the sky) and the other way on the other side of the meridian. This change in the rotational sense of the sun's disk confuses the observer into thinking the path of Venus is not straight. Observers using properly aligned equatorially mounted telescopes will not see this effect and will see Venus traveling in a straight line across the face of the sun.
Janet Anderson, 256-544-0034
Marshall Space Flight Center, Huntsville, Ala.