It.s coming. This June, in Africa,and the Middle East, among other places. Unluckily for us, on Bainbridge it will be over before we can see it, but probably it will be tracked on a number of internet sites. Undoubtedly there will be lots of photos. It is rare: In the six thousand years between 2000 BC and 4000 AD it will happen only 81 times. It is the .Transit of Venus..
What this means is that Venus will cross in front of the Sun on June 8th. It.s a miniature eclipse of the Sun, but an eclipse is usually when the Moon gets between us and the Sun. An eclipse by the Moon takes only a few minutes; the transit by Venus will last six hours (from about 5:15 UT to 11:30 UT). Venus will block only 0.1% of the light because it.s a lot farther away than the Moon, so most people won.t know it.s happening. Even so, this is a big event.
The Transit of Venus, 1882. (Courtesy U.S. Naval Observatory Library)
Back in the 17th century when telescopes were invented, widely separate places. There they each tried to measure exactly where they saw the planet against the Sun.s disk at precisely the same time, and then they compared their results. They also added to their calculations Kepler.s laws of planetary orbits, their relative motions, and their relative distances to the Sun. With the differences between their results, they learned how much farther the Sun is from the Earth than they.d thought before.
Doesn.t it sound easy?
They had a lot of problems. For one thing, they didn.t have clocks that kept precise time, so their comparisons were only approximate. Besides that, they had great trouble pinpointing an exact time when they should all make their measurements. After astronomers figured out what time the next transit would occur, where in the world they could see it, and how to get there, weather and political conditions complicated their work: In the 18th century the French astronomer Guillaume le Gentil had to make his measurements from the deck of a rocking ship because of a war where he.d hoped to land.
astronomers realized that, with a transit, they had a But astronomers are persistent. With several more powerful new measuring tool to learn about the Solar transits and with some additional help from radar in the 20th century, scientists now calculate that the mean the world. It.s an opportunity to test our own ability to distance to the Sun (the Astronomical Unit) is 149,597,870.691 km, or more or less 93 million miles.
Since we know how far it is to a place most of us have no desire to visit, why should we still be interested in this June.s transit?
One reason is that no one alive today observed the last one in 1882, and although the next time will be in 2012, the one after that won.t come until 2117.
Then, it.s an educational opportunity on many levels: It.s an event that.s helped us learn how big our Solar System is, how the planets move, and what is in our Solar System besides what we can see with our naked eyes. It.s a chance to share in a scientific project with astronomers around the world. It’s an opportunity to test our own ability to calculate the distance to the Sun. It’s a time to study the history of astronomy. And then, there are always chances of unexpected or incidental discoveries. Captain Cook made one such discovery when he sailed to Tahiti in 1769 to observe the transit and bumped into a string of islands previously unknown to the western world—Hawaii. But this measuring tool of the distance to the Sun has an even bigger value: It’s part of why Earth-bound people can calculate with such fine accuracy how to throw a bit of machinery into the sky, then guide it, and know that at a certain time it will land on Mars, or take their friends to the Moon—and bring them safely back!