On Tuesday afternoon, June 5, all of North America will be treated to another rare astronomical event a transit of the planet Venus across the face of the sun. Weather permitting, this will be the first Venus transit visible in the Southwest since June 8, 2004.
A Venus transit occurs when Venus passes directly between Earth and the sun, so that it is silhouetted against the face of the sun. Since Venus will be about 26 million miles from Earth at that time, it will appear only as a small black dot as it moves across the sun, its apparent diameter only about three percent of the suns apparent diameter.
Therefore, this event certainly will not be as spectacular as our recent annular solar eclipse of May 20. It is more of a curiosity, but Venus transits have their own lesson to teach us about the arrangement of our solar-system planets.
This lesson starts when we ask why Venus transits are so rare. They occur only in pairs eight years apart, then do not occur again for over a century! Thus, the next Venus transit will not occur until Dec. 11, 2117, 105 years from now. Thats a Saturday, so mark your calendar.
Since Venus orbits closer to the sun than Earth does, it travels faster around the sun, in only 225 Earth days, while Earth takes 365 days to travel around the sun. So why dont we see Venus transits every time Venus whizzes by Earth? We ought to see Venus transits once every year.
The answer to this riddle lies in the angular displacement between the orbits of Venus and Earth. Think of a huge, flat, round pizza with the sun at the center of the pizza and Earth traveling in its orbit around the sun on the circular edge of the pizza. That imaginary pizza forms a flat two-dimensional plane which astronomers call the plane of the ecliptic.
The plane of the ecliptic is an astronomical reference which we use to define the position of all other astronomical objects in our sky, including all other solar-system planets. Now, Venus has its own sun-centered pizza to describe its orbit around the sun, smaller than and contained entirely within Earths pizza, or orbit.
But the Venus pizza does not line up perfectly inside the larger Earth pizza. Instead, the Venus pizza is tilted at a slight angle to the Earth pizza, so Venus orbit is tilted 3.4 degrees to Earths orbit. In astronomical terms, Venus orbit is tilted to the official plane of the ecliptic.
This means that, each time Venus passes between Earth and the sun, it may be positioned vertically higher or lower than the sun rather than directly in front of it, so that a Venus transit cannot occur. Since Venus and Earth travel at different speeds around the sun, these two planets line up with the sun at different horizontal points in their orbits many times over the years. But both horizontal and vertical solar alignment must occur for Venus transits to be seen.
The time is right for another solar alignment. On June 5, the Venus transit will begin at 4:10 p.m. Mountain Daylight Time and last until 10:50 p.m., a duration of six hours, 40 minutes. That means the sun will set on our northwest horizon while the Venus transit still is underway, but you will see Venus cross the sun at an angle and off-center, which shows the tilt of its orbit.
As with any other solar observation, caution must be taken while viewing Venus transits. Since the sun is so bright and Venus will appear so small, it will be very hard to see Venus crossing the sun with the naked eye using only an approved solar filter or a #14 welders glass.
You might have better luck to poke a tiny hole in a cardboard sheet, then hold the sheet to project sunlight through the hole onto another sheet or a wall. An image of the sun appears on the second surface, and a pencil-point image of Venus might be visible.
The best way to see Venus transits is to aim a telescope or binoculars at the sun and project a solar image through the instrument onto a wall or cardboard. An image of Venus should be clearly visible as a black dot moving across the projected face of the sun.
Again, never view the sun directly through telescopes or binoculars without a correct solar filter. Solar rays are extremely concentrated as they pass through eyepieces, like a magnifying glass. Such intense rays will damage your eyes severely.
Using these safer methods, enjoy the last Venus transit well see for over a century.
Copyright © 2012 James F. Andrus; all rights reserved. Andrus earned a degree in meteorology from the University of Wisconsin, and an additional degree in computer science from Jan Juan College. He lives in Cortez, where he is a National Weather Service Cooperative Weather Observer. He has a lifelong interest in the physical sciences.
