It has been a busy month for solar enthusiasts, but tonight will host a once-in-a-lifetime event. Tonight is the last time that any of us will be able to view the transit of Venus across the solar plane. The planets won’t be in such perfect alignment for another 105 years, so head outside and carefully watch the celestial bodies dance before your eyes.
Planetary transit occurs when a planet passes directly between the sun and the Earth. This alignment can only occur between Earth and either Mercury or Venus since their planetary orbits lie closer to the sun than our own. Since Mercury orbits very close to the Sun, we can observe its transit every 13 to 14 years. Venus, on the other hand, has a tilted orbit with respect to the celestial plane, so being in the correct alignment to view a transit is a lot more rare, and won’t happen again till the year 2117.
The entire transit of Venus will be visible from Hawaii, Alaska, New Zealand, Japan, the Philippines, most of Australia, and parts of eastern Asia. Countries in the Western Hemisphere will see the transit on at sunset on Tuesday, while those in the Eastern Hemisphere will see it at sunrise Wednesday.
During the transit Venus will look like a small black dot crossing the suns surface. The diameter of Venus will be about 1/30th the diameter of the sun. In comparison it will look like a grape crossing in front of a watermelon. The transit might not seem visually exciting, but watching the black dot cross the solar surface is a really cool.
But beyond the “cool” factor, astronomers will be watching the transit with academic interests in mind. During the transit of Venus, the luminosity of the sun will be measured very carefully. When Venus covers a portion of the Sun, the Sun’s luminosity will dip slightly, and this dip can easily be measured. This dip can tell astronomers a lot about the size and distance of the object passing in front of the Sun. This is the same method that astronomers use to identify and observe extra-solar planets! By observing the transit of Venus, astronomers will be able to refine their current models quite a bit, since so many variables of the Sun-Earth-Venus system are already known, such as mass and relative distances. With its sensitive instrumentation designed to peer deep into the cosmos, Hubble Space Telescope can’t look directly at the sun. Instead astronomers will have the orbiting observatory aimed at the moon to watch for the slight drop in reflected sunlight during the transit. The hope is that this data will be a good companion to the data received by NASA’s Kepler spacecraft, which sole purpose is to search for new worlds using this light-transit method. Kepler has already confirmed 61 extra-solar planets and identified 2300 other possible candidates.
Astronomers also hope to use the transit as a chance to collect better data on the behavior of Venus’s atmosphere, in order to compare it to measurements that were taken by the European Space Agency’s Venus Express orbiter. ESA’s Venus Express has already returned detailed information about the weather patterns within Venus’ dense atmosphere, but with its close proximity to Venus, it can only observe one region of Venus at a time.
While the transit will be visible to the naked eye, it is NEVER recommended that you look directly at the sun. Use the same precautions as you would to observe the partial phases of a solar eclipse. You can use inexpensive “eclipse shades” to get a direct view of the event. Back in March, GeekMom Kathy described how to set-up a telescopes to project onto a simple white card or into a rear-projection screen. Perhaps the easiest way to watch the transit of Venus is to make a pinhole camera. To do so, cut a hole about a quarter-inch (0.6-centimeter) wide in a piece of cardboard paper, and use the hole to project an image of the sun onto a flat surface, such as a wall or sidewalk. (There are more details on Erik Weck’s post on GeekDad.)
And if clouds or geography keep you from viewing it in person, you can view the transit online. No matter how you observe, just be sure to observe safely — and enjoy!