Transit of Venus

Some astronomical phenomena will look very much the same regardless of your choice of vantage point. The Orion Nebula will look the same as seen from any point on the Earth from which it is visible. Indeed, it would look the same from anywhere in the solar system. It is so distant that moving around within the solar system or on the surface of the Earth will not change your perspective. But closer to home, what we see very much depends on where we are. This is certainly the case for the upcoming transit of Venus, the second of a pair eight years apart and the last of the twenty-first century.

A transit is the passage of one astronomical object across the face of another. Only an object that is between us and the sun can undergo a transit across it, and the only planets that can do so are Mercury and Venus. Mercury transits are relatively common with 13 or 14 per century; the last was in 2006 and the next will be in 2016. Transits of Venus, however, occur in pairs eight years apart, with the pairs separated by more than a century. Why are Venus transits so much more rare?

How frequently a transit occurs will depend on several factors:

1. How big is the object across which the object transits? This is the same (the sun) for both Mercury and Venus, so there is no difference here.

2. How big is the transiting object as seen from Earth? Venus is both absolutely and apparently larger than Mercury, so it has the advantage here. In the diagram below, two objects both follow the same path, but only the larger of the two can be seen on the face of the sun.

3. How close to the sun does the object orbit? The closer it is, the more often it will transit. Imagine an object that is orbiting right at the visible “surface” of the sun: you would see it transiting for half of each orbit around the sun. Mercury orbits the sun every 88 days and Venus every 225 days. But we are also orbiting around the sun, and the more relevant figure is how often the two planets pass between us and the sun. The time interval for Mercury varies, but averages about four months. Venus’ time interval is just over nineteen months. This is the factor which results in more frequent transits for Mercury than for Venus.

4. What is the planet’s inclination to the ecliptic plane? Say what? Now you’re just talking geek. Let’s translate that sentence into the vernacular. The ecliptic is the apparent path that the sun takes in our sky over the course of a year. This apparent motion is of course actually due to the Earth’s orbital motion around the sun. Take the Earth’s elliptical orbit, and put it down flat on your coffee table. (Use your imagination.) The flat surface of your coffee table is the ecliptic plane. Now imagine Venus’ smaller orbit. While it lies in an orbital plane very close to that of the Earth, it is not exactly the same. It is tilted (inclined) a little over three degrees from the ecliptic. This means that for most of the passages of Venus between the Earth and the sun, Venus passes either above or below the sun’s disk from our point of view. Only on rare occasions do the two planets line up with the sun and let us see a transit. Even though Mercury’s orbital inclination is greater (seven degrees), its closer-in orbit still means it transits more frequently than Venus.

It is a testament to the power of the laws of physics that Jeremiah Horrocks was able to correct some earlier calculations by Johannes Kepler, then predict and observe the Venus transit of December 1639 from his home in England. He is the first person ever known to have done so. The next transits occurred in 1761 and 1769.

They aroused great interest and occasioned some of the first international scientific collaboration, as expeditions were sent to various spots around the globe. The reason for this had to do with simple trigonometry. The relative spacing of the planets was well known by this time with the application once again of the laws of physics and planetary motion. Venus, for example, was known to orbit at 72% of the Earth’s distance from the sun. But these were relative distances only; the absolute value of the Earth’s distance was unknown. The average distance of the Earth from the sun is called the astronomical unit.

Geometry to the rescue! If the transit were observed from widely separated points on the Earth, the slight differences in Venus’ position could be used to determine how far away it was, and therefore the absolute distance scale for the planets.

The 1769 transit was observed by the British explorer Captain James Cook in Tahiti, from a location still known as Point Venus. The Apollo 15 command module and a space shuttle were both named Endeavour after Cook’s ship, and Star Trek’s Captain Kirk’s name was inspired by Cook. Observations from the 1761 and 1769 transits were used to calculate a new value for the astronomical unit that was within 2.3% of the currently accepted value. An even more accurate value was calculated when the new science of photography was put to use for the transits of 1874 and 1882.

Today we have far more accurate means for detecting distances to nearby planets. We can time reflected radar pulses and determine the distance to within a few meters. So what is the scientific value of a Venus transit in 2012, when the only people who travel across the globe to see it are wealthy tourists?

One method used by scientists looking for planets orbiting stars around other planets is to observe these planets as they transit across the faces of their host stars. The scientists hope to calibrate some of their instruments by observing a transit much closer at hand; you can read about that here.

Okay, enough with the history—how can you see the transit for yourself? It will begin around 6:15 p.m. EDT on June 5th. For each time zone west of this, subtract an hour; those of you in the Pacific time zone will see the start of the transit with the sun much higher in the sky. The transit will still be in progress at sunset from all locations in the continental U.S.; the entire transit will be visible from both Alaska and Hawaii, from Japan and the east coast of Australia and from much of the Pacific.

How can you view the sun safely during the transit? This article gives you six ways to do so.  Whatever you do, DO NOT look directly at the sun! Don’t imagine that sunglasses will adequately protect your eyes.

And keep an eye on the weather! The forecast for Virginia does not look too promising right now. Keep the car gassed up and prepare to travel if you need to. Your next chance for a Venus transit will be in 2117, so you’ll probably want to make this one.

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