Invited guest post by Summer Ash
On Tuesday June 5th, Venus will pass between the Earth and the Sun for the second time in eight years and the last time for another 105 years. The transit of Venus, as it is called, is a very rare event in our solar system due to the relative alignments of the orbits of Earth and Venus around the Sun. Venus’ orbit is inclined at an angle just under 3.5° with respect to Earth’s. This may seem small, but on astronomical scales, this means that Venus will only pass between the Earth and the Sun directly along our line of sight twice every 120 years or so. This occurred in 2004, but unfortunately was not visible from much of the U.S. and the next pair of transits won’t take place until 2117.
The importance of this event is now mostly historical, but at one time it was essential to understanding the scale of our Solar System. While Kepler’s laws of planetary motion and Issac Newton’s law of universal gravitation allowed for the calculation of the relative distances of the planets from the Sun, but nothing was known in absolute terms. In 1716, Edmund Halley was the first to propose that measurements of the transit of Venus would allow for the Sun-Earth distance to be estimated. However, it wasn’t until the 1761 and 1769 transits that astronomers were able to obtain useful measurements that led to the first calculations of the true distance between the Earth and the Sun.
(The above graphic originally appeared in The New York Times).
This was accomplished through the principles of parallax. As illustrated in the figure above, two observers on different points of the Earth were needed to observe the transit independently. Each would record the time Venus first crossed into the disk of the Sun and the time it left it. Using the known separation distance of the observers on Earth, the distance to the Sun could then be calculated.
Once the Earth-Sun distance was calculated, the distances to all the other planets known at the time could be derived. As successive transits have occurred, more precise measurements were made and the Earth-Sun distance was refined. It was the last transit in 1882 that yielded our current estimate of approximately 93 million miles and defined our standard distance of the Solar System: the astronomical unit (AU), i.e. 1 AU = 93 million miles.
As you can imagine, coordinating observers around the planet is a lot easier now then it was in the 1700’s and 1800’s. One poor French astronomer was sent to the southeast coast of India to observe the 1761 transit, but his destination was deemed too dangerous en route due to the Seven Year’s war between France and Britain. He tried desperately to reach a safe landing point, but was still at sea when the transit occurred and was unable to take any useful data on the rocking deck of the ship. Reasoning that the next transit was only eight years away, he decide to wait it out in the Indian Ocean, passing the time mapping part of Madagascar. When 1769 arrived, he set sail for the Philippines but was once again derailed by international conflict, this time with the Spanish. Retreating to his original target in India, he arrived in plenty of time to view the transit only to be thwarted at the last minute by overcast skies. If that weren’t enough, when he finally gathered the strength of mind to return home, his tumultuous journey home culminated in the unpleasant discoveries that he had been declare legally dead, his wife remarried, and his family had divided up his estate.
There have now been six transits of Venus observed by humans since the invention of the telescope in 1609. Here are a few tips for how you can be part of the seventh without having to set sail for a foreign land:
- First, check to make sure you are located in a place where the transit will be visible.
- Decide which method of observing the transit works best for you.
- If you are located somewhere outside of the viewing area or if mother nature decides not to cooperate, never fear, NASA will be webcasting the transit from Mauna Kea in Hawaii.
My method of choice (absent a telescope with a solar filter) is a pinhole projector as it requires little more than some cardboard, some aluminum foil, and a direct sightline to the Sun. It also enables more than one person to watch the transit at a time. The only tradeoff is that the larger you make the projected image of the Sun (i.e., the larger the distance between the two pieces of cardboard), the fainter it gets. You can get around this by using a very long box to enclose the pinhole projector on three sides, which will preserve some of the brightness.
Regardless of your plans for the transit, please remember that you should never look directly at the Sun without proper protection. Viewing the Sun through clouds, sunglasses, unfiltered telescopes, binoculars or cameras can result in instant and permanent damage.
That being said, happy transiting!
Summer Ash is Director of Outreach for the Department of Astronomy at Columbia University and an instructor for Frontiers of Science in the Core Curriculum. Her doctoral research was on the evolution of radio galaxies and active galactic nuclei. She values the power of the scientific method, the history of science and the necessity of skeptical inquiry. As a self-professed space cadet, Summer grew up dragging friends and family out at all hours of the day or night to look up at the sky. In her previous life she was a rocket scientist, but now enjoys getting paid to spread her love of space with anyone who will listen. She attempts to blog at Newtonianism for the Ladies, tweets as @Summer_Ash, and is the in-house Astrophysicist for The Rachel Maddow Show.