Last week’s transit of Venus made for some big scuttlebutt in nerdy astronomy circles. What’s the big deal with Venus’ orbit bringing it between us and the sun? After all, from Venus’ perspective, it’s just business as usual. Well, it’s our perspective that makes the event so interesting. It is, after all, the story of how we learned the scale of our solar system.
In the early 1600s, Johannes Kepler published his laws of planetary motion which allowed astronomers to calculate the relative distances of the planets from the Sun. In other words: how many Earth distances from the sun the planets were. We call this distance of the Earth from the sun, one astronomical unit. What we didn’t know was what the actual measurement of an AU was.
To demonstrate his laws, Kepler calculated that Venus would transit the sun and be visible from Earth as a black dot passing against the bright background in 1631 and again in 1761, 130 years later. Unfortunately, it wouldn’t be visible from Europe, where Kepler lived ... and then he died ... in 1630, one year before the transit. Everyone missed it.
Thankfully, a young astronomer named Jeremiah Horrocks was paying attention. Based on his own measurements, he realized Kepler’s calculations weren’t completely accurate, and that another transit of Venus would be visible from his England home in 1639.
On that cloudy winter day, minutes before sunset, Horrocks and his partner, William Crabtree, were able to briefly project the event onto the wall of his bedroom using a small telescope. The measurements they took from their single vantage point not only allowed them to calculate the size of Venus and details of its orbit, but to more accurately approximate the distance of the Earth from the sun than ever before.
Horrocks died two years later, and Humanity would have to wait a few more generations before refining these measurements and learning the true scale of our solar system.
In 1716, Edmond Halley devised a more precise way of using the Venus transit to discover the value of the AU. He posited that two people from different vantage points on the Earth could record the time it takes Venus to pass in front of the sun and compare the times of passage with the distance between their vantage points. From there, the distance from Venus to the Earth could be calculated. Then by using Kepler’s laws, they could extrapolate the distance of the Earth from the sun.
Halley died in 1742, 20 years before the next transit in 1761.
Fortunately, 20 years was enough time for that idea to make it around the civilized world and be improved upon. Joseph-Nicolas Delisle noted that if astronomers watching the transit knew their precise locations and recorded the times that the edge of the planet lined up with the edge of the sun, they wouldn’t need to have a clear view of the entire transit.
By this time, the Americas were in the picture, and astronomers from everywhere sailed all over the world to record the transit. Sea travel was expensive and time consuming, so many of the voyages took on additional scientific expeditions on their way to view the transit (one discovery of this era was that of Australia and New Zealand by Captain Cook).
From the transits in 1761 and 1769, astronomers were able to narrow down the distance of Earth from the sun between 94 and 96 million miles! Pretty close in astronomical terms, but everyone would have to die and humanity would have to wait out a few more generations before getting another chance to work out the precise distance.
One benefit of turning over multiple generations of humans is that you get multiple generations of science and technology. The transits of 1874 and 1882 were photographed!
The United States Naval Observatory sent out multiple expeditions that took more than 1,600 photographs of the transits of 1874 and 1882. After studying the data for nearly two decades, William Harkness came up with a distance of 92,797,000 miles, give or take 59,700 miles. Closer still, but not the answer we were looking for.
Thanks to radar, by the transit of 2004, we already knew the distance of the Earth from the sun was 92,955,589 miles. But the rarity of the event (remember, no living human in 2004 had ever seen a Venus transit before) and the sacrifices our ancestors made to uncover the mystery of the AU inspired another generation of enthusiasts to take new measurements. Over 2,500 astronomers recorded measurements of the transit and came only .0007% short of the accepted AU value.
Last week, more people than ever were able to witness the event. Telescopes, binoculars, and pinhole projectors all over the world pointed to the sun while millions looked on in person, and millions more watched online. One particular telescope, however, was not pointed in the direction of the sun. The Hubble Space Telescope, with it’s low Earth orbit vantage point, was aimed at our moon.
When Venus obscured a portion of the sun hitting our planet, it also obscured a portion of the sun hitting the moon. The Hubble telescope is sensitive enough to pick up those differences in light reflected from the moon when a Venus-sized object is obscuring the sun. Detecting these kinds of differences in light is the primary technique we use to find planets in other solar systems. Astronomers hope to use the information they collect from this Venus transit to find smaller, rocky planets that closely orbit their stars. These planets are the ones most likely to harbor life.
What will these astronomers find? Will they too come up just short of unlocking a key piece of information that will have to wait for future generations of astronomers? I expect it will be a number of years before we know the answers to these questions. And that is the point of why I’m writing all of this down. The journey is often times more inspiring than the discovery. After all, we are no less amazing than the spectacular things we see in the sky. We are made up of the same elements as everything else in the universe. We are the universe discovering itself. Our perspective matters.