How the transit of Venus opened the planet to our forefathers

Author Mark Anderson of 'The Day the World Discovered the Sun' explains how the transit of Venus allowed 18th-century astronomers to create an early GPS system.

Last weekend, many of us here on the West Coast dabbled in a bit in astronomy during a partial eclipse of the sun. With the sun low on the horizon, I stole a quick glance to see if I could glimpse a shadow.

Yup, I did. And then I saw something else: stars. My eyes!

My advice: Do not try this at home. And definitely don't even think about it at around sunset ET on June 5, when the planet Venus will do something really unusual. It will dart across the sun, and much of the world will be able to watch it make its move with the right (and safe) equipment.

It's called the transit of Venus. The planet will appear as little dark circle on the sun as it zips between us and that big yellow ball.

If you want to catch it, hope for a clear sky because you won't see it again. About every 120 years or so, transits of Venus come in clusters – two in eight years. The last one was in 2004, and we won't see another until well into the 22nd century. Astronomers long ago figured out when the transit of Venus would occur, and they discovered that timing it down to the second would give them tremendous insight into the solar system.

When the event occurred on June 3, 1769, scientists – including the famous Captain James Cook – fanned around the globe in an attempt to uncover the secrets of the universe.

In his new book The Day the World Discovered the Sun: An Extraordinary Story of Scientific Adventure and the Race to Track the Transit of Venus, author Mark Anderson uncovers the tales of the men who literally went to the ends of the earth in search of discovery.

In an interview, Anderson talked about the goals of the missions, the way 18th-century scientists managed to measure time to the second (hint: not with an iPhone stopwatch), and how clouds on the big date may be more than just a mood-buster.

Q: What was the big mystery that all these scientists were trying to solve?

A: In two words, it was "how far."

How far is the most important thing in our universe, the sun? And how far would you go to find that answer? Would you be willing to lay down your life? A host of scientists and explorers at this time did just that.

The other question is essential to the story: How far is the shore? That's the other question, the one that ship navigators could not answer. As a result, thousands of people died gruesome deaths at sea by shipwreck or being lost at sea.

These astronomers had invented an 18th-century GPS system that opened up the entire planet to seafaring nations. The voyages were the perfect testing grounds for this new technology.

The cool thing for the story is that to find the answer to both questions, you have to go to the ends of the earth. That sets the stage for an amazing adventure.

Q: How did this one event in the skies provide so much information?

A: Once you got to these far-flung locations, you could time – down to the second – the  duration of how long it takes for Venus to pass over the sun. It was about six hours, but it varied depending on where you were on earth. They had to send people to Arctic locations and tropical locations to get as different a set of answers as you could get.

When you measure it down to the second, then you can answer these sorts of astronomical, scientific, and even philosophical questions.

Q: How did the astronomers time the transit so efficiently before they had modern clocks?

A: They would take these big, hulking pendulum clocks with them. Once you landed and set up shop, one of the things you get set up is the pendulum clock. Then you can get a very accurate reading, down to fractions of a second.

Q: Were they like grandfather clocks?

A: Grandfather clocks are quaint versions of the precision clocks.

Q: What drew you to this story?

A: Books talk about the science and all these voyages, yet the thing that I found most wanting in all these stories was just a simple answer to the question why. Why did all of these people risk their lives, why did kings and emperors write checks for the equivalents of millions of dollars to send these voyages halfway across the planet?

The answer: The astronomers were providing the kings with the keys to the world, the solution to navigation. It wasn't just about the scientific question, although it motivated the scientists themselves. It was also about what you might call the founding-father era’s military industrial complex.

It was so fundamentally tied into these matters of the utmost national interest and national security matters. To say it was about life and death on the high seas is no exaggeration. If you can’t find the shore, then before morning is light, your ship could be dashed on the rocks.

Q: You had to see the sun to see Venus wander across it. If it was cloudy on that one day, were you doomed?

A: Yes, in so many words.

I don’t really delve into those stories, as they don’t provide a very satisfying resolution. But there was a Frenchman, this poor guy, who is the sad sack of these whole crazy, decade-long odysseys.

In 1761 (during another transit of Venus), he shipped off to India, but he discovered when he got there that his colony had been taken over by the Brits, and he had nowhere to land.

He stuck around for eight more years and finally found a satisfactory place to be on June 3, 1769. But he got nothing that time either. He'd spent a large chunk of his professional career chasing the transit of Venus, and he found nothing.

Q: What do the 1769 voyages mean in the big picture?

A: They really represent the first time that science has traversed national borders and has collaborated, not by a few here and there, but by the dozens and scores and all across the planet in the service of a singular set of goals.

For more about this year's transit of Venus, try NASA or Anderson's website.

Randy Dotinga is a Monitor contributor.