Every 122 years, Venus crosses the Sun as a black and fleeting teardrop, kindling the awe of all those who dare to know

by Ananya Palivela  BIO

The transit of Venus seen on 8 June 2004. Photo by David Cortner

is a science journalist. She is the physical sciences reporter at Chemical and Engineering News magazine, based in Washington, DC. She is also the author of the Substack Ananya Writes Physics.

Edited byPam Weintraub

There is something comical about the fact that the planet named for the goddess of love once drove the scientific world to its knees.

During a transit, Venus passes between Earth and the Sun, becoming a small black dot against the solar disk. These events are among astronomy’s rarest spectacles. They come in pairs eight years apart, then disappear for more than a century. The last one was in 2012, when Venus crossed the face of the Sun as a small black dot, its journey livestreamed around the world. Scientists and enthusiasts watched something they would never see again in their lifetimes – from terraces with eclipse glasses or on their laptops from the comfort of their homes. But a quarter of a millennium ago, scientists didn’t have that luxury. Astronomers risked everything to see this small shadow in the name of science. The quest was romantic in the classic sense: obsessive, impractical and grand. As with the era’s poetry and painting, its romance was inseparable from the empire that produced it.

Courtesy NASA/Goddard Space Flight Center/Wikipedia

It all started when I walked into a tiny museum at Harvard University.

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I’m a science writer, which is what happens when you love physics but physics doesn’t quite love you back. I have a master’s degree in it, but the intuition for research never really hit me right. Still, I couldn’t shake the feeling that there was something I could give science. Most people don’t see science the way I do – as something interesting, beautiful, even emotional. So, naturally, I did what no tortured artist has ever done before: I started writing about my unrequited love. I wanted people to see the science that I see, and feel it the way I do.

If I got lucky, I’d find one of those stories that make science feel human. Stories of people who cared too much. The ones who spent years chasing starlight or boiling mercury, convinced that, if they just measured carefully enough, everything would finally make sense. They believed the Universe could be solved with mathematics and patience.

I couldn’t tell if I was drawn to those old stories because they were profound or just nostalgic – and the Putnam Gallery in Cambridge, Massachusetts (less like a museum than a well-lit storage space) was where I would find out. The place was so little known that I was the only person there, aside from the woman at the front desk, and the quiet gave me the time to notice what became, to me, the funniest thing in the room: almost every instrument I looked at pointed to the same thing. A telescope built to track Venus. A map charting her path across the Sun. A pendulum clock precisely calibrated to catch the exact moment of her transit. Venus, Venus, Venus. The fact that so many astronomers seemed to be obsessed with Venus was ironic. Of all the astronomical bodies there were to observe, and all the other science there was to do, these old men were focused on the one planet named after the most beautiful woman in existence. Typical!

Venus would appear to trace different paths across the Sun depending on where on Earth you were watching from

Even though the joke was what pulled me in, I realised that the obsession had a purpose. It was about measuring the dimensions of the solar system. For centuries, astronomers knew the relative distances of the planets: Venus was closer to the Sun than Earth was, and Mars was further away. But they didn’t know the scale. They had the map but not the ruler. How many kilometres lay between Earth and the Sun? No one had a solid number.

The key, it turned out, was the transit of Venus across the Sun. If astronomers timed the transit from far-out parts of the globe, they could use a phenomenon called parallax (the apparent shift of an object when you view it from two locations) to calculate the distance to the Sun. (Picture the tiny shift you see when you hold your thumb in front of your face and close one eye, then the other.) Scale up that effect to the size of the solar system, and Venus would appear to trace slightly different paths across the Sun depending on where on Earth you were watching from. Thus, with enough observations and enough mathematics, astronomers could triangulate Earth’s distance from the Sun.

It was the English astronomer Edmond Halley who figured this out back in 1716. But he had a problem: as with all transits of Venus, opportunities came in pairs eight years apart, and then not again for more than a century. Halley knew he wouldn’t live to see the next one, so he left behind instructions urging future astronomers to observe the transits of 1761 and 1769.

Edmond Halley’s 1716 prediction of how to measure the dimensions of the solar system through observations of the transit of Venus. Courtesy the APS Collection

In 1761, even with wars raging across continents, astronomers took up Halley’s challenge. Backed by royal support and armed with the best instruments of the time, they set out for more than 70 observation sites around the globe. That was how all those telescopes, maps, clocks and quadrants had found their way, eventually, into this tiny museum: relics of a global campaign to measure a........

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