The Transit of Venus

Transits of Venus

There are many astronomical events that happens regularly some over years and some over hundreds of years and one such event is transit of Venus and there is a very nice background story of science. For a very long time the distance between Earth and Sun was not known properly. It was called Astronomical Unit and scientists used ratio to define many distances.  

Edmond Halley (1656–1742), known to almost anyone because of the Halley’s Comet, is one of the most important figures in the history of Venus transit astronomy — his contribution wasn’t just scientific, it was almost poetic: a man who knew he would never live to see the thing he was asking others to measure.

The problem Halley was trying to solve

In the late 17th century, astronomers had mapped the relative distances of the planets fairly well using Kepler’s laws, but they didn’t know the absolute scale of the Solar System — specifically, how far Earth was from the Sun. This single number, the Astronomical Unit (AU), was the key to unlocking everything else. Halley recognized that a Venus transit offered a rare and precise way to calculate it.

Halley’s method — parallax across the Earth

His approach was based on the principle of solar parallax. If two observers watch Venus cross the Sun from widely separated locations on Earth, they will see Venus trace slightly different paths across the solar disk — one slightly higher, one slightly lower. By precisely timing how long Venus takes to cross the Sun from each location, and knowing the distance between the observers on Earth, you could use trigonometry to calculate the distance from Earth to Venus, and from that — via Kepler’s ratios — the Earth-Sun distance.

The elegance of the method was that it relied on timing, not angular measurement. Even modest telescopes of the era could time an event to within a second, making the method far more practical than trying to directly measure tiny angles. Halley calculated that if observers spread across the globe timed the transit carefully, the AU could be determined to within 1 part in 500 — extraordinary accuracy for the time.

 

The letter to younger astronomers

Halley observed the 1677 transit of Mercury, which gave him the idea, and then worked out in 1716 — when he was around 60 years old — that the upcoming Venus transits of 1761 and 1769 would be the best opportunities to apply his method. He knew he would almost certainly be dead by then (he died in 1742, aged 85, so he did live remarkably long, but not long enough).

He wrote a passionate appeal, published in the Philosophical Transactions of the Royal Society, addressed explicitly to future astronomers — to the people who would be alive when he was not. The tone is remarkable: urgent, generous, almost fatherly. He described the method in detail, explained which locations on Earth would give the best parallax baseline, and calculated the expected durations of the transit from different vantage points.

His closing appeal was essentially: I cannot do this myself — I beg you, do not let the moment pass. He understood that these transits came in pairs separated by 8 years, then gaps of over a century, and that 1761 and 1769 would be the only chances for generations.

What happened next

The scientific world took his letter seriously. The 1761 transit prompted one of the first coordinated international scientific expeditions in history — over 150 observers were dispatched to more than 60 locations across the globe, from Siberia to St. Helena to Newfoundland. The 1769 transit saw Captain James Cook sail to Tahiti specifically to observe it, a voyage that also produced the European charting of Australia and New Zealand.

The results were imperfect — a notorious optical problem called the black drop effect made it hard to time the exact moment Venus touched the Sun’s edge — but by combining all observations, astronomers arrived at a figure for the AU of about 93 million miles, remarkably close to the modern value of 92.96 million miles.

Halley’s letter is one of the great examples in science of one generation deliberately laying the groundwork for the next — and trusting that the next generation would show up.

Why now?

Recently, at an AI Summit, a communication professor from Galgotias University—a private institution in Noida—claimed a Chinese-made robot dog as an indigenous Indian creation. This incident embarrassed the nation, especially with high-profile international visitors present to witness our local innovations. Far from an isolated case, such misrepresentation reflects a deeper norm in our education system. This is not to discourage genuine innovators pursuing original ideas in India—many have succeeded despite, not because of, the system.

The last Venus transit occurred in 2004 and 2012, events within our lifetimes, yet the next won’t happen until 2117. In 2004, as a third-year engineering student, I don’t recall a single discussion about it—we were too busy cramming to avoid failing exams. Though our curriculum didn’t cover astronomy, I wish someone had highlighted this rare phenomenon for future generations. It’s a small but telling regret.

Technological progress stems from centuries of incremental effort across generations. Edmund Halley predicted the 1761 transit, drawing observers from rival nations—French astronomers, despite enmity with Britain, joined the quest. Frenchman Guillaume Joseph Hyacinthe Jean-Baptiste Le Gentil sailed to Pondicherry, India, but clouds thwarted his observation.

As Indians, we’re often consumed by survival basics—money, food, shelter—overlooking pursuits of fundamental knowledge. We have the aptitude; what’s lacking is systemic support for big thinkers in schools, colleges, and universities. They need freedom from daily worries, with essentials secured for years.

I close with a Shloka from the Isha Upanishad (1st millennium BCE), contemplating infinity when the world feared zero and infinity

Its author pondered wholeness beyond mundane concerns like retirement funds, emigration, or AI’s disruptions—urging us to nurture such fearless curiosity today.