# How Did the Transit of Venus Shape Modern Astronomy?

• BobG
In summary, the transit of Venus will occur on June 5-6, and it is a significant event in the history of astronomy.
BobG
Homework Helper
Venus will cross the Sun on June 5-6 of this year.

Historically, the transit of Venus holds a significant place in the history of astronomy.

As most know, Kepler's third law stated the square of the period of a planet is proportional to the cube of it's mean distance. Newton's Laws of Motion explained why Kepler's law was true and even supplied an equation to Kepler's law:

$$\tau={\frac{2 \pi}{G M} \sqrt{a^3}$$

where G is the universal gravitational constant
M is the mass of the Earth
a is the average radius of the Earth (the semi-major axis)

The only problem is - G, M, and a weren't known. The orbital period of the planets were known, and from that, the distance of the other planets were determined in astronomical units - with astronomical units being the distance from the Earth to the Sun - whatever that turned out to be. Even worse, at least according to what Newton believed, G would wind up being such a small number no one would ever be able to calculate it. At least not unless someone could ever come up with a way to measure the mass of the Earth or the mass of the Sun.

Still, even if G and M weren't known, G*M was known for the Earth, since you could measure the distance from the Earth to the Moon by comparing the position of the Moon against the stars in two different locations at the same time. Worked great for the Moon, but the Sun is so bright that you can't see the stars behind it. Another method was needed.

And, Edmund Halley realized that Venus would transit the Sun in the year 1761 and again in 1769. (And then again in 1874 and 1882. And then again in 2004 and 2012. Etc.) If a team of astronomers were disbursed around the world, they could record key times of the Venus transit using the Sun as the reference point and determine how far away Venus was from the Earth - and then using Kepler's third law, figure out how far every planet in the Solar System, including Earth, was from the Sun. By covering various longitudes and latitudes, and recording chord widths and transit times, an accurate calculation of the distance surely could be made.

Halley initiated one of the first and greatest international scientific quests ever, even though he died long before the transits occurred. And, with an international effort, around 120 astronomers set out for various parts of the world to make their observation.

One of them was Guillarme LeGentil.

The eleven-year voyage of the French astronomer LeGentil to the Indian Ocean to observe the transits of Venus in 1761 and 1769 is probably the longest lasting astronomical expedition in history. In fact, it is quite possible that, except for interplanetary travel, there will never be astronomical expeditions to equal in duration and severity those made for that particular pair of transits.

In the first of these articles, we referred to the heart-breaking experiences of LeGentil who, for all his lengthy voyages, failed to achieve any useful observations of either transit.

Heart-breaking is an understatement. By time he returned to Paris, he'd been declared dead, his seat in the Royal Acedemy of Science was already occupied by his successor, his wife had remarried and was raising a family with her new husband, and his relatives had already divided his estate. Oh, and he also lost his luggage.

But, sometimes, the journey is the story - and the destination or goal is just a mere triviality (as is the wife and all of his property?).

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I always associate the transit with Captain Cook:

Every 120 years or so a dark spot glides across the Sun. Small, inky-black, almost perfectly circular, it's no ordinary sunspot. Not everyone can see it, but some who do get the strangest feeling, of standing, toes curled in the damp sand, on the beach of a South Pacific isle...
Sea gulls fluttered upward, screeching. City odors drifted in from Plymouth, across the ship, shoving aside the salt air. Sails snapped taut. The wind had changed and it was time to go.
On August 12, 1768, His Majesty's Bark Endeavour slipped out of harbor, Lt. James Cook in command, bound for Tahiti. The island had been "discovered" by Europeans only a year before in the South Pacific, a part of Earth so poorly explored mapmakers couldn't agree if there was a giant continent there ... or not. Cook might as well have been going to the Moon or Mars. He would have to steer across thousands of miles of open ocean, with nothing like GPS or even a good wristwatch to keep time for navigation, to find a speck of land only 20 miles across. On the way, dangerous storms could (and did) materialize without warning. Unknown life forms waited in the ocean waters. Cook fully expected half the crew to perish.
It was worth the risk, he figured, to observe a transit of Venus.

http://science.nasa.gov/science-news/science-at-nasa/2004/28may_cook/

James Cook is one of the historical inspirations for James Kirk.

On a side note: Cook's navigator on one of his voyages, William Bligh, went on to become a ship's captain and suffered more mutinies (three total) than any other British commander. The mutiny on the Bounty was directly against his command. Two others included men under his command but were against navy policy.

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LeGentil story makes an interesting read.

Sadly, forecasts for the weather in my neck of woods are not too good for the next few days. Is it is cloudy now and they predict it will be cloudy till at least Wednesday.

Here, too- forecast is cloudy weather on the 5th :(

I blame the Queen. It stands to reason that the UK gets an extra day's national holiday, the weather forecast is for heavy rain and temperatures below 10C (down from the mid 20s last week).

Never thought Her Majesty can influence weather patterns in distant parts of Europe, but I can't think of a better explanation. Few days ago weather was perfect here as well.

## What is the Transit of Venus across the Sun?

The Transit of Venus across the Sun is a rare astronomical event in which the planet Venus passes between the Earth and the Sun, appearing as a small black dot moving across the surface of the Sun. This phenomenon occurs in pairs, with each pair separated by 8 years, and then not again for over 100 years.

## When will the next Transit of Venus occur?

The next Transit of Venus will occur on December 11th, 2117. The last occurrence was on June 5th, 2012.

## Why is the Transit of Venus important?

The Transit of Venus is important because it allows scientists to measure the distance between the Earth and the Sun, known as the astronomical unit (AU). By observing the transit from different locations on Earth, scientists can use parallax to calculate the distance. This information is crucial for understanding the size and scale of our solar system.

## Can the Transit of Venus be viewed with the naked eye?

No, it is not safe to view the Transit of Venus with the naked eye. The intense solar radiation can cause permanent damage to the eyes. Special eye protection, such as solar eclipse glasses, must be used to safely view the event.

## How often does the Transit of Venus occur?

The Transit of Venus occurs in pairs, separated by 8 years. However, the pairs only occur every 105 or 121 years, with the next pair not occurring until 2117 and 2125. This makes the Transit of Venus a very rare event that only happens about twice every century.

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