# Transit of Venus across the Sun

1. May 1, 2012

### BobG

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.

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?).

Last edited by a moderator: May 6, 2017
2. May 2, 2012

### zoobyshoe

I always associate the transit with Captain Cook:

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.

3. May 3, 2012

### BobG

Last edited by a moderator: May 6, 2017
4. May 4, 2012

### BobG

Last edited by a moderator: May 6, 2017
5. Jun 2, 2012

### Staff: Mentor

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.

6. Jun 2, 2012

### Andy Resnick

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

7. Jun 2, 2012

### AlephZero

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).

8. Jun 2, 2012

### Staff: Mentor

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.