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pokerfan

(27,677 posts)
Sun Jun 3, 2012, 05:18 PM Jun 2012

The Man Who Knew Venus Would Transit the Sun

Venus will pass in front of the sun on Tuesday, June 5. The next transit will not occur again until 2117.

Jeremiah Horrocks was the only person in all of England—and probably in the entire world—convinced that the 1639 transit would take place. Son of craftspeople and perhaps farmers—there were also a few watchmakers among his forbears—Horrocks had been a local wunderkind who entered Emmanuel College in Cambridge at age 14 as a sizar—a poor student whose duties, along with studies, included the preparation of meals, waiting on tables, and custodial work. In 1635, three years after his arrival, Horrocks left the university—in all probability due to lack of funds--and returned to Lancashire, where he continued to observe the heavens with a small telescope he either purchased or received as a gift from one of the landed families whose children he tutored.

Horrocks knew that Venus had last crossed between the Earth and the sun eight years earlier, on December 6, 1631. That eclipse—and so many other astronomical events—had been accurately predicted in Johannes Kepler's Rudolphine Tables. According to Kepler, Venus would next cross the sun in 1761. There was no mention of a 1639 transit.

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Jim Lane

(11,175 posts)
2. One amateur historian of science has criticized this article for numerous errors.
Sun Jun 3, 2012, 09:43 PM
Jun 2012

You can read his takedown here: http://thonyc.wordpress.com/2012/06/02/scientific-american-craps-out/

I'm no expert in the history of science in the 17th century but one point he makes is so clear that it occurred to me even before reading his piece. The linked Scientific American article says that Copernicus's theory could not explain retrograde motion. In fact, retrograde motion is hard to explain if all the planets orbit the Earth, but is quite easy to explain -- in fact, is inevitable -- if the Earth and the other planets orbit the Sun.

So, on the one point where I feel qualified to adjudicate, the amateur is right and the highly prestigious magazine is wrong.

pokerfan

(27,677 posts)
3. The author defends himself in the comments
Mon Jun 4, 2012, 02:06 AM
Jun 2012

I'm guessing these two have a history...

Sure, Copernicus explained retrograde motion. So did Ptolemy, and Aristotle. They just didn't explain it right. Of course the heliocentric model is light years better than those with jury-rigged epicycles or nesting spheres to illustrate why planets seem to move backwards in their orbits. But the phenomenon of retrograde motion wasn’t fully understood until Kepler.

http://www.scientificamerican.com/article.cfm?id=man-who-knew-venus-would-transit-sun-excerpt#comments

lastlib

(22,978 posts)
4. "Venus will pass in front of the sun on Tuesday, June 5"....
Mon Jun 4, 2012, 02:32 PM
Jun 2012

...An omen for the Walker recall? (Hopefully a good omen...!)

No DUplicitous DUpe

(2,994 posts)
5. HallsofIvy explains it all (How to measure the Astronomical Unit from the transit of Venus)
Mon Jun 4, 2012, 03:37 PM
Jun 2012

"Each of the observers would measure the angle the eye-line to Venus makes with the horizon. The difference between the two would be E. Of course, that is also the angle the two lines make when they "cross" at Venus.

Now look at the triangle made by those two lines from observers on Earth and where they cross at Venus. You can measure the distance between the two observers- one leg of the triangle. The distance is called "d" in the picture. To within "experimental error" the distance from each observer to venus is the same: that is an isosceles triangle and dropping a perpendicular to the base forms two identical right triangles.
Now we have a right triangle with one angle V/2 and side opposite that angle d/2. You can calculate the length of the hypotenuse (distance from Earth to Venus) by sin(V/2)= (d/2)/hypotenuse.
Since we know by Kepler's third law ("The cube of a planet’s distance from the Sun is proportional to the square of its orbital period"- since we can measure the orbital period of a planet we can find the ratio of the distance from the sun to that of the Earth. ) that the distance from Venus to the Sun is about 0.72 times the distance from the Earth to the sun, We can use the distance from the Earth to Venus to calculate the distance from the sun to the Earth. Once we know that, we know the proportionality in Kepler's third law and can calculate the distance from the sun to any planet, knowing its period."
http://www.physicsforums.com/archive/index.php/t-101559.html

padruig

(133 posts)
6. and remember ...
Mon Jun 4, 2012, 04:53 PM
Jun 2012


these calculations were done by HAND !

proof of something Von Braun once said

that the most powerful computer (and most cheaply mass produced) is the human mind

eridani

(51,907 posts)
8. Some famous historian once said that it was a very good thing that Ptolemy didn't have a computer
Mon Jun 4, 2012, 09:07 PM
Jun 2012

That would have made it much harder to get to heliocentrism.

brush

(53,467 posts)
7. What time on Tuesday
Mon Jun 4, 2012, 07:49 PM
Jun 2012

Does anyone know what time the Venus transit will happen, Pacific Standard time?

pokerfan

(27,677 posts)
10. The precise times are location dependent
Mon Jun 4, 2012, 09:33 PM
Jun 2012

but in general, for North America, the transit will be in progress as the sun sets. Go here to get exact times for your location: http://transitofvenus.nl/wp/where-when/local-transit-times/

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