# An astronomical question **URL:** \ **Category:** Factual Questions\ **Created:** [September 10, 2014, 1:16am UTC](https://boards.straightdope.com/t/an-astronomical-question/698042 "2014-09-10T01:16:12Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![carnivorousplant](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnivorousplant/32/3563_2.png) [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)\ **Post date:** [September 10, 2014, 1:16am UTC](https://boards.straightdope.com/t/an-astronomical-question/698042/1 "2014-09-10T01:16:12Z")
In reading about the Royal Navy in the eighteenth century, I found that: “The Admiralty and the Royal Society, who were preparing a scientific expedition to the South Seas, decided that Tahiti would make an excellent location for the transit of Venus across the Sun-an exercise that would help determine the distance between the Earth and the Sun.” How so? Michaelson and Morley had yet to perform the experiment that would show the speed pf light to be constant; Einstein had not suggested the theory of relativity that would be proven by light bending about an astronomical object. Thanks. ---
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [September 10, 2014, 1:21am UTC](https://boards.straightdope.com/t/an-astronomical-question/698042/2 "2014-09-10T01:21:21Z")
The speed of light had nothing to do with it. The intent was to get a more precise measurement of solar parallax (by taking measurements at widely spaced points on the Earth) and thus calculate the distance by triangulation. ---
**Author:** ![chacoguy](https://avatars.discourse-cdn.com/v4/letter/c/50afbb/32.png) [@chacoguy](https://boards.straightdope.com/u/chacoguy)\ **Post date:** [September 10, 2014, 1:25am UTC](https://boards.straightdope.com/t/an-astronomical-question/698042/3 "2014-09-10T01:25:49Z")
[I found this.](http://spiff.rit.edu/classes/phys445/gettys/venus_ex/venus_ex.html) I can’t say I understand it completely. ---
**Author:** ![carnivorousplant](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnivorousplant/32/3563_2.png) [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)\ **Post date:** [September 10, 2014, 1:32am UTC](https://boards.straightdope.com/t/an-astronomical-question/698042/4 "2014-09-10T01:32:02Z")
It is trigonometry! Given the distance between two observation points, and the angle at while Venus moved between the sun and Earth, they had a triangle and could determine the distance to Venus, but the Sun? ---
**Author:** ![Giles](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/giles/32/60_2.png) [@Giles](https://boards.straightdope.com/u/Giles)\ **Post date:** [September 10, 2014, 1:39am UTC](https://boards.straightdope.com/t/an-astronomical-question/698042/5 "2014-09-10T01:39:14Z")
If you know the ratio of the distances from Earth to Venus and the Sun, then measuring one of those distances will give you the other. ---
**Author:** ![carnivorousplant](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnivorousplant/32/3563_2.png) [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)\ **Post date:** [September 10, 2014, 1:50am UTC](https://boards.straightdope.com/t/an-astronomical-question/698042/6 "2014-09-10T01:50:47Z")
> [@Giles](#): > > If you know the ratio of the distances from Earth to Venus and the Sun, then measuring one of those distances will give you the other. How did they know that in 1768? Trigonometry again? ---
**Author:** ![Giles](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/giles/32/60_2.png) [@Giles](https://boards.straightdope.com/u/Giles)\ **Post date:** [September 10, 2014, 2:02am UTC](https://boards.straightdope.com/t/an-astronomical-question/698042/7 "2014-09-10T02:02:14Z")
> [@carnivorousplant](#): > > How did they know that in 1768? > > Trigonometry again? They did it by calculating the orbits of Earth and Venus around the Sun, again using trigonometry, and Newton’s law of gravity. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [September 10, 2014, 2:57am UTC](https://boards.straightdope.com/t/an-astronomical-question/698042/8 "2014-09-10T02:57:25Z")
> [@carnivorousplant](#): > > How did they know that in 1768? > > Trigonometry again? You can call it trigonometry, or you can call it [parallax](http://en.wikipedia.org/wiki/Parallax). Either way, it’s simple geometry. If you observe the same object from 2 different locations, its position against the background will be shifted. If you know how far apart those 2 locations are, you now know the distance to that object. Here, the “object” is Venus, and “background” is the sun. ---
**Author:** ![septimus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/septimus/32/410_2.png) [@septimus](https://boards.straightdope.com/u/septimus)\ **Post date:** [September 10, 2014, 3:53am UTC](https://boards.straightdope.com/t/an-astronomical-question/698042/9 "2014-09-10T03:53:18Z")
[Trivia item](http://journals.cambridge.org/download.php?file=%2FIAU%2FIAU2004_IAUC196%2FS1743921305001262a.pdf&code=d0f46939d4c4f7420e7fa9d615b280fd): > [@](#): > > With the transit behind him Cook was now free to open the sealed orders from the Admiralty, and these revealed that this scientific venture would now become a voyage of exploration, in search of the mooted southern continent, _Terra Australis Incognita_. I already knew this, but while Googling for a cite learned that [ul][li] Astronomer Green having died (too much partying in Tahiti? :rolleyes: ), the great explorer Cook had an important role in presenting and annotating the transit data.[/li][li] The secret mission was to find Antartica, not Australia. The cited pdf calls the mission “unsuccessful”, but in fact James Cook did explore part of Australia on this trip and claimed New South Wales for King George III.[/li][/ul] ---
**Author:** ![jrepka](https://avatars.discourse-cdn.com/v4/letter/j/ee59a6/32.png) [@jrepka](https://boards.straightdope.com/u/jrepka)\ **Post date:** [September 10, 2014, 5:48am UTC](https://boards.straightdope.com/t/an-astronomical-question/698042/10 "2014-09-10T05:48:46Z")
> [@carnivorousplant](#): > > How did they know that in 1768? > > Trigonometry again? When Venus reaches elongation (the point on its orbit when it is farthest in the sky from the sun, when viewed from Earth) it forms a right triangle with the Earth and the sun, in which Earth’s distance from the sun is the hypotenuse. The sine of the angle S-E-V (which can easily be determined by measuring Venus’ altitude at sunrise/sunset) is the ratio of the Venus-sun distance to the Earth-sun distance. Simple trig like this allowed early astronomers to determine relative distances to the known planets. Observing the Venus transit (after many failed attempts) gave astronomers the actual value of an astronomical unit (AU), they could also determine the distances to the planets. Bonus: once the value of the AU was determined, we could bootstrap that to determine the distance to nearby stars (using the diameter of Earth’s orbit as the baseline for measuring stellar parallax). ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [September 10, 2014, 1:20pm UTC](https://boards.straightdope.com/t/an-astronomical-question/698042/11 "2014-09-10T13:20:49Z")
> [@septimus](#): > > The secret mission was to find Antartica … Sort of. A popular theory of the time was that there had to be a great southern continent to balance the large land area known to exist north of the equator. It turns out, of course, there is no such thing - Antarctica doesn’t come close to filling that role. Cook did a great deal of careful exploring, including charting New Zealand and part of the eastern coast of Australia. He did not sight Antarctica, but did seriously circumscribe its possible extent. ---
**Author:** ![carnivorousplant](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnivorousplant/32/3563_2.png) [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)\ **Post date:** [September 10, 2014, 2:25pm UTC](https://boards.straightdope.com/t/an-astronomical-question/698042/12 "2014-09-10T14:25:27Z")
Thanks, jrepka. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 10, 2014, 2:29pm UTC](https://boards.straightdope.com/t/an-astronomical-question/698042/13 "2014-09-10T14:29:54Z")
The measurements of Tycho Brahe, combined with the mathematical analysis of Kepler, were enough to pin down the _relative_ sizes of all orbits about the Sun. That is, if you define the radius of Earth’s orbit to be 1 AU, then you can get the radius of all the other orbits in AU also. Measuring any one distance (such as the distance from Earth to Venus) then suffices to give you all of them. Incidentally, this is a large part of why AU are used. Even today, when we know the distances through many different experiments, we still don’t know the absolute distances as precisely as we know the relative distances. So any calculation which can be done in AUs will give better results that way.