Sun-Earth distance

1. Sep 29, 2004

T@P

I was always wondering how astronomers in the 1800's were able to determine the distance to the sun, and similarly how they deduced that the earth is rotating around the sun.

(I am not asking these questions from a doubt over the answers, I simply want to know how)

2. Sep 29, 2004

mathman

The distance to the sun can be dtermined by triangulation. You need to know the earth's size. After that it is a simple measurement problem.

The earth revolving around the sun, rather than vice versa, was Copernicus's theory. Newton's gravity gave a quantitative explanation.

3. Sep 29, 2004

marcus

the history of that measurement is interesting
in 1675, when Olaus Roemer measured the speed of light, they already had measured the sun-earth distance but they were about 10 percent wrong
and consequently Roemer's figure for the speed of light (gotten by timing a jovian moon) was about 10 percent wrong

a hundred years later in 1770s Captain James Cook was sent to Tahiti to observe the transit of Venus across the face of the sun, in an attempt to get a more accurate measurement of the sun-earth distance

somewhere there is probably a plot of the measurements people made from like 1600 onwards, getting progressively more accurate.
there should also be an account of the different methods

humans have a long history of asking how far the sun is, and contriving methods to measure it. Around 250 BC a Greek named Aristarchus determined that the sun was some 10 times farther than the moon and much bigger than the earth. So he decided that the earth was probably revolving around the sun----Copernicus rediscovered this and at one point gave Aristarchus credit for being the first to realize it.

4. Sep 29, 2004

marcus

Found out some history from a Nasa site

---quote---Historical Background: Tycho Brahe estimated the distance between the Sun and the Earth at 8 million kilometers (5 million miles). Later, Johannes Kepler estimated the AU was at 24 million kilometers (15 million miles). In 1672, Giovanni Cassini made a much better estimate by using Mars. By observing Mars from Paris and having a colleague, Jean Richer, also observe Mars at the same time in French Guiana in South America, Cassini determined the parallax of Mars. From that Cassini was able to calculate the distance from Earth to Mars, and then the distance from Earth to the Sun. Cassini calculated the AU to be at 140 million kilometers (87 million miles), which is lower, but very close to the modern day number.
---end quote---
http://neo.jpl.nasa.gov/glossary/au.html

5. Sep 30, 2004

Staff Emeritus
Then in the 18th century there were two transits of Venus, by observing the passage of Venus across the sun from two different latitudes, you can do trigonometry and calculate the earth-sun distance. Until the space age, these measurements and refinements of Cassini's method were the source of the figurses given. Notice that the distance varies slightly, as the earth's orbit is not circular but slightly elliptica.l