h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Could any body explain how AU was measured and at when
was the distance first estimated with reasonable accuracy
( is it done by a geometrical procedure )

I am sorry being a simple engineer I find it a somewhat
chicken and egg type problem

Interesing question! I've always taken it for granted.

Well, this is probably not the way it was done, but it would work: When there is exact half-moon, you can measure the angle between moon and sun and can calculate the ratio of the distances Earth--moon and Earth--Sun. The Earth's shadow on the moon and the radius of the Earth and the above ratio yields the distance moon--Earth. The you can use the ratio for a second time to calculate the distance Earth--sun.

Alternatively you can measure the speed of light on Earth and use Ole Rømer's method to measure the speed of light using celestial objects, but in the inverted way. Then you get the distance to eg Jupiter and can use that as a starting point to measure the solar system.

Today, at least the distance Earth--Moon can be measured very accurately which makes the whole thing much easier. Or you just send probes.


I don't know anything about the When though. But probably Brahe or at least Kepler knew it.

But if it is geometry you must know something accurately and was that info available in 16th century

According to NASA first estimate was 8 million miles and later improved by using PARALLAX of MARS from 2 sites in France and they
got the distance as 92 million

But the description was still a bit vague...can you assist

The radius of the Earth was accurately known rather early; surely in the 16th century. The other things can be done with proper angular measurement equipment which was also available to at least Brahe and Kepler.

The parallax method is very simple. You measure the position of mars at the same time from two locations (as far away from each other as possible) and calculate the angle between both positions (usually very small). The distance is then (approximately) d/angle, with d as the distance between the two sites, and 'angle' given in rad.

With the distance to mars you can calculate the mass of the sun and then you have everything you need. (Not yet completely though; actually the mass of Earth is still unknown but negligible.)

The AU was first measured accurately by measurements of the transit of venus across the sun's disk. This is where Venus eclipses the sun, so seen (with a telescope) from the Earth, the observer sees a small dark circle cross the solar disk. Observing this phenomenon from different locations on the Earth gives you a parallax distance to Venus, and since Kepler's time, the ratio of the distance of Venus and Earth is known by their orbital periods, the distance to the sun can be determined. Solar transit experiments were part of Captain Cook's scientific undertakings. Transits occurred in 1761, 1769, 1874, and 1884 and won't occur again until June 8, 2004. A good description can be obtained here (the author also has a book on the subject):

http://www.dsellers.demon.co.uk/venus/ven_ch1.htm

I seem to recall that the transit of Venus was the reason behind the scientist's expedition to the jungle in Edgar Rice Burrough's Tarzan of the Apes.

Sorry. I was wrong about Tarzan.

Don't worry, the rest was interesting enough.