How far is Earth from the sun?

Catastrophe

"Science begets knowledge, opinion ignorance.
' "The new definition is much simpler than the old one," said astronomer Sergei Klioner of the Technical University of Dresden in Germany. Both Klioner and Capitaine were both part of the International Astronomical Union group that worked to refine the definition. '

But it is still an arbitrary figure and subject to variations which give rise to aphelion and perihelion.

Cat :)
 
This article lists the various values obtained for the astronomical unit, showing in earth radii and the modern au measurement. A long, and difficult history and astronomy to measure the distance between Earth and the Sun.

Six stages in the history of the astronomical unit, Six stages in the history of the astronomical unit - NASA/ADS (harvard.edu)

The PDF report with tables showing the different values for defining the au or AU distance is in the link. Also 2001JAHH....4...15H (harvard.edu)

Early astronomy taught a much smaller solar system and the distance to the fixed stars were closer than the Sun distance today using the modern, heliocentric solar system value. The distance between Earth and Sun began to change greatly once the Mars parallax was measured in 1672 like the article indicates. In the 1700s and 1800s, Venus and Mercury transits helped establish the solar parallax that defines the au.
 
' "The new definition is much simpler than the old one," said astronomer Sergei Klioner of the Technical University of Dresden in Germany. Both Klioner and Capitaine were both part of the International Astronomical Union group that worked to refine the definition. '

But it is still an arbitrary figure and subject to variations which give rise to aphelion and perihelion.

Cat :)

Good observation. When it comes to the orbit of Earth, we enjoy a very small eccentricity in the heliocentric solar system. Exoplanet studies show great variation here more than 900 exoplanets with e >=0.10. Climate change would not be an issue if we orbited the Sun in a larger eccentricity :)
 

Catastrophe

"Science begets knowledge, opinion ignorance.
All this talk, of units reminds me of a story. Discussing French homework in class, one boy read out his answer to a swimming pool, which was approximately a metre deep.

You have guessed! He read out "the pool was approximately 3.280839895 feet deep".

If you say that Neptune is 30 AU distant from the Sun, you are not expecting a reply in feet and fractions of an inch. The new AU, imho, is no more useful in practice than the old 'one'.

Cat :)
 
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All this talk, of units reminds me of a story. Discussing French homework in class, one boy read out his answer to a swimming pool, which was approximately a metre deep.

You have guessed! He read out "the pool was approximately 3.280839895 feet deep".

If you say that Neptune is 30 AU distant from the Sun, you are not expecting a reply in feet and fractions of an inch. The new AU, imho, is no more useful in practice than the old 'one'.

Cat :)
Use c.g.s. units, express in centimeters :) 1 au > 10^13 cm :)
 
Here are a few more interesting tid bits about the history of the AU.

1582. Tycho brilliantly realized that he could affirm or debunk the Copernican model by using parallax for Mars. [The Cop model would put Mars very close to Earth during opposition, whereas this would be impossible from the Ptolemy model.] His method was sound, but he needed a telescope, which awaited invention. It would have soon produced the value of the AU. [Fortuitously, it was Tycho's extreme efforts in getting accuracy for Mars (~1/2 arcmin. -- the eye is only ~1 arcminute) that pushed Kepler to trust Tycho's measurements enough to eventually discover that orbits are elliptical.]

What was needed most would be transits. Kepler was first to predict a transit. He predicted a Mercury transit for 11/7/1631, followed by a transit of Venus on 12/6/1631. [He died, however, in 1630.]

Along came Jeremiah Horrocks, who, at the age of 20, calculated with improved accuracy the transit of Venus for Dec. 4th, 1639. He notified a student a Cambridge (Crabtree). They both observed the transit through their little refractors. Horrocks calculated the angular diameter of Venus to be 1.3 arcminutes, “10x smaller than expected”. [Given the old estimates of a short distance to the Sun, this makes sense.]

In 1653, Christiaan Huygens estimated the Sun’s distance using the phases of Venus.

In 1672, Cassini used the parallax method on Mars to determine the AU.

Next came our friend Halley who observed a 1677 transit of Mercury. He later presented the idea of using parallax of transits as a method to measure the distance to the Sun (ie 1 AU). He published his idea in 1716, and he wrote it in Latin to make it something for the world to read. He did not live long enough to see the experiment conducted.
 

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