why is it important to precisely measure 1 AU?

[Ted Sung]

Could someone explain why a precise measurement of an AU is important?
I faintly remember reading something about how Shapiro's radar
reflecting expermiments (in the 60s/70s) increased the precision of
this measurement dramatically but forgot why it had an impact
(presumably it was needed to precisely calculate orbits so that we
could sent the various explorers to other planets.).

Thanks,

Ted

[Mod. note: because it determines the baseline for parallax
measurements, an accurate value for the AU is also vital to accurate
measurements of distances outside our solar system -- mjh]

[Jonathan Silverlight]

In message >, Ted Sung
> writes
>Could someone explain why a precise measurement of an AU is important?
>I faintly remember reading something about how Shapiro's radar
>reflecting expermiments (in the 60s/70s) increased the precision of
>this measurement dramatically but forgot why it had an impact
>(presumably it was needed to precisely calculate orbits so that we
>could sent the various explorers to other planets.).
>
>Thanks,
>
>Ted
>
>[Mod. note: because it determines the baseline for parallax
>measurements, an accurate value for the AU is also vital to accurate
>measurements of distances outside our solar system -- mjh]

But isn't the error in measurements outside the solar system almost
entirely due to errors in the small angles involved? For instance we
know the distance to Deneb to within 36%, but the distance to alpha
Centuari to 0.23%.
--
Save the Hubble Space Telescope!
Replies to jsilverlight AT merseia.fsnet.co.uk are welcome.

[Gordon D. Pusch]

(Ted Sung) writes:

> Could someone explain why a precise measurement of an AU is important?
> I faintly remember reading something about how Shapiro's radar
> reflecting expermiments (in the 60s/70s) increased the precision of
> this measurement dramatically but forgot why it had an impact
> (presumably it was needed to precisely calculate orbits so that we
> could sent the various explorers to other planets.).
>
> [Mod. note: because it determines the baseline for parallax
> measurements, an accurate value for the AU is also vital to accurate
> measurements of distances outside our solar system -- mjh]

It is also important in that once we know the semimajor axis and the period
of the Earth's orbit (or that of the other planets), we can determine from
those two data the mass of the Sun, which is one of the most important
data required to calculate the Sun's internal structure and evolution.


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'

[Martin Hardcastle]

In article >,
Jonathan Silverlight > wrote:
(quoting a moderator's note)
>>[Mod. note: because it determines the baseline for parallax
>>measurements, an accurate value for the AU is also vital to accurate
>>measurements of distances outside our solar system -- mjh]
>
>But isn't the error in measurements outside the solar system almost
>entirely due to errors in the small angles involved? For instance we
>know the distance to Deneb to within 36%, but the distance to alpha
>Centuari to 0.23%.

Different sort of error. Not knowing the AU precisely gives you a
systematic error -- at least if you want to convert distances into
metres and do physics with them. Not being able to measure parallaxes
precisely gives you a measurement error, which is what you're talking
about.

Martin
--
Martin Hardcastle Department of Physics, University of Bristol