# How to Calculate the AU

1. Apr 29, 2004

### recon

I have just read a recent Scientific American article about the June 2004 transit of Venus. In the article, past attempts of deriving the AU from Venus's transit was explained. It was also stated that there since have been more accurate ways of determining the AU. I would like to know how the current value for 1 AU is determined and how accurate it is. Could someone please help?

2. Apr 29, 2004

### marcus

this post is for people who dont get the Scientific American
and are still curious as to how the AU is told by observing a transit
of venus

this webpage explains it:
http://www.exploratorium.edu/venus/question4c.html [Broken]
if I can find one that does it better I will post it too

Last edited by a moderator: May 1, 2017
3. Apr 29, 2004

### marcus

recon, all I can offer is a guess. I think that we can tell the distances to space probes with extreme accuracy by timing signals generated on board the spacecraft. By keeping in touch with spacecraft we can survey the solar system in standard SI units---meters, kilometers---because these are defined in terms of light traveltime.

the relative sizes of orbits in AU can be determined from orbit characteristics, essentially by Keplers laws.

I would guess the AU is determined by comparing the distances to spacecraft measured in meters with the same distances calculated in AU.

there are several people here at PF who would know for sure (Nereid, Enigma, Phobos, too many to name them all) and could tell how accurately it is known.

my guess is that distances in the solar system are now known at least to within one part per billion-----because spacecraft navigation is so
consistently successful.

they used to use radar to measure distances, before there was a lot of data from navigating probes.

sorry I dont have an authoritative answer, if I come across anything I will edit it in later

4. Apr 29, 2004

### marcus

wow,
I looked up AU on google and
the definition is the radius of a circular orbit
with period of one year---for a pointmass
http://neo.jpl.nasa.gov/glossary/au.html

and it appears to be known to at least 9 decimal places (even more)
or as accurately as the gaussian gravitational constant k
is known (better than 9 places)

technically the AU is not actually equal to the average distance
between the earth and sun

it is a derived quantity from the Gaussian constant k
(a way of presenting the sun's mass)

this NASA site says
1 AU = 149,597,870.691 kilometers

that is, 12 place accuracy, to the nearest meter.
this is fantastic
it means that the mass of the sun, not in kilograms but in the
units of the Gaussian k, is known to 12 place accuracy

I cant sort all this out right now. it is incredible accuracy.

Last edited: Apr 29, 2004
5. Apr 29, 2004

### marcus

wow again!

look at this, a NASA page called "Astrophysical Constants"

http://ssd.jpl.nasa.gov/astro_constants.html

it gives the Gaussian gravitational constant k to many places accuracy

(the AU is just a derived thing from k)

Last edited by a moderator: Apr 20, 2017
6. May 3, 2004

### recon

marcus, thanks for the links! I guess you enjoyed the information as much as I enjoyed them! I liked the link http://www.exploratorium.edu/venus/question4c.html [Broken]. The Scientific American did not go into that much detail to explain it and was mostly historical, but I managed to figure it out nonetheless. The link confirmed my theory. Is there an online community of amateur astronomers preparing for this event to gather information to come up with their own rough estimate of the AU?

Last edited by a moderator: May 1, 2017