# How fast is the Earth-Sun distance changing

This is inspired by Evidence that the Solar System is expanding like the Universe?, which referenced an article by G. A. Krasinsky and V. A. Brumberg, "Secular Increase of Astronomical Unit from Analysis of the Major Planet Motions, and its Interpretation," pdf available here.

The article seeks to explain the increasing Earth-Sun separation (orbit-averaged separation, as in semi-major axis), and a large part of it is focused on the expansion of the universe. Now Ben Crowell gave a great explanation here as to why the effect of cosmic expansion only comes in with the third time-derivative of the scale factor, noting that the effect is "undetectably small."

So then I wonder: What is the rate at which the Earth-Sun distance is changing? Krasinsky and Brumberg cite something like $15\ \mathrm{cm/yr}$, but given the number of wrong calculations they do elsewhere, I don't entirely trust this claim. Who has done this analysis, and what do they find? Also, what methods are used? I would guess that measuring the precise distance to the Sun's surface would be challenging, due to all the activity it undergoes.

Note that discussion of cosmic expansion was background motivation for what sparked this question - I fully understand that any secular change in the Earth's orbit will be dominated by other effects.

Also note there is a related question on this site: How is distance between sun and earth calculated? However, that does not go into detail about the method. Moreover, it is not clear that the answers provide work for measuring small changes in the separation. For example is radar really that precise? Perhaps it is, in which case justification of that would serve as an answer here.

-
Do you mean average distance (equivalently semi-major axis) or instantaneous distance? The Earth's orbit is not a perfect circle so the distance changes measurably on an annual basis. I forget the numbers, just look up "Earth orbital elements" if you want to see how much it varies. A long term secular change in the orbit, of the kind you probably mean, would manifest as changes in the semi-major axis and/or eccentricity. – Michael Brown Jul 20 '13 at 2:04
Re previous comment: I didn't even look at who posted this. Of course you know about eccentricity! Please don't be offended. Just the wording of the question was a little less than precise on that point, so maybe the comment would be useful to someone else someday. :) – Michael Brown Jul 20 '13 at 2:49
@MichaelBrown No problem ;) Eccentricity would be interesting to know about too, especially given that I suspect it can change on shorter timescales. – Chris White Jul 20 '13 at 2:57
Potentially relevant: what fraction of sun's mass gets ejected annually beyond earth's orbit. – Johannes Jul 20 '13 at 5:38
The Krasinsky & Brumberg paper has 59 citations on ADS. That should be a good starting point. – Pulsar Jul 20 '13 at 11:54

According to E. V. Petjeva (2011), the measured rate of change of the Earth-Sun distance (astronomical unit) is (1.2 +/- 3.2) cm/yr, with the uncertainty value representing 3 standard deviations. In other words, any change is within the uncertainty of the measurement. She specifically addresses the Krasinsky and Brumberg value. E. M. Standish has also addressed this issue.

The measurements are by radar echos off other planets and radio signals from space craft. See the below references for details.

(Note that "astronomical unit" is not technically the same as Earth-Sun distance, see Standish reference for details, Krasinsky and the others are all measuring the "astronomical unit"; also "astronomical unit" was redefined to be a constant in 2012, see Nature reference for details)

http://syrte.obspm.fr/jsr/journees2011/pdf/pitjeva.pdf