Earth-Sun Distance Over Time: Stability & Changes Explained

[BillJx]

Is the average radius of the Earth's orbit stable over eons of time, or is it slowly increasing or decreasing? How much would it have changed in, say, the 80 million years since Barney's demise?

 

[Chronos]

Scarcely at all since the first blue green algae blossomed. The paleontology record indicates the average temperature on Earth has varied only a handful of degrees since life first originated. The Earth receeds very slowly from the sun. To put 'very slowly' into perspective, think meters per mega-year.

 

[Janus]

Scarcely at all since the first blue green algae blossomed. The paleontology record indicates the average temperature on Earth has varied only a handful of degrees since life first originated. The Earth receeds very slowly from the sun. To put 'very slowly' into perspective, think meters per mega-year.

Well, it can't quite be put qute as simply as that. If you look at the period of 1800 AD-2050 AD you will see that the Earth-moon barycenter recedes at a rate of .00000562 AU/century or about 2000 km over that period of time.

However, during the period of 3000 BC - 3000 AD,(Which includes the above time period) the E-M barycenter actually approaches the sun at an average rate of -.00000003 AU/century or about 270 km over that time.

 

[tony873004]

Semi-major axis is a periodic orbital element, just like the other orbital elements. But it has a very small amplitude. As far as the Earth's average distance from the Sun, there's 2 ways to define average: average with respect to position, and average with respect to time. The semi-major axis is the average with respect to position. Perihelion is as far interior to semi-major axis as Aphelion is exterior to it. But objects move slower at aphelion, and hence, spend more time there. So the time-averaged distance is a function of eccentricity, which has a much larger amplitude than semi-major axis.

But the more eccentric the orbit, the more solar flux we receive over the course of a year. Even though the Earth tends to loiter a little longer at aphelion, the extra flux received at perihelion, governed by inverse square, more than makes up for it.

 

[BillJx]

I did eventually find an answer to my own question in a Wikipedia article.
"The Sun, as part of its solar lifespan, will expand to a red giant in 5 Gyr. Models predict that the Sun will expand out to about 99% of the distance to the Earth's present orbit (1 astronomical unit, or AU). However, by that time, the orbit of the Earth may have expanded to about 1.7 AUs because of the diminished mass of the Sun." http://en.wikipedia.org/wiki/Red_giant

So the Earth-Sun distance doesn't increase at a regular rate (thanks Janus) but on average, over extremely long periods of time, conservation of angular momentum causes the distance to increase as the sun loses mass. That makes sense. There are obviously other factors at work too. I'll keep looking.

Thanks to all.

 

[pervect]

Now all we have to do is to decide if we believe the Wikipedia article :-(. Unfortunately, that particular statement doesn't appear to be sourced.

If we assume angular momentum is conserved, then GMmr = (ang mom)^2 = constant, so the article is implying the sun's mass drops to 1/1.7 = .58.

But is this the correct assumption?

Trying to find independent confirmation, I ran across http://www.astronomycafe.net/qadir/q1491.html, which seems to suggest that this number is a bit high, and also suggests using conservation of energy rather than angular momentum.

The source above isn't peer reviewed, but is by an astronomer and is in genuine print, so it's at the low end of the confidence scale IMO.

[add]
There was also some useful information on the Wikipedia talk page at http://www-astronomy.mps.ohio-state.edu/~pogge/Lectures/vistas97.html , but if I'm reading it right, the increased mass loss due to enhanced solar wind won't happen until after the sun has already become a red giant.

I also didn't see the 1.7 figure in the current version of the wikipedia article (I was going to flag it with a citation needed).

So at this point I don't know what to think, there may be room for considerably more discussion. At this point, I'm not even positive whether it's angular momentum or energy that should be conserved, though I'm leaning towards angular momentum.

 

[Chronos]

The Earth sun distance has not measurably increased or decreased over the past billion years, how about that? Put some numbers to that assertion. The wiki article assumes facts not in evidence, IMO. The sun has consistently brightened over the past 4.5 billion years by about 30%, and will continue to do so. The oceans may well boil off in about 1.2 billion years due to a runaway greenhouse effect. Is that important . . . probably not to us.

 

[Old Smuggler]

The Earth sun distance has not measurably increased or decreased over the past billion years, how about that?

That is what standard theory says. However, there are observational anomalies,
so this assertion is not particulary convincing. See

G.A. Krasinsky and V.A. Brumberg, Celes. Mech. & Dyn. Astron. 90, 267 (2004).